{"gene":"CCNT1","run_date":"2026-06-09T22:57:17","timeline":{"discoveries":[{"year":1998,"finding":"The cyclin domain of human CycT1 is necessary and sufficient to interact with HIV-1 Tat and promote cooperative binding to TAR RNA in vitro and mediate Tat transactivation in vivo. A Tat-TAR recognition motif (TRM) was identified at the carboxy-terminal edge of the cyclin domain. The interaction between Tat and hCycT1 requires zinc and essential cysteine residues in both proteins (C261 in hCycT1); murine CycT1 lacks C261 and forms only a weak, zinc-independent complex with HIV-1 Tat. A point mutation in mCycT1 (Y261C) restores high-affinity, zinc-dependent binding to Tat and TAR in vitro and rescues Tat transactivation in vivo. hCycT1 can interact simultaneously with Tat and CDK9 on TAR RNA in vitro.","method":"Alanine-scanning mutagenesis, in vitro binding/co-IP, TAR RNA binding assays, transactivation reporter assays, cloning and characterization of murine CycT1","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal methods (mutagenesis, in vitro reconstitution, in vivo reporter), replicated across species comparisons, foundational mechanistic paper","pmids":["9832504"],"is_preprint":false},{"year":1999,"finding":"Recruitment of CycT1/P-TEFb to the HIV-1 LTR promoter-proximal RNA target is fully sufficient to activate transcription elongation from that promoter; Tat and TAR become dispensable when CycT1 is artificially tethered. Activation by CycT1 required the ability to bind the CDK9 component of P-TEFb. The sole role of the Tat/TAR axis is to permit recruitment of CycT1/P-TEFb.","method":"Artificial RNA tethering assay, reporter gene assays, dominant-negative CDK9 binding mutants, nuclear run-on transcription assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal functional assays (artificial tethering, nuclear run-on, mutant complementation) establishing epistatic position of CycT1 in HIV transcription elongation","pmids":["10393900"],"is_preprint":false},{"year":2000,"finding":"CDK9 autophosphorylation at multiple C-terminal Ser/Thr residues is required for high-affinity binding of the Tat-P-TEFb complex to TAR RNA. The C-terminal half of CycT1 (beyond residue 303) masks Tat-TAR binding in the absence of ATP/phosphorylation; full-length CycT1 (1-728) is also phosphorylated by CDK9. Replacing CDK9 phosphorylation sites with negatively charged residues restores TAR binding to a catalytically inactive CDK9 mutant. P-TEFb also phosphorylates RNA polymerase II CTD substrates in vitro.","method":"In vitro kinase assays, TAR RNA binding assays, ATP-dependency studies, phosphomimetic mutations, affinity-purified complexes","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with mutagenesis and phosphomimetics, single lab but multiple orthogonal methods","pmids":["10958691"],"is_preprint":false},{"year":2002,"finding":"The histidine-rich stretch in the C-terminal region of CycT1 (positions 481-551) directly binds the CTD of RNA polymerase II, and this binding is required for P-TEFb to activate transcription of full-length transcripts from target genes when tethered to DNA upstream or downstream of promoters.","method":"Tethering assay to DNA, deletion/chimera constructs of CycT1 C-terminus, transcriptional activation reporter assays","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional tethering assays with deletion constructs, single lab, two complementary methods","pmids":["11739744"],"is_preprint":false},{"year":2002,"finding":"TAR RNA loop residues 252-260 of CycT1 directly contact the TAR RNA loop, enhancing the interaction of Tat residue K50 with the opposite side of the loop, as shown by systematic RNA-protein photocross-linking and protein footprinting. TAR RNA provides a scaffold for assembly of the Tat-CycT1 regulatory complex.","method":"RNA-protein photocross-linking, Western blot analysis, protein footprinting","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct cross-linking and footprinting experiments mapping contact residues, single lab, two orthogonal structural/biochemical methods","pmids":["12048247"],"is_preprint":false},{"year":2002,"finding":"CycT1-Tat binding to TAR RNA is highly cooperative (Hill coefficient 2.7, KD ~2.45 nM for the ternary complex). CycT1 does not bind TAR RNA in the absence of Tat. The C30-G34 base pair and functional groups O6 and N7 at positions 32 and 34 in the TAR loop are essential for CycT1-Tat interactions with TAR RNA.","method":"Gel electrophoresis mobility shift assay with comprehensive TAR loop mutants, quantitative binding analysis","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic mutagenesis with quantitative binding, single lab, defines molecular contacts","pmids":["12009901"],"is_preprint":false},{"year":2002,"finding":"Both human and murine CycT1, when DNA-tethered, can activate the HIV-1 LTR in a Tat- and TAR-independent manner, and this activity requires Sp1. This demonstrates that P-TEFb can stimulate HIV-1 LTR transcription via a DNA/Sp1-dependent mechanism distinct from the RNA-based Tat/TAR pathway.","method":"DNA tethering of CycT1, Sp1 depletion/rescue, reporter assays in murine cells","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional epistasis (Sp1 requirement), two complementary reporter assay approaches, single lab","pmids":["12458222"],"is_preprint":false},{"year":2003,"finding":"CTD analog heptapeptide repeats containing alanine substitutions (non-phosphorylatable) bind CycT1 via its histidine-rich region and block P-TEFb-mediated transcription elongation when placed near transcription units. The C. elegans transcriptional repressor PIE-1 similarly binds CycT1 via an alanine-containing heptapeptide repeat to inhibit transcriptional elongation.","method":"CTD analog competition, tethering reporter assays, pulldown/binding assays with PIE-1","journal":"Genes & development","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional competition and heterologous protein binding assay establishing CycT1 CTD-binding domain as the repression target, single lab","pmids":["12651893"],"is_preprint":false},{"year":2004,"finding":"CycC:CDK8 (not CycT1:CDK9/P-TEFb) phosphorylates the Notch ICD within its TAD and PEST domains; Mastermind (MAM) interacts directly with CDK8 and recruits both CycC:CDK8 and CycT1:CDK9/P-TEFb to the HES1 promoter. CycT1:CDK9/P-TEFb is recruited with Notch and coactivators to HES1 in signaling cells as part of the transcription elongation machinery.","method":"Chromatin immunoprecipitation (ChIP), Co-IP, recombinant CycC:CDK8 in vitro kinase assay on Notch ICD, point mutations of PEST Ser residues, subnuclear localization by immunofluorescence","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro kinase assay, ChIP, Co-IP, mutagenesis all in one paper; CycT1 role established by ChIP recruitment and genetic context","pmids":["15546612"],"is_preprint":false},{"year":2004,"finding":"The TRM (Tat-TAR recognition motif) region of CycT1 (C-terminal ~20 residues of the cyclin domain) is conformationally flexible or disordered in isolation, as assessed by partial proteolysis and circular dichroism, suggesting that conformational rearrangements accompany CycT1-Tat-TAR ternary complex formation. A cysteine at position 261 (absent in bovine CycT1) is required for metal binding in HIV ternary complex formation.","method":"Partial proteolysis, circular dichroism spectroscopy, mutagenesis of TRM residues","journal":"Virology","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — biophysical characterization with two complementary methods, single lab study of isolated domain","pmids":["14972556"],"is_preprint":false},{"year":2008,"finding":"Crystal structure of CDK9/CycT1 determined at high resolution. CycT1 is rotated ~26° relative to CDK9 compared to CDK2/CycA, showing plasticity in CDK-cyclin interactions. CDK9 autophosphorylates on Thr186 in the activation segment and three C-terminal sites; all autophosphorylation occurs in cis. The CycT1 C-terminal helix is flexible, which may be important for interactions with HIV TAT and HEXIM.","method":"X-ray crystallography, autophosphorylation assays with recombinant CDK9/CycT1","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus in vitro biochemical validation of autophosphorylation, rigorous mechanistic paper","pmids":["18566585"],"is_preprint":false},{"year":2008,"finding":"SC35 (splicing factor) affects transcriptional elongation by facilitating P-TEFb (CycT1-CDK9) recruitment to target genes. SC35 depletion induced Pol II accumulation within gene bodies, reduced CTD Ser2 phosphorylation, and was correlated with defective P-TEFb recruitment. Recombinant SC35 rescued the elongation defect in nuclear run-on experiments.","method":"siRNA depletion, ChIP, nuclear run-on, recombinant SC35 rescue, CTD phosphorylation analysis","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (ChIP, run-on, depletion, rescue), establishes SC35 as upstream regulator of P-TEFb recruitment","pmids":["18641664"],"is_preprint":false},{"year":2009,"finding":"Tat acetylation of Lys-28 by PCAF modulates the affinity and stability of HIV-1 Tat-CycT1-TAR complexes by enhancing interaction with the CycT1 Tat-TAR recognition motif (TRM). High-affinity assembly of the ternary complex correlates strongly with stimulation of transcription elongation in vitro and Tat activation in vivo. Bovine lentiviral Tat proteins evolved high-affinity TAR interaction independent of PCAF acetylation or CycT1.","method":"Tat acetylation assays, ternary complex assembly assays, transcription elongation in vitro, reporter assays in vivo, viral replication assays with compensatory mutations","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution of ternary complex, in vitro transcription elongation, in vivo reporter, and viral evolution assays in one rigorous study","pmids":["19223581"],"is_preprint":false},{"year":2012,"finding":"AFF4 serves as the central scaffold of the HIV-1 Tat super elongation complex (SEC), directly binding CycT1 (along with ELL2 and ENL/AF9) through short hydrophobic regions along its disordered axis. Mapping of binding sites was performed in vitro and in vivo, establishing that CycT1 is a direct binding partner of AFF4 and a bridging component linking P-TEFb to the larger elongation complex.","method":"In vitro binding assays mapping contact regions, in vivo co-IP, identification of binding partners","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal binding assays in vitro and in vivo, single lab, maps direct contacts","pmids":["23251033"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of a tripartite P-TEFb–AFF4 complex shows AFF4 meanders over the surface of the CycT1 subunit but makes no stable contacts with CDK9. Interface mutations reduced CycT1-AFF4 binding and AFF4-dependent transcription. AFF4 is positioned to make direct contacts with HIV Tat, and Tat enhances P-TEFb affinity for AFF4.","method":"X-ray crystallography, interface mutagenesis, transcription reporter assays, in vitro binding assays","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure combined with mutagenesis and functional assays in one rigorous study","pmids":["23471103"],"is_preprint":false},{"year":2014,"finding":"AFF4 binding to Tat-P-TEFb partially orders the CycT1 Tat-TAR recognition motif (TRM) and increases the affinity of Tat-P-TEFb for TAR RNA 30-fold. Crystal structure of a quaternary Tat-P-TEFb-AFF4 complex determined; AFF4 and Tat interact directly on the CycT1 surface, and AFF4 helix 2 stabilization (even without RNA contact) explains SEC preference.","method":"X-ray crystallography, in vitro TAR binding assays, interface mutagenesis of AFF1/AFF4, Tat-dependent transcription reporter assays","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure combined with quantitative binding assays and functional mutagenesis, explains mechanism of SEC preference","pmids":["24843025"],"is_preprint":false},{"year":2016,"finding":"5.9 Å cryo-EM/crystal structure of HIV-1 TAR in complex with Tat, AFF4, CDK9, and CycT1. The TAR central loop contacts the CycT1 TRM and the second Tat Zn2+-binding loop. AFF4 helix 2 is stabilized in the TAR complex (shown by HDX), explaining its 50-fold enhancement of TAR binding to the SEC. The Tat ARM enters the TAR major groove between the bulge and central loop.","method":"X-ray crystallography (5.9 Å), hydrogen-deuterium exchange (HDX), RNA SHAPE, SAXS, functional binding assays","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Strong — integrative structural analysis with multiple orthogonal methods (crystal structure, HDX, SAXS, SHAPE) in one study","pmids":["27731797"],"is_preprint":false},{"year":2018,"finding":"Crystal structure of TAR loop in complex with Tat and the SEC core (CycT1 TRM) at 3.5 Å resolution. The TAR central loop is stabilized by cross-loop hydrogen bonds and makes structure-specific contacts with CycT1 TRM side chains and Tat zinc-coordinating loop. Mutational analysis confirmed that TAR loop-CycT1 TRM interactions contribute importantly to binding affinity, and these interactions dominate over TAR bulge-Tat ARM interactions.","method":"X-ray crystallography, mutational analysis of CycT1 TRM and Tat, binding assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure with mutagenesis and quantitative binding validation","pmids":["30514815"],"is_preprint":false},{"year":2018,"finding":"CDK7-mediated phosphorylation of CDK9 at Ser-175 (confirmed by selective CDK7 inhibitor THZ1 and in vitro kinase assays) is required for Tat binding and proviral HIV reactivation but does not affect CDK9 kinase activity or P-TEFb assembly. Thr-186 phosphorylation stabilizes CDK9/CycT1 interface via arginine triad and E96 of CycT1; disruption of this interface causes accumulation of dephosphorylated CDK9 associated with cytoplasmic Hsp90/Cdc37 chaperone. Hsp90 inhibition blocks P-TEFb assembly and Thr-186 phosphorylation.","method":"Molecular dynamics simulations, CDK7 inhibitor (THZ1), in vitro kinase assays, co-IP, Hsp90 inhibition, site-directed mutagenesis of arginine triad","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro kinase assay plus mutagenesis plus pharmacological validation in one study, multiple orthogonal methods","pmids":["29743242"],"is_preprint":false},{"year":2021,"finding":"PKC-mediated phosphorylation of CycT1 promotes productive CycT1:CDK9 interactions (P-TEFb assembly) in human cells, while dephosphorylation of CycT1 by PP1 reverses this process. CycT1 not bound to CDK9 is rapidly degraded in resting/quiescent cells. PKC inhibitors or chronic PKC depletion cause P-TEFb disassembly and CycT1 degradation, recapitulating P-TEFb depletion in resting CD4+ T cells and anergic T cells.","method":"PKC inhibitor treatment, PP1 dephosphorylation assays, CycT1 degradation assays, primary cell experiments, co-IP","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal approaches (pharmacological inhibition, phosphatase treatment, primary cell experiments), mechanistically defines the PKC-PP1 switch controlling P-TEFb assembly","pmids":["34821217"],"is_preprint":false},{"year":2022,"finding":"Siah1 and Siah2 are the E3 ubiquitin ligases responsible for ubiquitination and degradation of free CycT1 (not bound to CDK9) in resting and quiescent cells. The ubiquitination and degradation of free CycT1 was mapped to its N-terminal region (positions 1-280) and involves six lysine residues. Inhibition of Siah1/2 rescued CycT1 expression in both proliferating and resting primary cells.","method":"Ubiquitination mapping, domain deletion analysis, Siah1/2 inhibition, co-IP, primary cell experiments","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Strong — identification of E3 ligases with ubiquitination site mapping and pharmacological rescue in primary cells, multiple complementary methods","pmids":["35524561"],"is_preprint":false},{"year":2022,"finding":"PARP1, activated by DNA damage, binds to transcriptionally engaged P-TEFb and poly(ADP-ribosyl)ates CycT1 at multiple positions including histidine residues. This modification prevents CycT1 from undergoing liquid-liquid phase separation required for CDK9 to hyperphosphorylate Pol II and stimulate elongation. Poly(ADP-ribosyl)ation of CycT1 promotes DNA repair and cell survival after DNA damage.","method":"PARP1 binding assays, PARylation site mapping (including histidine residues), phase separation assays, Pol II phosphorylation assays, cell survival assays","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — identifies writer (PARP1), modification sites, and functional consequence (phase separation disruption) with in vitro and cellular assays; multiple orthogonal methods","pmids":["35393539"],"is_preprint":false},{"year":2022,"finding":"Under stress, the CDK9/CycT1 heterodimer released from 7SK snRNP completely dissociates into monomers. Brd4 or SEC then recruits monomeric CDK9 and CycT1 to reassemble active P-TEFb on chromatin, inducing CDK9 autophosphorylation of T186. The same mechanism operates during entry into G1 phase after nocodazole release.","method":"Biochemical fractionation, co-IP, ChIP, cell cycle synchronization (nocodazole), stress treatment, pulldown assays","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple methods (Co-IP, ChIP, cell biology), single lab; establishes stepwise reassembly mechanism","pmids":["34935961"],"is_preprint":false},{"year":2023,"finding":"CCNT1 knockout (via CRISPR) prevents HIV-1 latency reactivation by a wide variety of latency reversal agents in primary CD4+ T cells and latently infected cell lines, without affecting T cell activation itself. RNA-seq showed CCNT1 regulates HIV-1 proviral genes to a larger extent than any other host gene but has no significant effects on host RNA transcripts in primary T cells after activation. CCNT1 function is non-essential for T cell biology but absolutely required for HIV latency reversal.","method":"CRISPR gene knockout screen, primary CD4+ T cell latency model, RNA-seq, multiple latency reversal agents","journal":"Viruses","confidence":"High","confidence_rationale":"Tier 2 / Strong — CRISPR KO in primary cells with transcriptome-wide analysis and multiple functional readouts; replicated as preprint and peer-reviewed paper","pmids":["37766271"],"is_preprint":false},{"year":2023,"finding":"7SK RNA methylation by METTL3 (induced by EGF signaling via phosphorylation of METTL3) enhances 7SK binding to heterogeneous nuclear ribonucleoproteins, causing release of the HEXIM1/P-TEFb complex (CDK9/CycT1) from the inhibitory 7SK snRNP and inducing transcriptional elongation. This establishes an m6A-based RNA methylation switch controlling CycT1-containing P-TEFb availability.","method":"METTL3 phosphorylation assays, 7SK methylation assays, P-TEFb release assays, transcription elongation assays, EGF signaling pathway analysis","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — identifies upstream signaling-to-P-TEFb mechanism, single lab, multiple assays but novel and not yet independently replicated","pmids":["37163588"],"is_preprint":false},{"year":2014,"finding":"A CycT1 mutant (V107E) eliminates binding to Hexim1, CDK9, and assembly on HIV TAR RNA or 7SK snRNA, yet retains strong AFF4 binding and slightly impaired Tat association. CycT1-V107E potently inhibits HIV replication and enforces viral latency in primary CD4+ T cells, defining Val107 in the N-terminal cyclin box as essential for P-TEFb assembly with its major partners.","method":"Mutagenesis, co-IP/pulldown binding assays for Hexim1/CDK9/AFF4/Tat, HIV replication and latency reporter assays in primary CD4+ T cells","journal":"Retrovirology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic mutagenesis with binding and functional assays, single lab, primary cell validation","pmids":["24985467"],"is_preprint":false},{"year":2012,"finding":"Nine novel CDK9/CCNT1-associated protein complexes (CCAPs) were identified in HeLa cell nuclei. siRNA depletion of five CCAPs (PPP1R10/TOX3/WDR82; TTF2; TPR; WRNIP1; FBXO11/CUL1/SKP1) enhanced Tat activation of integrated HIV-1 LTR, suggesting they negatively regulate P-TEFb availability similar to the 7SK snRNP.","method":"Co-IP/complexome mining, siRNA depletion, integrated HIV-1 LTR-luciferase reporter assays","journal":"Retrovirology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — complexome mining plus single siRNA reporter assay, indirect readout of CycT1 function, single lab","pmids":["23110726"],"is_preprint":false},{"year":2014,"finding":"A cell-based genetic analysis of 7SK snRNP mapped 7SK HEXIM1-binding to the 5'U-U bulge and central loop of stem-loop I; CycT1 and HEXIM1 form a combinatorial binding surface for 7SK. HEXIM1 inhibits CDK9 kinase activity via interactions between 7SK, HEXIM1, and CycT1. A tyrosine-to-alanine mutation in HEXIM1 (critical for CDK9 inhibition) converts HEXIM1 into a transcriptional activator.","method":"RNA tethering cell-based assay, HEXIM1/CycT1 7SK-binding mutants, reporter gene assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-based genetic epistasis with multiple mutant constructs mapping interaction surfaces, single lab","pmids":["24917669"],"is_preprint":false},{"year":2011,"finding":"Residues Q46, Q50, and F176 in the N-terminal region of human CycT1 play critical and distinctive roles in Tat-mediated transactivation. Q46 and Q50 are required for binding to Tat (in vitro pull-down); Q50 and F176 (along with C261) are required for Tat transactivation activity. A triple mutant (Q46A/Q50A/F176A) completely abolishes transcriptional activity.","method":"Alanine substitution mutagenesis based on 3D crystal structure, in vitro pull-down, luciferase reporter assays on HIV-1 LTR","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — structure-guided mutagenesis with in vitro binding and in vivo functional assays, single lab","pmids":["21763494"],"is_preprint":false},{"year":2012,"finding":"A CCNT1 splice variant (CYCT1b/dE7) that lacks exon 7 (generating a frameshift and premature stop codon) encodes a ~23 kDa protein covering ~70% of the cyclin box. This protein physically interacts with CDK9 and competes with full-length CCNT1 for CDK9 binding, acting dominant-negatively to inhibit HIV-1 transcription and Tat/LTR-driven transcription. The dE7/FL transcript ratio is high in quiescent PBMCs and low in proliferating cells.","method":"RT-PCR, ectopic expression, co-IP of dE7 with CDK9, luciferase reporter assays for Tat/LTR transcription, HIV-1 replication assays, NMD analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP of splice variant with CDK9, functional dominant-negative reporter and viral assays, single lab","pmids":["23569210"],"is_preprint":false},{"year":2005,"finding":"HEXIM1 inhibits Tat transactivation by repressing transcription mediated through direct activation of P-TEFb (via GAL4-CycT1 tethering). Effective Tat-inhibition requires the 7SK snRNA basic recognition motif of HEXIM1 as well as its C-terminus region required for interaction with CycT1. HEXIM1-mediated repression of Tat is not due to global inhibition of cellular transcription.","method":"HEXIM1/HEXIM2 co-expression, GAL4-CycT1 artificial tethering assay, HEXIM1 deletion/mutation analysis, reporter assays","journal":"Retrovirology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — tethering assay plus mutational dissection of HEXIM1-CycT1 interaction, single lab, two complementary approaches","pmids":["15992410"],"is_preprint":false},{"year":2026,"finding":"CycT1 (CCNT1) forms a hypoxia-specific, chromatin-associated interaction with nuclear-localized mitochondrial chaperone Tim8-Tim13 complexes and the hypoxia-inducible transcription factor BHLHE40. Tim8-Tim13 complex disruption and BHLHE40 silencing both impair the transcriptional response to acute hypoxia. This interaction is HIF-independent.","method":"Biochemical purification, co-IP validation across multiple human cell lines, Tim8-Tim13 disruption, BHLHE40 siRNA knockdown, transcriptional response assays","journal":"Science advances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical purification plus co-IP in multiple cell lines with functional knockdown assays, single lab, newly reported","pmids":["42160428"],"is_preprint":false},{"year":2002,"finding":"CycT1 is required as an essential cofactor for HIV-2 Tat and SIVmnd Tat function, in addition to HIV-1 Tat. Tat2 and Tat-M can also bind CycT2, but Tat-CycT2 complexes fail to bind TAR and are abortive. A single mutation in CycT2 (N260C) rescues TAR binding and Tat function for all three Tat proteins, demonstrating that C260/C261 is the key CycT1 residue for TAR binding in the context of multiple lentiviral Tats.","method":"Co-IP, in vitro binding assays (Tat-CycT1 and Tat-CycT2), TAR RNA binding assay, Tat transactivation reporter assay, site-directed mutagenesis of CycT2","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple binding assays and mutagenesis with functional validation, single lab extending findings from PMID 9832504","pmids":["10364329"],"is_preprint":false},{"year":2021,"finding":"FACT complex protein SUPT16H interferes with the association of Cyclin T1 (CCNT1) with the Tat-LTR axis. Depletion of SUPT16H enhances Tat-mediated HIV-1 LTR activity and spontaneously reverses HIV-1 latency, suggesting FACT suppresses HIV transcription in part by blocking CycT1/P-TEFb recruitment to the LTR.","method":"Co-IP (HIV Tat with SUPT16H), siRNA depletion, HIV-1 LTR reporter assays, HIV latency reversal assays in U1/HIV and J-LAT cells and primary CD4+ T cells","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP mapping interaction, siRNA depletion with functional consequence on CycT1 recruitment, multiple cell models","pmids":["26378236"],"is_preprint":false},{"year":2024,"finding":"Poly(ADP-ribosyl)ation of CycT1 is elevated in ionizing radiation (IR)-resistant nasopharyngeal carcinoma (NPC) cells, disrupting CycT1 phase separation, increasing RNA Pol II pausing, and contributing to IR resistance. Mutation of the major PARylation sites of CycT1 decreases RNAPII pausing and restores IR sensitivity in NPC cells.","method":"ChIP-seq, CycT1 PARylation site mutation, RNAPII pausing analysis, IR resistance assays in NPC cells","journal":"Journal of molecular cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — site-directed mutagenesis of PARylation sites with functional phenotype (RNAPII pausing, IR resistance), extends findings from PMID 35393539 to oncology context, single lab","pmids":["37407287"],"is_preprint":false},{"year":2014,"finding":"Cyclin T1 (CCNT1, CycT) is synthetically lethal with TSC1 and TSC2 mutations in Drosophila and mammalian cells. Individual knockdown of CycT (ortholog of CCNT1) reduced population growth rate of Drosophila cells lacking TSC1 or TSC2 but not wild-type cells; the same was true for mammalian TSC2-deficient cells including human tumor-derived cells.","method":"CRISPR-based Drosophila cell knockouts combined with RNAi screen, cross-species validation in mammalian TSC2-deficient and human tumor cells","journal":"Science signaling","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis (synthetic lethality) established by CRISPR/RNAi in two species, cross-species validation","pmids":["26350902"],"is_preprint":false},{"year":2014,"finding":"Both Cyclin T1 (Ccnt1) and its catalytic partner Cdk9 are required for development of Th1 cells and short-lived effector CTLs during viral infection; inhibiting Ccnt1 expression impaired these effector fates and enhanced Tfh and memory precursor CTL formation in vivo. Demonstrated by in vivo shRNAmir screens in TCR-transgenic CD4+ and CD8+ T cells responding to LCMV.","method":"In vivo pooled shRNAmir screen in TCR-transgenic T cells during LCMV infection, flow cytometric analysis of T cell differentiation phenotypes","journal":"Immunity","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo loss-of-function with specific T cell differentiation readout, two parallel screens (CD4+ and CD8+), single lab","pmids":["25148027"],"is_preprint":false},{"year":2021,"finding":"Fosl1 interacts with JunB (shown by co-IP) and promotes expression of Cyclin T1 (Ccnt1) during Xenopus tropicalis and mouse heart regeneration, as demonstrated by ChIP and luciferase reporter assays. Overexpression of Fosl1 promoted cardiomyocyte proliferation, and its knockdown suppressed it; Ccnt1 is a downstream transcriptional target of the Fosl1/JunB complex in this regenerative context.","method":"Co-immunoprecipitation (Fosl1-JunB interaction), luciferase reporter assay (Fosl1/JunB driving Ccnt1 promoter), ChIP analysis, in vivo overexpression/knockdown in Xenopus and neonatal mouse","journal":"NPJ Regenerative medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP, Co-IP, and reporter assays showing JunB/Fosl1 transcriptional control of Ccnt1 promoter; functional consequence in heart regeneration established by in vivo models","pmids":["34188056"],"is_preprint":false}],"current_model":"CCNT1 (Cyclin T1) is the regulatory cyclin subunit of P-TEFb (CDK9/CycT1), a kinase complex that phosphorylates RNA polymerase II CTD Ser2 to release promoter-proximal pausing and drive transcription elongation; its cyclin domain directly binds HIV-1 Tat and, together with Tat, cooperatively engages TAR RNA via a conformationally flexible Tat-TAR recognition motif (TRM) requiring zinc and C261, while its histidine-rich C-terminal region directly contacts the RNAPII CTD; P-TEFb assembly and activity are regulated by PKC-mediated phosphorylation of CycT1 (promoting CDK9 binding), dephosphorylation by PP1 (disassembly), ubiquitination and degradation of free CycT1 by Siah1/2 E3 ligases in resting cells, sequestration in the inactive 7SK snRNP (inhibited by HEXIM1/7SK via CycT1 contacts), release and reactivation via AFF4/Brd4-driven SEC reassembly on chromatin, and PARP1-mediated poly(ADP-ribosyl)ation of CycT1 histidine residues after DNA damage that disrupts its liquid-liquid phase separation and stalls elongation; structurally, CDK9/CycT1 shows a distinct 26° rotation compared to CDK2/CycA, with CycT1's C-terminal helix being flexible and important for regulatory interactions."},"narrative":{"mechanistic_narrative":"CCNT1 (Cyclin T1) is the regulatory cyclin subunit of the P-TEFb kinase complex, which drives RNA polymerase II transcription elongation, and is the host cofactor that is absolutely required for HIV-1 latency reversal [PMID:37766271, PMID:10393900]. CycT1 partners with the kinase CDK9, and a histidine-rich stretch in its C-terminal region (residues 481-551) directly binds the RNAPII CTD, an interaction required for P-TEFb to activate productive elongation [PMID:11739744]; the same CTD-binding region is the target of repressive CTD analogs and the elongation repressor PIE-1 [PMID:12651893]. In HIV-1, the cyclin domain of CycT1 is necessary and sufficient to bind Tat and to cooperatively engage TAR RNA through a conformationally flexible Tat-TAR recognition motif (TRM) at the C-terminal edge of the cyclin domain, a zinc-dependent interaction requiring Cys261 [PMID:9832504, PMID:14972556, PMID:12009901]; recruitment of CycT1/P-TEFb is the sole rate-limiting event, since artificial tethering of CycT1 renders Tat and TAR dispensable [PMID:10393900]. High-resolution structures define how the TRM, Tat, and the SEC scaffold AFF4 assemble cooperatively on TAR, with AFF4 ordering the TRM and increasing TAR affinity, and CDK9 autophosphorylation gating high-affinity complex formation [PMID:23471103, PMID:24843025, PMID:27731797, PMID:30514815, PMID:10958691]. P-TEFb availability is controlled by a layered regulatory network: PKC-mediated phosphorylation of CycT1 promotes CDK9 binding while PP1 reverses it [PMID:34821217], free CycT1 not bound to CDK9 is ubiquitinated and degraded by Siah1/Siah2 [PMID:35524561], CycT1 is sequestered in the inhibitory 7SK snRNP via a combinatorial CycT1-HEXIM1 surface [PMID:24917669, PMID:15992410], and stress- or cell-cycle-triggered dissociation allows Brd4/SEC-driven reassembly of active P-TEFb on chromatin [PMID:34935961]. Following DNA damage, PARP1 poly(ADP-ribosyl)ates CycT1 histidine residues, disrupting its liquid-liquid phase separation and stalling elongation, a mechanism co-opted in radioresistant nasopharyngeal carcinoma [PMID:35393539, PMID:37407287]. Beyond viral and transcriptional roles, CCNT1 is synthetically lethal with TSC1/TSC2 loss [PMID:26350902] and required for Th1 and short-lived effector CTL differentiation [PMID:25148027].","teleology":[{"year":1998,"claim":"Established the molecular basis for HIV-1 Tat function by showing CycT1's cyclin domain is the species-specific host factor that bridges Tat to TAR RNA, explaining the human-specific permissiveness of HIV transcription.","evidence":"Alanine-scanning mutagenesis, in vitro binding/co-IP, TAR binding and transactivation reporters, with cross-species (human vs murine CycT1) comparison","pmids":["9832504"],"confidence":"High","gaps":["Structural geometry of the ternary complex not resolved","Role within general (non-viral) transcription not yet addressed"]},{"year":1999,"claim":"Defined the epistatic position of CycT1 by showing recruitment of CycT1/P-TEFb is the sole rate-limiting step, making Tat and TAR dispensable when CycT1 is tethered directly.","evidence":"Artificial RNA tethering, nuclear run-on, dominant-negative CDK9-binding mutants, reporter assays","pmids":["10393900"],"confidence":"High","gaps":["Endogenous recruitment mechanism at native promoters not addressed","Identity of CTD-contacting region not yet mapped"]},{"year":2000,"claim":"Showed that CDK9 autophosphorylation licenses high-affinity Tat-P-TEFb-TAR assembly and that the CycT1 C-terminus masks Tat-TAR binding until phosphorylation occurs, coupling kinase activity to complex formation.","evidence":"In vitro kinase assays, phosphomimetic mutations, ATP-dependency TAR binding assays","pmids":["10958691"],"confidence":"High","gaps":["Precise phosphosites unmapped at this stage","In vivo relevance of masking not demonstrated"]},{"year":2002,"claim":"Identified the histidine-rich C-terminal region of CycT1 (481-551) as the direct RNAPII CTD-binding module required for productive elongation, linking CycT1 architecture to its core transcriptional output.","evidence":"DNA tethering with deletion/chimera constructs and transcription reporters","pmids":["11739744"],"confidence":"Medium","gaps":["Single-lab functional mapping without structural confirmation","Phospho-dependence of CTD contact not defined"]},{"year":2002,"claim":"Mapped the molecular contacts of the Tat-CycT1-TAR ternary complex, showing CycT1 loop residues contact the TAR loop and that assembly is highly cooperative with nanomolar affinity, establishing TAR as a scaffold.","evidence":"Photocross-linking, footprinting, EMSA with comprehensive TAR loop mutants and quantitative binding analysis","pmids":["12048247","12009901"],"confidence":"Medium","gaps":["Atomic-resolution structure not yet available","Conformational changes inferred indirectly"]},{"year":2002,"claim":"Revealed an alternative DNA/Sp1-dependent route for P-TEFb to activate the HIV LTR, distinct from the RNA-based Tat/TAR pathway, broadening the recruitment logic of CycT1.","evidence":"DNA tethering of CycT1, Sp1 depletion/rescue, reporters in murine cells","pmids":["12458222"],"confidence":"Medium","gaps":["Physiological prevalence of the Sp1 route unclear","Mechanism of Sp1-CycT1 connection not defined"]},{"year":2003,"claim":"Demonstrated that the CycT1 CTD-binding region is itself a regulatory target, since CTD analogs and the repressor PIE-1 bind it to block elongation, identifying a mode of P-TEFb inhibition.","evidence":"CTD analog competition, tethering reporters, PIE-1 pulldown/binding","pmids":["12651893"],"confidence":"Medium","gaps":["Whether endogenous repressors use this surface in mammals unaddressed","Single-lab functional study"]},{"year":2004,"claim":"Provided biophysical evidence that the CycT1 TRM is intrinsically flexible/disordered in isolation, supporting a coupled folding-binding model for ternary complex assembly and confirming Cys261's role in metal-dependent assembly.","evidence":"Partial proteolysis, circular dichroism, TRM mutagenesis","pmids":["14972556"],"confidence":"Medium","gaps":["Disorder shown only for isolated domain","Folding transition not captured structurally"]},{"year":2004,"claim":"Placed CycT1/P-TEFb within physiological signaling by showing its recruitment with Notch ICD and Mastermind to the HES1 promoter as the elongation arm of Notch transcriptional output.","evidence":"ChIP, Co-IP, in vitro kinase assay (CycC:CDK8 on Notch), mutagenesis, immunofluorescence","pmids":["15546612"],"confidence":"High","gaps":["Direct CycT1 contribution inferred from recruitment, not biochemical necessity","Generality across Notch targets not tested"]},{"year":2008,"claim":"Solved the CDK9/CycT1 crystal structure, revealing a distinctive ~26° cyclin rotation and a flexible CycT1 C-terminal helix, and demonstrated cis autophosphorylation on T186 plus C-terminal sites.","evidence":"X-ray crystallography and autophosphorylation assays with recombinant complex","pmids":["18566585"],"confidence":"High","gaps":["TRM and full C-terminus not resolved in structure","How flexibility enables regulatory interactions shown only indirectly"]},{"year":2008,"claim":"Identified the splicing factor SC35 as an upstream factor facilitating P-TEFb recruitment, connecting CycT1/CDK9 elongation activity to splicing machinery via CTD Ser2 phosphorylation.","evidence":"siRNA depletion, ChIP, nuclear run-on, recombinant SC35 rescue","pmids":["18641664"],"confidence":"High","gaps":["Direct SC35-CycT1 contact not established","Genome-wide scope of SC35-P-TEFb dependence not mapped"]},{"year":2009,"claim":"Showed that PCAF acetylation of Tat tunes ternary complex affinity at the CycT1 TRM, linking post-translational modification of the viral partner to CycT1-dependent elongation efficiency.","evidence":"Acetylation assays, ternary complex assembly, in vitro elongation, reporters, viral replication with compensatory mutations","pmids":["19223581"],"confidence":"High","gaps":["Acetylation effect specific to HIV-1 Tat","Structural basis of acetyl-TRM enhancement not resolved here"]},{"year":2012,"claim":"Identified AFF4 as the SEC scaffold that directly binds CycT1 through short hydrophobic regions, establishing CycT1 as the bridging subunit linking P-TEFb to the larger super elongation complex.","evidence":"In vitro and in vivo binding/co-IP mapping of contact regions","pmids":["23251033"],"confidence":"Medium","gaps":["Structural detail of interface not yet defined at this stage","Functional consequence on TAR binding not quantified"]},{"year":2013,"claim":"Resolved the P-TEFb-AFF4 interface crystallographically, showing AFF4 contacts only CycT1 and not CDK9 and is positioned to engage Tat, explaining how the SEC organizes around the cyclin surface.","evidence":"X-ray crystallography, interface mutagenesis, transcription reporters, binding assays","pmids":["23471103"],"confidence":"High","gaps":["TAR RNA not included in this structure","TRM ordering by AFF4 not yet demonstrated"]},{"year":2014,"claim":"Demonstrated that AFF4 binding allosterically orders the CycT1 TRM and increases Tat-P-TEFb affinity for TAR ~30-fold, providing the structural mechanism for SEC preference in HIV transcription.","evidence":"Crystallography of the quaternary complex, quantitative TAR binding, AFF1/AFF4 interface mutagenesis, reporters","pmids":["24843025"],"confidence":"High","gaps":["TAR RNA modeled but not co-crystallized at high resolution here","Dynamics of TRM ordering inferred"]},{"year":2014,"claim":"Defined Val107 in the N-terminal cyclin box as essential for P-TEFb assembly with CDK9, HEXIM1, and 7SK while sparing AFF4 binding, separating assembly from scaffold engagement and providing a latency-enforcing mutant.","evidence":"Mutagenesis, co-IP/pulldown for multiple partners, HIV replication/latency assays in primary CD4+ T cells","pmids":["24985467"],"confidence":"Medium","gaps":["Structural basis of V107 selectivity not resolved","Single-lab mutant characterization"]},{"year":2014,"claim":"Mapped the combinatorial CycT1-HEXIM1 surface on 7SK stem-loop I, establishing how the 7SK snRNP inhibits CDK9 kinase activity through CycT1 contacts.","evidence":"RNA tethering cell-based assay with HEXIM1/CycT1 7SK-binding mutants and reporters","pmids":["24917669"],"confidence":"Medium","gaps":["Atomic structure of the 7SK-HEXIM1-CycT1 surface not determined","Cell-based genetic readout indirect"]},{"year":2016,"claim":"Delivered an integrative structure of the Tat-P-TEFb-AFF4-TAR complex, showing the TAR loop simultaneously contacts the CycT1 TRM and Tat zinc loop and that AFF4 helix 2 stabilization underlies enhanced TAR binding.","evidence":"Cryo-EM/crystallography at 5.9 Å, HDX, SHAPE, SAXS, binding assays","pmids":["27731797"],"confidence":"High","gaps":["Limited resolution for side-chain detail","Dynamics during transcription cycle not captured"]},{"year":2018,"claim":"Refined the TAR-TRM interface to 3.5 Å, showing TAR loop-CycT1 TRM contacts dominate over Tat ARM-bulge interactions, redefining the affinity determinants of the complex.","evidence":"X-ray crystallography, TRM/Tat mutational analysis, binding assays","pmids":["30514815"],"confidence":"High","gaps":["Full SEC context not in structure","Conformational dynamics not addressed"]},{"year":2018,"claim":"Established kinase-cascade and chaperone control of P-TEFb assembly: CDK7 phosphorylates CDK9 Ser175 for Tat binding, T186 phosphorylation stabilizes the CDK9/CycT1 interface via E96 of CycT1, and Hsp90/Cdc37 buffer unassembled CDK9.","evidence":"MD simulations, CDK7 inhibitor THZ1, in vitro kinase assays, co-IP, Hsp90 inhibition, arginine-triad mutagenesis","pmids":["29743242"],"confidence":"High","gaps":["In vivo dynamics of the chaperone-assembly cycle not fully resolved","CycT1-specific phospho-regulation not separated here"]},{"year":2021,"claim":"Identified a PKC-PP1 phosphorylation switch on CycT1 that toggles P-TEFb assembly, explaining loss of P-TEFb in resting and anergic T cells through CycT1 destabilization.","evidence":"PKC inhibitors, PP1 dephosphorylation, degradation assays, co-IP in primary cells","pmids":["34821217"],"confidence":"High","gaps":["Exact CycT1 PKC phosphosites not enumerated here","Coupling to degradation machinery defined separately"]},{"year":2022,"claim":"Identified Siah1/Siah2 as the E3 ligases degrading free CycT1 via its N-terminal region, defining the proteostatic mechanism that limits P-TEFb in resting cells.","evidence":"Ubiquitination mapping, domain deletion, Siah1/2 inhibition, co-IP, primary cell rescue","pmids":["35524561"],"confidence":"High","gaps":["Upstream signals controlling Siah activity not defined","Specific degron lysines among the six not individually resolved"]},{"year":2022,"claim":"Showed that P-TEFb fully dissociates into monomers under stress and is reassembled on chromatin by Brd4/SEC with CDK9 T186 autophosphorylation, defining a stepwise activation route shared with G1 entry.","evidence":"Fractionation, co-IP, ChIP, nocodazole synchronization, stress treatment, pulldowns","pmids":["34935961"],"confidence":"Medium","gaps":["Single-lab reassembly model","Quantitative kinetics of monomer pools not established"]},{"year":2022,"claim":"Revealed that PARP1 poly(ADP-ribosyl)ates CycT1 histidine residues after DNA damage, disrupting CycT1 phase separation and stalling elongation to favor DNA repair and survival.","evidence":"PARP1 binding, PARylation site mapping, phase separation assays, Pol II phosphorylation, survival assays","pmids":["35393539"],"confidence":"High","gaps":["Composition of CycT1 condensates not fully defined","Reversal/erasure of the modification not characterized"]},{"year":2023,"claim":"Established by CRISPR knockout that CCNT1 is dispensable for T cell biology yet absolutely required for HIV-1 latency reversal, nominating it as a host-directed target whose loss does not perturb the host transcriptome.","evidence":"CRISPR knockout in primary CD4+ T cells and latent cell lines, RNA-seq, multiple latency reversal agents","pmids":["37766271"],"confidence":"High","gaps":["Apparent host transcriptome neutrality contrasts with developmental roles elsewhere","Compensation by paralogs not examined"]},{"year":2023,"claim":"Connected EGF signaling to P-TEFb release via METTL3-dependent m6A methylation of 7SK, defining an RNA-methylation switch controlling availability of CycT1-containing P-TEFb.","evidence":"METTL3 phosphorylation, 7SK methylation, P-TEFb release and elongation assays, EGF pathway analysis","pmids":["37163588"],"confidence":"Medium","gaps":["Not independently replicated","Direct effect on CycT1 versus snRNP architecture not separated"]},{"year":2024,"claim":"Extended the PARylation-phase separation axis to oncology, showing elevated CycT1 PARylation enforces RNAPII pausing and radioresistance in nasopharyngeal carcinoma, reversible by mutating PARylation sites.","evidence":"ChIP-seq, CycT1 PARylation-site mutation, RNAPII pausing analysis, IR resistance assays in NPC cells","pmids":["37407287"],"confidence":"Medium","gaps":["Single-lab oncology extension","Therapeutic targetability not tested in vivo"]},{"year":2026,"claim":"Uncovered a hypoxia-specific, HIF-independent chromatin role for CycT1 in complex with nuclear Tim8-Tim13 chaperones and BHLHE40, implicating CycT1 in the acute hypoxic transcriptional response.","evidence":"Biochemical purification, co-IP across multiple cell lines, Tim8-Tim13 disruption, BHLHE40 knockdown, transcription assays","pmids":["42160428"],"confidence":"Medium","gaps":["Newly reported, single lab","Mechanism linking CycT1 to hypoxic gene selection unresolved"]},{"year":null,"claim":"How CycT1 phase separation, partner selection (7SK/SEC/Brd4), and post-translational switches are integrated to direct gene-specific elongation in physiological (non-viral) contexts remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["Endogenous gene targets of CycT1-driven elongation largely uncatalogued","How competing regulatory inputs (PKC/PP1, Siah, PARP1, 7SK) are coordinated in vivo is unknown","Physiological role of the hypoxia and developmental interactions mechanistically incomplete"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,3,19,10]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[0,4,5,16,17]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[1,3,8]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[8,11,22,26]},{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[22,31,21]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[1,3,11]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,1,23]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[21,34]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[36,23]}],"complexes":["P-TEFb (CDK9/CycT1)","super elongation complex (SEC)","7SK snRNP"],"partners":["CDK9","TAT (HIV-1)","AFF4","HEXIM1","BRD4","PARP1","SIAH1","BHLHE40"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O60563","full_name":"Cyclin-T1","aliases":[],"length_aa":726,"mass_kda":80.7,"function":"Regulatory subunit of the cyclin-dependent kinase pair (CDK9/cyclin-T1) complex, also called positive transcription elongation factor B (P-TEFb), which facilitates the transition from abortive to productive elongation by phosphorylating the CTD (C-terminal domain) of the large subunit of RNA polymerase II (RNA Pol II) (PubMed:16109376, PubMed:16109377, PubMed:30134174, PubMed:35393539). Required to activate the protein kinase activity of CDK9: acts by mediating formation of liquid-liquid phase separation (LLPS) that enhances binding of P-TEFb to the CTD of RNA Pol II (PubMed:29849146, PubMed:35393539) (Microbial infection) In case of HIV or SIV infections, binds to the transactivation domain of the viral nuclear transcriptional activator, Tat, thereby increasing Tat's affinity for the transactivating response RNA element (TAR RNA). Serves as an essential cofactor for Tat, by promoting RNA Pol II activation, allowing transcription of viral genes","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/O60563/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CCNT1","classification":"Not Classified","n_dependent_lines":129,"n_total_lines":1208,"dependency_fraction":0.10678807947019868},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CDK9","stoichiometry":10.0},{"gene":"LARP7","stoichiometry":10.0},{"gene":"DDOST","stoichiometry":0.2},{"gene":"DDX21","stoichiometry":0.2},{"gene":"OST4","stoichiometry":0.2},{"gene":"PSPC1","stoichiometry":0.2},{"gene":"RPS16","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/CCNT1","total_profiled":1310},"omim":[{"mim_id":"615695","title":"HEXAMETHYLENE BIS ACETAMIDE-INDUCIBLE PROTEIN 2; HEXIM2","url":"https://www.omim.org/entry/615695"},{"mim_id":"615488","title":"KAT8 REGULATORY NSL COMPLEX, SUBUNIT 2; KANSL2","url":"https://www.omim.org/entry/615488"},{"mim_id":"607328","title":"HEXAMETHYLENE BIS ACETAMIDE-INDUCIBLE PROTEIN 1; HEXIM1","url":"https://www.omim.org/entry/607328"},{"mim_id":"603251","title":"CYCLIN-DEPENDENT KINASE 9; CDK9","url":"https://www.omim.org/entry/603251"},{"mim_id":"143055","title":"CYCLIN T1; CCNT1","url":"https://www.omim.org/entry/143055"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CCNT1"},"hgnc":{"alias_symbol":["CCNT","CYCT1"],"prev_symbol":["HIVE1"]},"alphafold":{"accession":"O60563","domains":[{"cath_id":"1.10.472.10","chopping":"2-150","consensus_level":"medium","plddt":92.5434,"start":2,"end":150},{"cath_id":"1.10.472.10","chopping":"152-273","consensus_level":"medium","plddt":85.5301,"start":152,"end":273}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O60563","model_url":"https://alphafold.ebi.ac.uk/files/AF-O60563-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O60563-F1-predicted_aligned_error_v6.png","plddt_mean":59.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CCNT1","jax_strain_url":"https://www.jax.org/strain/search?query=CCNT1"},"sequence":{"accession":"O60563","fasta_url":"https://rest.uniprot.org/uniprotkb/O60563.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O60563/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O60563"}},"corpus_meta":[{"pmid":"15546612","id":"PMC_15546612","title":"Mastermind 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A Tat-TAR recognition motif (TRM) was identified at the carboxy-terminal edge of the cyclin domain. The interaction between Tat and hCycT1 requires zinc and essential cysteine residues in both proteins (C261 in hCycT1); murine CycT1 lacks C261 and forms only a weak, zinc-independent complex with HIV-1 Tat. A point mutation in mCycT1 (Y261C) restores high-affinity, zinc-dependent binding to Tat and TAR in vitro and rescues Tat transactivation in vivo. hCycT1 can interact simultaneously with Tat and CDK9 on TAR RNA in vitro.\",\n      \"method\": \"Alanine-scanning mutagenesis, in vitro binding/co-IP, TAR RNA binding assays, transactivation reporter assays, cloning and characterization of murine CycT1\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal methods (mutagenesis, in vitro reconstitution, in vivo reporter), replicated across species comparisons, foundational mechanistic paper\",\n      \"pmids\": [\"9832504\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Recruitment of CycT1/P-TEFb to the HIV-1 LTR promoter-proximal RNA target is fully sufficient to activate transcription elongation from that promoter; Tat and TAR become dispensable when CycT1 is artificially tethered. Activation by CycT1 required the ability to bind the CDK9 component of P-TEFb. The sole role of the Tat/TAR axis is to permit recruitment of CycT1/P-TEFb.\",\n      \"method\": \"Artificial RNA tethering assay, reporter gene assays, dominant-negative CDK9 binding mutants, nuclear run-on transcription assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal functional assays (artificial tethering, nuclear run-on, mutant complementation) establishing epistatic position of CycT1 in HIV transcription elongation\",\n      \"pmids\": [\"10393900\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"CDK9 autophosphorylation at multiple C-terminal Ser/Thr residues is required for high-affinity binding of the Tat-P-TEFb complex to TAR RNA. The C-terminal half of CycT1 (beyond residue 303) masks Tat-TAR binding in the absence of ATP/phosphorylation; full-length CycT1 (1-728) is also phosphorylated by CDK9. Replacing CDK9 phosphorylation sites with negatively charged residues restores TAR binding to a catalytically inactive CDK9 mutant. P-TEFb also phosphorylates RNA polymerase II CTD substrates in vitro.\",\n      \"method\": \"In vitro kinase assays, TAR RNA binding assays, ATP-dependency studies, phosphomimetic mutations, affinity-purified complexes\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with mutagenesis and phosphomimetics, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"10958691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The histidine-rich stretch in the C-terminal region of CycT1 (positions 481-551) directly binds the CTD of RNA polymerase II, and this binding is required for P-TEFb to activate transcription of full-length transcripts from target genes when tethered to DNA upstream or downstream of promoters.\",\n      \"method\": \"Tethering assay to DNA, deletion/chimera constructs of CycT1 C-terminus, transcriptional activation reporter assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional tethering assays with deletion constructs, single lab, two complementary methods\",\n      \"pmids\": [\"11739744\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"TAR RNA loop residues 252-260 of CycT1 directly contact the TAR RNA loop, enhancing the interaction of Tat residue K50 with the opposite side of the loop, as shown by systematic RNA-protein photocross-linking and protein footprinting. TAR RNA provides a scaffold for assembly of the Tat-CycT1 regulatory complex.\",\n      \"method\": \"RNA-protein photocross-linking, Western blot analysis, protein footprinting\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct cross-linking and footprinting experiments mapping contact residues, single lab, two orthogonal structural/biochemical methods\",\n      \"pmids\": [\"12048247\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CycT1-Tat binding to TAR RNA is highly cooperative (Hill coefficient 2.7, KD ~2.45 nM for the ternary complex). CycT1 does not bind TAR RNA in the absence of Tat. The C30-G34 base pair and functional groups O6 and N7 at positions 32 and 34 in the TAR loop are essential for CycT1-Tat interactions with TAR RNA.\",\n      \"method\": \"Gel electrophoresis mobility shift assay with comprehensive TAR loop mutants, quantitative binding analysis\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic mutagenesis with quantitative binding, single lab, defines molecular contacts\",\n      \"pmids\": [\"12009901\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Both human and murine CycT1, when DNA-tethered, can activate the HIV-1 LTR in a Tat- and TAR-independent manner, and this activity requires Sp1. This demonstrates that P-TEFb can stimulate HIV-1 LTR transcription via a DNA/Sp1-dependent mechanism distinct from the RNA-based Tat/TAR pathway.\",\n      \"method\": \"DNA tethering of CycT1, Sp1 depletion/rescue, reporter assays in murine cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional epistasis (Sp1 requirement), two complementary reporter assay approaches, single lab\",\n      \"pmids\": [\"12458222\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"CTD analog heptapeptide repeats containing alanine substitutions (non-phosphorylatable) bind CycT1 via its histidine-rich region and block P-TEFb-mediated transcription elongation when placed near transcription units. The C. elegans transcriptional repressor PIE-1 similarly binds CycT1 via an alanine-containing heptapeptide repeat to inhibit transcriptional elongation.\",\n      \"method\": \"CTD analog competition, tethering reporter assays, pulldown/binding assays with PIE-1\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional competition and heterologous protein binding assay establishing CycT1 CTD-binding domain as the repression target, single lab\",\n      \"pmids\": [\"12651893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"CycC:CDK8 (not CycT1:CDK9/P-TEFb) phosphorylates the Notch ICD within its TAD and PEST domains; Mastermind (MAM) interacts directly with CDK8 and recruits both CycC:CDK8 and CycT1:CDK9/P-TEFb to the HES1 promoter. CycT1:CDK9/P-TEFb is recruited with Notch and coactivators to HES1 in signaling cells as part of the transcription elongation machinery.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), Co-IP, recombinant CycC:CDK8 in vitro kinase assay on Notch ICD, point mutations of PEST Ser residues, subnuclear localization by immunofluorescence\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro kinase assay, ChIP, Co-IP, mutagenesis all in one paper; CycT1 role established by ChIP recruitment and genetic context\",\n      \"pmids\": [\"15546612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The TRM (Tat-TAR recognition motif) region of CycT1 (C-terminal ~20 residues of the cyclin domain) is conformationally flexible or disordered in isolation, as assessed by partial proteolysis and circular dichroism, suggesting that conformational rearrangements accompany CycT1-Tat-TAR ternary complex formation. A cysteine at position 261 (absent in bovine CycT1) is required for metal binding in HIV ternary complex formation.\",\n      \"method\": \"Partial proteolysis, circular dichroism spectroscopy, mutagenesis of TRM residues\",\n      \"journal\": \"Virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — biophysical characterization with two complementary methods, single lab study of isolated domain\",\n      \"pmids\": [\"14972556\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Crystal structure of CDK9/CycT1 determined at high resolution. CycT1 is rotated ~26° relative to CDK9 compared to CDK2/CycA, showing plasticity in CDK-cyclin interactions. CDK9 autophosphorylates on Thr186 in the activation segment and three C-terminal sites; all autophosphorylation occurs in cis. The CycT1 C-terminal helix is flexible, which may be important for interactions with HIV TAT and HEXIM.\",\n      \"method\": \"X-ray crystallography, autophosphorylation assays with recombinant CDK9/CycT1\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus in vitro biochemical validation of autophosphorylation, rigorous mechanistic paper\",\n      \"pmids\": [\"18566585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SC35 (splicing factor) affects transcriptional elongation by facilitating P-TEFb (CycT1-CDK9) recruitment to target genes. SC35 depletion induced Pol II accumulation within gene bodies, reduced CTD Ser2 phosphorylation, and was correlated with defective P-TEFb recruitment. Recombinant SC35 rescued the elongation defect in nuclear run-on experiments.\",\n      \"method\": \"siRNA depletion, ChIP, nuclear run-on, recombinant SC35 rescue, CTD phosphorylation analysis\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (ChIP, run-on, depletion, rescue), establishes SC35 as upstream regulator of P-TEFb recruitment\",\n      \"pmids\": [\"18641664\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Tat acetylation of Lys-28 by PCAF modulates the affinity and stability of HIV-1 Tat-CycT1-TAR complexes by enhancing interaction with the CycT1 Tat-TAR recognition motif (TRM). High-affinity assembly of the ternary complex correlates strongly with stimulation of transcription elongation in vitro and Tat activation in vivo. Bovine lentiviral Tat proteins evolved high-affinity TAR interaction independent of PCAF acetylation or CycT1.\",\n      \"method\": \"Tat acetylation assays, ternary complex assembly assays, transcription elongation in vitro, reporter assays in vivo, viral replication assays with compensatory mutations\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution of ternary complex, in vitro transcription elongation, in vivo reporter, and viral evolution assays in one rigorous study\",\n      \"pmids\": [\"19223581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"AFF4 serves as the central scaffold of the HIV-1 Tat super elongation complex (SEC), directly binding CycT1 (along with ELL2 and ENL/AF9) through short hydrophobic regions along its disordered axis. Mapping of binding sites was performed in vitro and in vivo, establishing that CycT1 is a direct binding partner of AFF4 and a bridging component linking P-TEFb to the larger elongation complex.\",\n      \"method\": \"In vitro binding assays mapping contact regions, in vivo co-IP, identification of binding partners\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal binding assays in vitro and in vivo, single lab, maps direct contacts\",\n      \"pmids\": [\"23251033\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of a tripartite P-TEFb–AFF4 complex shows AFF4 meanders over the surface of the CycT1 subunit but makes no stable contacts with CDK9. Interface mutations reduced CycT1-AFF4 binding and AFF4-dependent transcription. AFF4 is positioned to make direct contacts with HIV Tat, and Tat enhances P-TEFb affinity for AFF4.\",\n      \"method\": \"X-ray crystallography, interface mutagenesis, transcription reporter assays, in vitro binding assays\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure combined with mutagenesis and functional assays in one rigorous study\",\n      \"pmids\": [\"23471103\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"AFF4 binding to Tat-P-TEFb partially orders the CycT1 Tat-TAR recognition motif (TRM) and increases the affinity of Tat-P-TEFb for TAR RNA 30-fold. Crystal structure of a quaternary Tat-P-TEFb-AFF4 complex determined; AFF4 and Tat interact directly on the CycT1 surface, and AFF4 helix 2 stabilization (even without RNA contact) explains SEC preference.\",\n      \"method\": \"X-ray crystallography, in vitro TAR binding assays, interface mutagenesis of AFF1/AFF4, Tat-dependent transcription reporter assays\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure combined with quantitative binding assays and functional mutagenesis, explains mechanism of SEC preference\",\n      \"pmids\": [\"24843025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"5.9 Å cryo-EM/crystal structure of HIV-1 TAR in complex with Tat, AFF4, CDK9, and CycT1. The TAR central loop contacts the CycT1 TRM and the second Tat Zn2+-binding loop. AFF4 helix 2 is stabilized in the TAR complex (shown by HDX), explaining its 50-fold enhancement of TAR binding to the SEC. The Tat ARM enters the TAR major groove between the bulge and central loop.\",\n      \"method\": \"X-ray crystallography (5.9 Å), hydrogen-deuterium exchange (HDX), RNA SHAPE, SAXS, functional binding assays\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — integrative structural analysis with multiple orthogonal methods (crystal structure, HDX, SAXS, SHAPE) in one study\",\n      \"pmids\": [\"27731797\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Crystal structure of TAR loop in complex with Tat and the SEC core (CycT1 TRM) at 3.5 Å resolution. The TAR central loop is stabilized by cross-loop hydrogen bonds and makes structure-specific contacts with CycT1 TRM side chains and Tat zinc-coordinating loop. Mutational analysis confirmed that TAR loop-CycT1 TRM interactions contribute importantly to binding affinity, and these interactions dominate over TAR bulge-Tat ARM interactions.\",\n      \"method\": \"X-ray crystallography, mutational analysis of CycT1 TRM and Tat, binding assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure with mutagenesis and quantitative binding validation\",\n      \"pmids\": [\"30514815\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CDK7-mediated phosphorylation of CDK9 at Ser-175 (confirmed by selective CDK7 inhibitor THZ1 and in vitro kinase assays) is required for Tat binding and proviral HIV reactivation but does not affect CDK9 kinase activity or P-TEFb assembly. Thr-186 phosphorylation stabilizes CDK9/CycT1 interface via arginine triad and E96 of CycT1; disruption of this interface causes accumulation of dephosphorylated CDK9 associated with cytoplasmic Hsp90/Cdc37 chaperone. Hsp90 inhibition blocks P-TEFb assembly and Thr-186 phosphorylation.\",\n      \"method\": \"Molecular dynamics simulations, CDK7 inhibitor (THZ1), in vitro kinase assays, co-IP, Hsp90 inhibition, site-directed mutagenesis of arginine triad\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro kinase assay plus mutagenesis plus pharmacological validation in one study, multiple orthogonal methods\",\n      \"pmids\": [\"29743242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PKC-mediated phosphorylation of CycT1 promotes productive CycT1:CDK9 interactions (P-TEFb assembly) in human cells, while dephosphorylation of CycT1 by PP1 reverses this process. CycT1 not bound to CDK9 is rapidly degraded in resting/quiescent cells. PKC inhibitors or chronic PKC depletion cause P-TEFb disassembly and CycT1 degradation, recapitulating P-TEFb depletion in resting CD4+ T cells and anergic T cells.\",\n      \"method\": \"PKC inhibitor treatment, PP1 dephosphorylation assays, CycT1 degradation assays, primary cell experiments, co-IP\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal approaches (pharmacological inhibition, phosphatase treatment, primary cell experiments), mechanistically defines the PKC-PP1 switch controlling P-TEFb assembly\",\n      \"pmids\": [\"34821217\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Siah1 and Siah2 are the E3 ubiquitin ligases responsible for ubiquitination and degradation of free CycT1 (not bound to CDK9) in resting and quiescent cells. The ubiquitination and degradation of free CycT1 was mapped to its N-terminal region (positions 1-280) and involves six lysine residues. Inhibition of Siah1/2 rescued CycT1 expression in both proliferating and resting primary cells.\",\n      \"method\": \"Ubiquitination mapping, domain deletion analysis, Siah1/2 inhibition, co-IP, primary cell experiments\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — identification of E3 ligases with ubiquitination site mapping and pharmacological rescue in primary cells, multiple complementary methods\",\n      \"pmids\": [\"35524561\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PARP1, activated by DNA damage, binds to transcriptionally engaged P-TEFb and poly(ADP-ribosyl)ates CycT1 at multiple positions including histidine residues. This modification prevents CycT1 from undergoing liquid-liquid phase separation required for CDK9 to hyperphosphorylate Pol II and stimulate elongation. Poly(ADP-ribosyl)ation of CycT1 promotes DNA repair and cell survival after DNA damage.\",\n      \"method\": \"PARP1 binding assays, PARylation site mapping (including histidine residues), phase separation assays, Pol II phosphorylation assays, cell survival assays\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — identifies writer (PARP1), modification sites, and functional consequence (phase separation disruption) with in vitro and cellular assays; multiple orthogonal methods\",\n      \"pmids\": [\"35393539\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Under stress, the CDK9/CycT1 heterodimer released from 7SK snRNP completely dissociates into monomers. Brd4 or SEC then recruits monomeric CDK9 and CycT1 to reassemble active P-TEFb on chromatin, inducing CDK9 autophosphorylation of T186. The same mechanism operates during entry into G1 phase after nocodazole release.\",\n      \"method\": \"Biochemical fractionation, co-IP, ChIP, cell cycle synchronization (nocodazole), stress treatment, pulldown assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple methods (Co-IP, ChIP, cell biology), single lab; establishes stepwise reassembly mechanism\",\n      \"pmids\": [\"34935961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CCNT1 knockout (via CRISPR) prevents HIV-1 latency reactivation by a wide variety of latency reversal agents in primary CD4+ T cells and latently infected cell lines, without affecting T cell activation itself. RNA-seq showed CCNT1 regulates HIV-1 proviral genes to a larger extent than any other host gene but has no significant effects on host RNA transcripts in primary T cells after activation. CCNT1 function is non-essential for T cell biology but absolutely required for HIV latency reversal.\",\n      \"method\": \"CRISPR gene knockout screen, primary CD4+ T cell latency model, RNA-seq, multiple latency reversal agents\",\n      \"journal\": \"Viruses\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — CRISPR KO in primary cells with transcriptome-wide analysis and multiple functional readouts; replicated as preprint and peer-reviewed paper\",\n      \"pmids\": [\"37766271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"7SK RNA methylation by METTL3 (induced by EGF signaling via phosphorylation of METTL3) enhances 7SK binding to heterogeneous nuclear ribonucleoproteins, causing release of the HEXIM1/P-TEFb complex (CDK9/CycT1) from the inhibitory 7SK snRNP and inducing transcriptional elongation. This establishes an m6A-based RNA methylation switch controlling CycT1-containing P-TEFb availability.\",\n      \"method\": \"METTL3 phosphorylation assays, 7SK methylation assays, P-TEFb release assays, transcription elongation assays, EGF signaling pathway analysis\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — identifies upstream signaling-to-P-TEFb mechanism, single lab, multiple assays but novel and not yet independently replicated\",\n      \"pmids\": [\"37163588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A CycT1 mutant (V107E) eliminates binding to Hexim1, CDK9, and assembly on HIV TAR RNA or 7SK snRNA, yet retains strong AFF4 binding and slightly impaired Tat association. CycT1-V107E potently inhibits HIV replication and enforces viral latency in primary CD4+ T cells, defining Val107 in the N-terminal cyclin box as essential for P-TEFb assembly with its major partners.\",\n      \"method\": \"Mutagenesis, co-IP/pulldown binding assays for Hexim1/CDK9/AFF4/Tat, HIV replication and latency reporter assays in primary CD4+ T cells\",\n      \"journal\": \"Retrovirology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic mutagenesis with binding and functional assays, single lab, primary cell validation\",\n      \"pmids\": [\"24985467\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Nine novel CDK9/CCNT1-associated protein complexes (CCAPs) were identified in HeLa cell nuclei. siRNA depletion of five CCAPs (PPP1R10/TOX3/WDR82; TTF2; TPR; WRNIP1; FBXO11/CUL1/SKP1) enhanced Tat activation of integrated HIV-1 LTR, suggesting they negatively regulate P-TEFb availability similar to the 7SK snRNP.\",\n      \"method\": \"Co-IP/complexome mining, siRNA depletion, integrated HIV-1 LTR-luciferase reporter assays\",\n      \"journal\": \"Retrovirology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — complexome mining plus single siRNA reporter assay, indirect readout of CycT1 function, single lab\",\n      \"pmids\": [\"23110726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A cell-based genetic analysis of 7SK snRNP mapped 7SK HEXIM1-binding to the 5'U-U bulge and central loop of stem-loop I; CycT1 and HEXIM1 form a combinatorial binding surface for 7SK. HEXIM1 inhibits CDK9 kinase activity via interactions between 7SK, HEXIM1, and CycT1. A tyrosine-to-alanine mutation in HEXIM1 (critical for CDK9 inhibition) converts HEXIM1 into a transcriptional activator.\",\n      \"method\": \"RNA tethering cell-based assay, HEXIM1/CycT1 7SK-binding mutants, reporter gene assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-based genetic epistasis with multiple mutant constructs mapping interaction surfaces, single lab\",\n      \"pmids\": [\"24917669\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Residues Q46, Q50, and F176 in the N-terminal region of human CycT1 play critical and distinctive roles in Tat-mediated transactivation. Q46 and Q50 are required for binding to Tat (in vitro pull-down); Q50 and F176 (along with C261) are required for Tat transactivation activity. A triple mutant (Q46A/Q50A/F176A) completely abolishes transcriptional activity.\",\n      \"method\": \"Alanine substitution mutagenesis based on 3D crystal structure, in vitro pull-down, luciferase reporter assays on HIV-1 LTR\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — structure-guided mutagenesis with in vitro binding and in vivo functional assays, single lab\",\n      \"pmids\": [\"21763494\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"A CCNT1 splice variant (CYCT1b/dE7) that lacks exon 7 (generating a frameshift and premature stop codon) encodes a ~23 kDa protein covering ~70% of the cyclin box. This protein physically interacts with CDK9 and competes with full-length CCNT1 for CDK9 binding, acting dominant-negatively to inhibit HIV-1 transcription and Tat/LTR-driven transcription. The dE7/FL transcript ratio is high in quiescent PBMCs and low in proliferating cells.\",\n      \"method\": \"RT-PCR, ectopic expression, co-IP of dE7 with CDK9, luciferase reporter assays for Tat/LTR transcription, HIV-1 replication assays, NMD analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP of splice variant with CDK9, functional dominant-negative reporter and viral assays, single lab\",\n      \"pmids\": [\"23569210\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"HEXIM1 inhibits Tat transactivation by repressing transcription mediated through direct activation of P-TEFb (via GAL4-CycT1 tethering). Effective Tat-inhibition requires the 7SK snRNA basic recognition motif of HEXIM1 as well as its C-terminus region required for interaction with CycT1. HEXIM1-mediated repression of Tat is not due to global inhibition of cellular transcription.\",\n      \"method\": \"HEXIM1/HEXIM2 co-expression, GAL4-CycT1 artificial tethering assay, HEXIM1 deletion/mutation analysis, reporter assays\",\n      \"journal\": \"Retrovirology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — tethering assay plus mutational dissection of HEXIM1-CycT1 interaction, single lab, two complementary approaches\",\n      \"pmids\": [\"15992410\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"CycT1 (CCNT1) forms a hypoxia-specific, chromatin-associated interaction with nuclear-localized mitochondrial chaperone Tim8-Tim13 complexes and the hypoxia-inducible transcription factor BHLHE40. Tim8-Tim13 complex disruption and BHLHE40 silencing both impair the transcriptional response to acute hypoxia. This interaction is HIF-independent.\",\n      \"method\": \"Biochemical purification, co-IP validation across multiple human cell lines, Tim8-Tim13 disruption, BHLHE40 siRNA knockdown, transcriptional response assays\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical purification plus co-IP in multiple cell lines with functional knockdown assays, single lab, newly reported\",\n      \"pmids\": [\"42160428\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"CycT1 is required as an essential cofactor for HIV-2 Tat and SIVmnd Tat function, in addition to HIV-1 Tat. Tat2 and Tat-M can also bind CycT2, but Tat-CycT2 complexes fail to bind TAR and are abortive. A single mutation in CycT2 (N260C) rescues TAR binding and Tat function for all three Tat proteins, demonstrating that C260/C261 is the key CycT1 residue for TAR binding in the context of multiple lentiviral Tats.\",\n      \"method\": \"Co-IP, in vitro binding assays (Tat-CycT1 and Tat-CycT2), TAR RNA binding assay, Tat transactivation reporter assay, site-directed mutagenesis of CycT2\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple binding assays and mutagenesis with functional validation, single lab extending findings from PMID 9832504\",\n      \"pmids\": [\"10364329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FACT complex protein SUPT16H interferes with the association of Cyclin T1 (CCNT1) with the Tat-LTR axis. Depletion of SUPT16H enhances Tat-mediated HIV-1 LTR activity and spontaneously reverses HIV-1 latency, suggesting FACT suppresses HIV transcription in part by blocking CycT1/P-TEFb recruitment to the LTR.\",\n      \"method\": \"Co-IP (HIV Tat with SUPT16H), siRNA depletion, HIV-1 LTR reporter assays, HIV latency reversal assays in U1/HIV and J-LAT cells and primary CD4+ T cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP mapping interaction, siRNA depletion with functional consequence on CycT1 recruitment, multiple cell models\",\n      \"pmids\": [\"26378236\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Poly(ADP-ribosyl)ation of CycT1 is elevated in ionizing radiation (IR)-resistant nasopharyngeal carcinoma (NPC) cells, disrupting CycT1 phase separation, increasing RNA Pol II pausing, and contributing to IR resistance. Mutation of the major PARylation sites of CycT1 decreases RNAPII pausing and restores IR sensitivity in NPC cells.\",\n      \"method\": \"ChIP-seq, CycT1 PARylation site mutation, RNAPII pausing analysis, IR resistance assays in NPC cells\",\n      \"journal\": \"Journal of molecular cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — site-directed mutagenesis of PARylation sites with functional phenotype (RNAPII pausing, IR resistance), extends findings from PMID 35393539 to oncology context, single lab\",\n      \"pmids\": [\"37407287\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Cyclin T1 (CCNT1, CycT) is synthetically lethal with TSC1 and TSC2 mutations in Drosophila and mammalian cells. Individual knockdown of CycT (ortholog of CCNT1) reduced population growth rate of Drosophila cells lacking TSC1 or TSC2 but not wild-type cells; the same was true for mammalian TSC2-deficient cells including human tumor-derived cells.\",\n      \"method\": \"CRISPR-based Drosophila cell knockouts combined with RNAi screen, cross-species validation in mammalian TSC2-deficient and human tumor cells\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis (synthetic lethality) established by CRISPR/RNAi in two species, cross-species validation\",\n      \"pmids\": [\"26350902\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Both Cyclin T1 (Ccnt1) and its catalytic partner Cdk9 are required for development of Th1 cells and short-lived effector CTLs during viral infection; inhibiting Ccnt1 expression impaired these effector fates and enhanced Tfh and memory precursor CTL formation in vivo. Demonstrated by in vivo shRNAmir screens in TCR-transgenic CD4+ and CD8+ T cells responding to LCMV.\",\n      \"method\": \"In vivo pooled shRNAmir screen in TCR-transgenic T cells during LCMV infection, flow cytometric analysis of T cell differentiation phenotypes\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo loss-of-function with specific T cell differentiation readout, two parallel screens (CD4+ and CD8+), single lab\",\n      \"pmids\": [\"25148027\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Fosl1 interacts with JunB (shown by co-IP) and promotes expression of Cyclin T1 (Ccnt1) during Xenopus tropicalis and mouse heart regeneration, as demonstrated by ChIP and luciferase reporter assays. Overexpression of Fosl1 promoted cardiomyocyte proliferation, and its knockdown suppressed it; Ccnt1 is a downstream transcriptional target of the Fosl1/JunB complex in this regenerative context.\",\n      \"method\": \"Co-immunoprecipitation (Fosl1-JunB interaction), luciferase reporter assay (Fosl1/JunB driving Ccnt1 promoter), ChIP analysis, in vivo overexpression/knockdown in Xenopus and neonatal mouse\",\n      \"journal\": \"NPJ Regenerative medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP, Co-IP, and reporter assays showing JunB/Fosl1 transcriptional control of Ccnt1 promoter; functional consequence in heart regeneration established by in vivo models\",\n      \"pmids\": [\"34188056\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CCNT1 (Cyclin T1) is the regulatory cyclin subunit of P-TEFb (CDK9/CycT1), a kinase complex that phosphorylates RNA polymerase II CTD Ser2 to release promoter-proximal pausing and drive transcription elongation; its cyclin domain directly binds HIV-1 Tat and, together with Tat, cooperatively engages TAR RNA via a conformationally flexible Tat-TAR recognition motif (TRM) requiring zinc and C261, while its histidine-rich C-terminal region directly contacts the RNAPII CTD; P-TEFb assembly and activity are regulated by PKC-mediated phosphorylation of CycT1 (promoting CDK9 binding), dephosphorylation by PP1 (disassembly), ubiquitination and degradation of free CycT1 by Siah1/2 E3 ligases in resting cells, sequestration in the inactive 7SK snRNP (inhibited by HEXIM1/7SK via CycT1 contacts), release and reactivation via AFF4/Brd4-driven SEC reassembly on chromatin, and PARP1-mediated poly(ADP-ribosyl)ation of CycT1 histidine residues after DNA damage that disrupts its liquid-liquid phase separation and stalls elongation; structurally, CDK9/CycT1 shows a distinct 26° rotation compared to CDK2/CycA, with CycT1's C-terminal helix being flexible and important for regulatory interactions.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CCNT1 (Cyclin T1) is the regulatory cyclin subunit of the P-TEFb kinase complex, which drives RNA polymerase II transcription elongation, and is the host cofactor that is absolutely required for HIV-1 latency reversal [#23, #1]. CycT1 partners with the kinase CDK9, and a histidine-rich stretch in its C-terminal region (residues 481-551) directly binds the RNAPII CTD, an interaction required for P-TEFb to activate productive elongation [#3]; the same CTD-binding region is the target of repressive CTD analogs and the elongation repressor PIE-1 [#7]. In HIV-1, the cyclin domain of CycT1 is necessary and sufficient to bind Tat and to cooperatively engage TAR RNA through a conformationally flexible Tat-TAR recognition motif (TRM) at the C-terminal edge of the cyclin domain, a zinc-dependent interaction requiring Cys261 [#0, #9, #5]; recruitment of CycT1/P-TEFb is the sole rate-limiting event, since artificial tethering of CycT1 renders Tat and TAR dispensable [#1]. High-resolution structures define how the TRM, Tat, and the SEC scaffold AFF4 assemble cooperatively on TAR, with AFF4 ordering the TRM and increasing TAR affinity, and CDK9 autophosphorylation gating high-affinity complex formation [#14, #15, #16, #17, #2]. P-TEFb availability is controlled by a layered regulatory network: PKC-mediated phosphorylation of CycT1 promotes CDK9 binding while PP1 reverses it [#19], free CycT1 not bound to CDK9 is ubiquitinated and degraded by Siah1/Siah2 [#20], CycT1 is sequestered in the inhibitory 7SK snRNP via a combinatorial CycT1-HEXIM1 surface [#27, #30], and stress- or cell-cycle-triggered dissociation allows Brd4/SEC-driven reassembly of active P-TEFb on chromatin [#22]. Following DNA damage, PARP1 poly(ADP-ribosyl)ates CycT1 histidine residues, disrupting its liquid-liquid phase separation and stalling elongation, a mechanism co-opted in radioresistant nasopharyngeal carcinoma [#21, #34]. Beyond viral and transcriptional roles, CCNT1 is synthetically lethal with TSC1/TSC2 loss [#35] and required for Th1 and short-lived effector CTL differentiation [#36].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Established the molecular basis for HIV-1 Tat function by showing CycT1's cyclin domain is the species-specific host factor that bridges Tat to TAR RNA, explaining the human-specific permissiveness of HIV transcription.\",\n      \"evidence\": \"Alanine-scanning mutagenesis, in vitro binding/co-IP, TAR binding and transactivation reporters, with cross-species (human vs murine CycT1) comparison\",\n      \"pmids\": [\"9832504\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural geometry of the ternary complex not resolved\", \"Role within general (non-viral) transcription not yet addressed\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Defined the epistatic position of CycT1 by showing recruitment of CycT1/P-TEFb is the sole rate-limiting step, making Tat and TAR dispensable when CycT1 is tethered directly.\",\n      \"evidence\": \"Artificial RNA tethering, nuclear run-on, dominant-negative CDK9-binding mutants, reporter assays\",\n      \"pmids\": [\"10393900\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous recruitment mechanism at native promoters not addressed\", \"Identity of CTD-contacting region not yet mapped\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Showed that CDK9 autophosphorylation licenses high-affinity Tat-P-TEFb-TAR assembly and that the CycT1 C-terminus masks Tat-TAR binding until phosphorylation occurs, coupling kinase activity to complex formation.\",\n      \"evidence\": \"In vitro kinase assays, phosphomimetic mutations, ATP-dependency TAR binding assays\",\n      \"pmids\": [\"10958691\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise phosphosites unmapped at this stage\", \"In vivo relevance of masking not demonstrated\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identified the histidine-rich C-terminal region of CycT1 (481-551) as the direct RNAPII CTD-binding module required for productive elongation, linking CycT1 architecture to its core transcriptional output.\",\n      \"evidence\": \"DNA tethering with deletion/chimera constructs and transcription reporters\",\n      \"pmids\": [\"11739744\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab functional mapping without structural confirmation\", \"Phospho-dependence of CTD contact not defined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapped the molecular contacts of the Tat-CycT1-TAR ternary complex, showing CycT1 loop residues contact the TAR loop and that assembly is highly cooperative with nanomolar affinity, establishing TAR as a scaffold.\",\n      \"evidence\": \"Photocross-linking, footprinting, EMSA with comprehensive TAR loop mutants and quantitative binding analysis\",\n      \"pmids\": [\"12048247\", \"12009901\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Atomic-resolution structure not yet available\", \"Conformational changes inferred indirectly\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Revealed an alternative DNA/Sp1-dependent route for P-TEFb to activate the HIV LTR, distinct from the RNA-based Tat/TAR pathway, broadening the recruitment logic of CycT1.\",\n      \"evidence\": \"DNA tethering of CycT1, Sp1 depletion/rescue, reporters in murine cells\",\n      \"pmids\": [\"12458222\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological prevalence of the Sp1 route unclear\", \"Mechanism of Sp1-CycT1 connection not defined\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstrated that the CycT1 CTD-binding region is itself a regulatory target, since CTD analogs and the repressor PIE-1 bind it to block elongation, identifying a mode of P-TEFb inhibition.\",\n      \"evidence\": \"CTD analog competition, tethering reporters, PIE-1 pulldown/binding\",\n      \"pmids\": [\"12651893\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether endogenous repressors use this surface in mammals unaddressed\", \"Single-lab functional study\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Provided biophysical evidence that the CycT1 TRM is intrinsically flexible/disordered in isolation, supporting a coupled folding-binding model for ternary complex assembly and confirming Cys261's role in metal-dependent assembly.\",\n      \"evidence\": \"Partial proteolysis, circular dichroism, TRM mutagenesis\",\n      \"pmids\": [\"14972556\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Disorder shown only for isolated domain\", \"Folding transition not captured structurally\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Placed CycT1/P-TEFb within physiological signaling by showing its recruitment with Notch ICD and Mastermind to the HES1 promoter as the elongation arm of Notch transcriptional output.\",\n      \"evidence\": \"ChIP, Co-IP, in vitro kinase assay (CycC:CDK8 on Notch), mutagenesis, immunofluorescence\",\n      \"pmids\": [\"15546612\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct CycT1 contribution inferred from recruitment, not biochemical necessity\", \"Generality across Notch targets not tested\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Solved the CDK9/CycT1 crystal structure, revealing a distinctive ~26° cyclin rotation and a flexible CycT1 C-terminal helix, and demonstrated cis autophosphorylation on T186 plus C-terminal sites.\",\n      \"evidence\": \"X-ray crystallography and autophosphorylation assays with recombinant complex\",\n      \"pmids\": [\"18566585\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"TRM and full C-terminus not resolved in structure\", \"How flexibility enables regulatory interactions shown only indirectly\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identified the splicing factor SC35 as an upstream factor facilitating P-TEFb recruitment, connecting CycT1/CDK9 elongation activity to splicing machinery via CTD Ser2 phosphorylation.\",\n      \"evidence\": \"siRNA depletion, ChIP, nuclear run-on, recombinant SC35 rescue\",\n      \"pmids\": [\"18641664\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct SC35-CycT1 contact not established\", \"Genome-wide scope of SC35-P-TEFb dependence not mapped\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Showed that PCAF acetylation of Tat tunes ternary complex affinity at the CycT1 TRM, linking post-translational modification of the viral partner to CycT1-dependent elongation efficiency.\",\n      \"evidence\": \"Acetylation assays, ternary complex assembly, in vitro elongation, reporters, viral replication with compensatory mutations\",\n      \"pmids\": [\"19223581\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Acetylation effect specific to HIV-1 Tat\", \"Structural basis of acetyl-TRM enhancement not resolved here\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Identified AFF4 as the SEC scaffold that directly binds CycT1 through short hydrophobic regions, establishing CycT1 as the bridging subunit linking P-TEFb to the larger super elongation complex.\",\n      \"evidence\": \"In vitro and in vivo binding/co-IP mapping of contact regions\",\n      \"pmids\": [\"23251033\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural detail of interface not yet defined at this stage\", \"Functional consequence on TAR binding not quantified\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Resolved the P-TEFb-AFF4 interface crystallographically, showing AFF4 contacts only CycT1 and not CDK9 and is positioned to engage Tat, explaining how the SEC organizes around the cyclin surface.\",\n      \"evidence\": \"X-ray crystallography, interface mutagenesis, transcription reporters, binding assays\",\n      \"pmids\": [\"23471103\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"TAR RNA not included in this structure\", \"TRM ordering by AFF4 not yet demonstrated\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstrated that AFF4 binding allosterically orders the CycT1 TRM and increases Tat-P-TEFb affinity for TAR ~30-fold, providing the structural mechanism for SEC preference in HIV transcription.\",\n      \"evidence\": \"Crystallography of the quaternary complex, quantitative TAR binding, AFF1/AFF4 interface mutagenesis, reporters\",\n      \"pmids\": [\"24843025\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"TAR RNA modeled but not co-crystallized at high resolution here\", \"Dynamics of TRM ordering inferred\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined Val107 in the N-terminal cyclin box as essential for P-TEFb assembly with CDK9, HEXIM1, and 7SK while sparing AFF4 binding, separating assembly from scaffold engagement and providing a latency-enforcing mutant.\",\n      \"evidence\": \"Mutagenesis, co-IP/pulldown for multiple partners, HIV replication/latency assays in primary CD4+ T cells\",\n      \"pmids\": [\"24985467\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of V107 selectivity not resolved\", \"Single-lab mutant characterization\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Mapped the combinatorial CycT1-HEXIM1 surface on 7SK stem-loop I, establishing how the 7SK snRNP inhibits CDK9 kinase activity through CycT1 contacts.\",\n      \"evidence\": \"RNA tethering cell-based assay with HEXIM1/CycT1 7SK-binding mutants and reporters\",\n      \"pmids\": [\"24917669\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Atomic structure of the 7SK-HEXIM1-CycT1 surface not determined\", \"Cell-based genetic readout indirect\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Delivered an integrative structure of the Tat-P-TEFb-AFF4-TAR complex, showing the TAR loop simultaneously contacts the CycT1 TRM and Tat zinc loop and that AFF4 helix 2 stabilization underlies enhanced TAR binding.\",\n      \"evidence\": \"Cryo-EM/crystallography at 5.9 Å, HDX, SHAPE, SAXS, binding assays\",\n      \"pmids\": [\"27731797\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Limited resolution for side-chain detail\", \"Dynamics during transcription cycle not captured\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Refined the TAR-TRM interface to 3.5 Å, showing TAR loop-CycT1 TRM contacts dominate over Tat ARM-bulge interactions, redefining the affinity determinants of the complex.\",\n      \"evidence\": \"X-ray crystallography, TRM/Tat mutational analysis, binding assays\",\n      \"pmids\": [\"30514815\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full SEC context not in structure\", \"Conformational dynamics not addressed\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Established kinase-cascade and chaperone control of P-TEFb assembly: CDK7 phosphorylates CDK9 Ser175 for Tat binding, T186 phosphorylation stabilizes the CDK9/CycT1 interface via E96 of CycT1, and Hsp90/Cdc37 buffer unassembled CDK9.\",\n      \"evidence\": \"MD simulations, CDK7 inhibitor THZ1, in vitro kinase assays, co-IP, Hsp90 inhibition, arginine-triad mutagenesis\",\n      \"pmids\": [\"29743242\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo dynamics of the chaperone-assembly cycle not fully resolved\", \"CycT1-specific phospho-regulation not separated here\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identified a PKC-PP1 phosphorylation switch on CycT1 that toggles P-TEFb assembly, explaining loss of P-TEFb in resting and anergic T cells through CycT1 destabilization.\",\n      \"evidence\": \"PKC inhibitors, PP1 dephosphorylation, degradation assays, co-IP in primary cells\",\n      \"pmids\": [\"34821217\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Exact CycT1 PKC phosphosites not enumerated here\", \"Coupling to degradation machinery defined separately\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identified Siah1/Siah2 as the E3 ligases degrading free CycT1 via its N-terminal region, defining the proteostatic mechanism that limits P-TEFb in resting cells.\",\n      \"evidence\": \"Ubiquitination mapping, domain deletion, Siah1/2 inhibition, co-IP, primary cell rescue\",\n      \"pmids\": [\"35524561\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Upstream signals controlling Siah activity not defined\", \"Specific degron lysines among the six not individually resolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Showed that P-TEFb fully dissociates into monomers under stress and is reassembled on chromatin by Brd4/SEC with CDK9 T186 autophosphorylation, defining a stepwise activation route shared with G1 entry.\",\n      \"evidence\": \"Fractionation, co-IP, ChIP, nocodazole synchronization, stress treatment, pulldowns\",\n      \"pmids\": [\"34935961\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab reassembly model\", \"Quantitative kinetics of monomer pools not established\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Revealed that PARP1 poly(ADP-ribosyl)ates CycT1 histidine residues after DNA damage, disrupting CycT1 phase separation and stalling elongation to favor DNA repair and survival.\",\n      \"evidence\": \"PARP1 binding, PARylation site mapping, phase separation assays, Pol II phosphorylation, survival assays\",\n      \"pmids\": [\"35393539\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Composition of CycT1 condensates not fully defined\", \"Reversal/erasure of the modification not characterized\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Established by CRISPR knockout that CCNT1 is dispensable for T cell biology yet absolutely required for HIV-1 latency reversal, nominating it as a host-directed target whose loss does not perturb the host transcriptome.\",\n      \"evidence\": \"CRISPR knockout in primary CD4+ T cells and latent cell lines, RNA-seq, multiple latency reversal agents\",\n      \"pmids\": [\"37766271\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Apparent host transcriptome neutrality contrasts with developmental roles elsewhere\", \"Compensation by paralogs not examined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Connected EGF signaling to P-TEFb release via METTL3-dependent m6A methylation of 7SK, defining an RNA-methylation switch controlling availability of CycT1-containing P-TEFb.\",\n      \"evidence\": \"METTL3 phosphorylation, 7SK methylation, P-TEFb release and elongation assays, EGF pathway analysis\",\n      \"pmids\": [\"37163588\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Not independently replicated\", \"Direct effect on CycT1 versus snRNP architecture not separated\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Extended the PARylation-phase separation axis to oncology, showing elevated CycT1 PARylation enforces RNAPII pausing and radioresistance in nasopharyngeal carcinoma, reversible by mutating PARylation sites.\",\n      \"evidence\": \"ChIP-seq, CycT1 PARylation-site mutation, RNAPII pausing analysis, IR resistance assays in NPC cells\",\n      \"pmids\": [\"37407287\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab oncology extension\", \"Therapeutic targetability not tested in vivo\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Uncovered a hypoxia-specific, HIF-independent chromatin role for CycT1 in complex with nuclear Tim8-Tim13 chaperones and BHLHE40, implicating CycT1 in the acute hypoxic transcriptional response.\",\n      \"evidence\": \"Biochemical purification, co-IP across multiple cell lines, Tim8-Tim13 disruption, BHLHE40 knockdown, transcription assays\",\n      \"pmids\": [\"42160428\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Newly reported, single lab\", \"Mechanism linking CycT1 to hypoxic gene selection unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CycT1 phase separation, partner selection (7SK/SEC/Brd4), and post-translational switches are integrated to direct gene-specific elongation in physiological (non-viral) contexts remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Endogenous gene targets of CycT1-driven elongation largely uncatalogued\", \"How competing regulatory inputs (PKC/PP1, Siah, PARP1, 7SK) are coordinated in vivo is unknown\", \"Physiological role of the hypoxia and developmental interactions mechanistically incomplete\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 3, 19, 10]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 4, 5, 16, 17]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [1, 3, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [8, 11, 22, 26]},\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [22, 31, 21]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [1, 3, 11]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 1, 23]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [21, 34]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [36, 23]}\n    ],\n    \"complexes\": [\n      \"P-TEFb (CDK9/CycT1)\",\n      \"super elongation complex (SEC)\",\n      \"7SK snRNP\"\n    ],\n    \"partners\": [\n      \"CDK9\",\n      \"Tat (HIV-1)\",\n      \"AFF4\",\n      \"HEXIM1\",\n      \"Brd4\",\n      \"PARP1\",\n      \"Siah1\",\n      \"BHLHE40\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}