{"gene":"CDKN1B","run_date":"2026-06-09T22:57:18","timeline":{"discoveries":[{"year":2002,"finding":"hKIS (human kinase interacting stathmin) is the nuclear kinase responsible for phosphorylating p27(Kip1) on serine 10 (S10). hKIS binds the C-terminal domain of p27(Kip1) and phosphorylates it on S10 in vitro and in vivo, promoting its nuclear export to the cytoplasm. hKIS is activated by mitogens during G0/G1, and siRNA depletion of KIS inhibits S10 phosphorylation and enhances growth arrest.","method":"In vitro kinase assay, co-immunoprecipitation, siRNA knockdown, cell cycle analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — in vitro kinase assay plus in vivo validation, siRNA epistasis, single lab with multiple orthogonal methods","pmids":["12093740"],"is_preprint":false},{"year":1999,"finding":"Cyclins D1 and D2, induced by Myc, sequester p27(Kip1) and p21(Cip1) into cyclin D-CDK complexes, thereby releasing cyclin E-CDK2 from inhibition and promoting cell cycle progression. Primary fibroblasts from cyclin D1(-/-) or cyclin D2(-/-) mice do not proliferate in response to Myc. A kinase-defective cyclin D1 mutant that still binds CKIs restores Myc sensitivity, demonstrating that the sequestration function—not kinase activity—is required.","method":"Genetic knockout mouse embryo fibroblasts, retroviral complementation with kinase-dead mutants, biochemical co-immunoprecipitation","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — genetic epistasis with multiple knockouts, biochemical validation, mutant complementation","pmids":["10508164"],"is_preprint":false},{"year":2001,"finding":"c-Myc transcriptionally represses the p27(Kip1) gene. The repression requires c-Myc homology box II, maps to a region -20 to +20 of the p27 promoter that contains a putative Inr element, and is facilitated by Max. c-Myc represses p27 promoter activity in multiple cell types including B cells, T cells, smooth muscle cells, and breast cancer cells.","method":"Promoter-luciferase reporter assays, co-transfection of c-Myc expression vectors, deletion/mutation analysis, Max co-transfection","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reporter assays with mutagenesis in multiple cell types, single lab","pmids":["11313917"],"is_preprint":false},{"year":1999,"finding":"The Ah receptor (AhR) directly induces p27(Kip1) transcription without requiring ongoing protein synthesis or cell proliferation, and this induction mediates dioxin-induced suppression of cell proliferation. p27(Kip1) antisense-expressing cells are resistant to dioxin-mediated growth arrest, and thymuses from Kip1-deficient mice are largely resistant to dioxin-induced proliferation inhibition.","method":"Antisense expression, Kip1-knockout mouse tissue culture, transcription assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic rescue with antisense and knockout, multiple orthogonal approaches","pmids":["10398686"],"is_preprint":false},{"year":2011,"finding":"p27(Kip1) and p57(Kip2) cooperatively maintain hematopoietic stem cell (HSC) quiescence by retaining Hsc70/cyclin D1 complex in the cytoplasm. Combined deficiency of p57 and p27 in HSCs allows nuclear import of the Hsc70/cyclin D1 complex with concomitant Rb phosphorylation, causing severe defects in HSC quiescence and reduced transplantation capacity.","method":"Conditional double knockout mice, co-immunoprecipitation of Hsc70 with p57/p27, subcellular fractionation, transplantation assay","journal":"Cell stem cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic double-knockout with defined molecular mechanism (co-IP, fractionation, rescue), multiple orthogonal methods","pmids":["21885020"],"is_preprint":false},{"year":2001,"finding":"Retinoblastoma protein (pRb) causes posttranscriptional accumulation of p27(KIP1) specifically bound to cyclin E, decreasing cyclin E-associated kinase activity, and this is required for pRb-mediated senescence. Ablation of p27(KIP1) expression by antisense reversibly abrogates pRb-induced cell cycle arrest and senescence.","method":"pRb reintroduction into RB(-/-) cells, western blot, kinase assays, antisense knockdown of p27","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — gain-of-function and loss-of-function with biochemical readouts, single lab","pmids":["11340156"],"is_preprint":false},{"year":2004,"finding":"Quaking (QKI) RNA binding proteins bind and stabilize p27(Kip1) mRNA, leading to increased p27(Kip1) protein accumulation in oligodendrocytes, thereby promoting cell cycle arrest and oligodendrocyte differentiation. QKI isoforms absent from Qk(v) mutant mice oligodendrocytes are required for this effect.","method":"RNA immunoprecipitation (mRNA binding assay), retroviral expression of QKI isoforms, mRNA stability assay, in vivo retroviral injection","journal":"Nature neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct mRNA binding demonstrated, in vitro and in vivo validation with mutant mouse model","pmids":["15568022"],"is_preprint":false},{"year":2006,"finding":"Cytoplasmic p27(Kip1) promotes cell motility, survival, and tumorigenicity through modulation of Akt stability, independently of its cell cycle inhibitory function. A p27 mutant restricted to the cytoplasm (p27deltaNLS) downregulates RhoA and increases motility. RNA interference of cytoplasmic p27 in U87 glioma cells inhibits motility and survival and reduces Akt levels and tumorigenicity in vivo.","method":"Nuclear localization signal deletion mutant, RNAi knockdown, Akt turnover assays, xenograft tumor model","journal":"Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutant construct plus RNAi with in vivo validation, single lab","pmids":["16489017"],"is_preprint":false},{"year":2005,"finding":"p27(Kip1) and p21(Cip1) inhibit cell cycle progression and tumor suppression independently of CDK2; ectopic expression of p27(Kip1) efficiently inhibits cell cycle progression in Cdk2(-/-) fibroblasts, and loss of p27(Kip1) confers similar proliferative advantages regardless of Cdk2 status. Ablation of Cdk2 does not suppress phenotypic defects including pituitary tumors in p27(Kip1) null mice.","method":"Cdk2(-/-) mouse embryo fibroblasts, ectopic expression, double knockout mice, tumor analysis","journal":"Cancer cell","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — genetic epistasis in multiple cell and tissue contexts with replicated approaches","pmids":["15950907"],"is_preprint":false},{"year":2012,"finding":"p27(Kip1) controls cytokinesis through a CDK-independent mechanism by binding to and regulating citron kinase (citron-K), a Rho effector. p27 and citron-K colocalize at the contractile ring and mid-body during telophase and cytokinesis; p27 prevents citron-K interaction with its activator RhoA. A p27 mutant unable to bind or inhibit cyclin-CDK complexes (p27CK-) causes multinucleation and cytokinesis defects, rescued by expression of the minimal citron-K p27-binding domain.","method":"Co-immunoprecipitation, co-localization studies, rescue experiments with domain mutants, p27(CK-) knock-in mice","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro binding, in vivo co-IP, genetic rescue with specific mutants, multiple orthogonal methods in single study","pmids":["22293177"],"is_preprint":false},{"year":2012,"finding":"p27 directly associates with the SRR2 enhancer of the Sox2 gene together with a p130-E2F4-SIN3A repressive complex to repress Sox2 transcription during differentiation. Sox2 haploinsufficiency genetically rescues gigantism, pituitary hyperplasia, pituitary tumors, and retinal defects characteristic of p27 null mice.","method":"ChIP assay, genetic epistasis (p27-null × Sox2 haploinsufficient mice), iPSC reprogramming without ectopic Sox2","journal":"Cell stem cell","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — ChIP demonstrating direct chromatin association, genetic epistasis in vivo, multiple orthogonal methods","pmids":["23217425"],"is_preprint":false},{"year":2015,"finding":"Autophagy selectively degrades p27(Kip1) (CDKN1B) after TCR stimulation to allow T lymphocyte proliferation. p27(Kip1) forms polymers that associate physiologically with the autophagy receptor SQSTM1/p62. Genetic deletion of one allele of CDKN1B in autophagy-deficient T cells restores proliferative capability.","method":"Atg7-deficient T cells, adoptive transfer model, co-immunoprecipitation with SQSTM1/p62, genetic rescue by CDKN1B haploinsufficiency","journal":"Autophagy","confidence":"High","confidence_rationale":"Tier 2 / Strong — co-IP for direct interaction, genetic epistasis rescue, in vivo infection model, multiple orthogonal methods","pmids":["26569626"],"is_preprint":false},{"year":2008,"finding":"Protein kinase CK2-alpha' phosphorylates p27(Kip1) in response to angiotensin II (a cardiac growth factor), leading to proteasomal degradation of p27 in cardiomyocytes. Unphosphorylated p27 potently inhibits CK2-alpha', forming a regulatory feedback loop in differentiated cardiomyocytes that controls cardiac hypertrophic growth.","method":"In vitro kinase assay, co-immunoprecipitation, proteasome inhibitor experiments, CK2-alpha' knockdown/overexpression in cardiomyocytes","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro kinase assay plus in vivo validation, bidirectional regulation demonstrated with multiple methods","pmids":["18311148"],"is_preprint":false},{"year":2006,"finding":"BRCA1 and FOXA1 cooperatively activate p27(Kip1) promoter transcription. FOXA1 binds a specific site within the BRCA1-responsive element of the p27(Kip1) promoter (-545 to -511), and co-expression of BRCA1 and FOXA1 synergistically activates the promoter. BRCA1 increases FOXA1 protein stability. BRCA1 siRNA reduces both FOXA1 expression and p27(Kip1) promoter activity.","method":"Promoter-luciferase reporter assay, EMSA with antibody supershift, siRNA knockdown, co-transfection, protein half-life assay","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — EMSA demonstrating direct DNA binding, reporter assays with mutagenesis, siRNA validation; single lab","pmids":["16331276"],"is_preprint":false},{"year":2005,"finding":"MLL-AF4 fusion protein directly binds the CDKN1B (p27kip1) promoter and regulates its transcriptional activity in a cell-type-dependent manner (activating or repressing). Chromatin immunoprecipitation (ChIP) confirmed MLL-AF4 binding to the CDKN1B promoter in MLL-AF4 and MLL-AF9 leukemia cell lines.","method":"Chromatin immunoprecipitation (ChIP), luciferase reporter gene assay, inducible MLL-AF4 cell lines","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP demonstrating direct promoter binding, reporter assay validation, replicated in multiple cell lines but single lab","pmids":["16169901"],"is_preprint":false},{"year":2012,"finding":"p27(Kip1) integrates sequential phosphorylation signals from non-receptor tyrosine kinases (NRTKs) to regulate CDK4 activity and cell cycle entry. Abl phosphorylates p27 on Tyr88, facilitating a second modification on Tyr74 by Src; doubly tyrosine-phosphorylated p27 causes partial reactivation of Cdk4 within ternary complexes. Unmodified p27 inhibits Cdk4/cyclin D with IC50 ~1 nM.","method":"In vitro kinase assay, isothermal titration calorimetry, phosphorylation-specific analysis, Cdk4 activity reconstitution","journal":"ACS chemical biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with kinetic measurements, sequential phosphorylation mechanism established biochemically, single lab","pmids":["22276948"],"is_preprint":false},{"year":2017,"finding":"Cytoplasmic p27(Kip1) promotes cell invasion by binding to Cortactin and facilitating its interaction with PAK1. p27 localizes to invadopodia and limits their number and stability. PAK1-mediated phosphorylation of Cortactin promotes invadopodia turnover, and expression of Cortactin mutants at PAK-targeted phosphorylation sites abolishes p27's effects on invadopodia dynamics.","method":"Co-immunoprecipitation, invadopodium formation assays, Cortactin phospho-mutants, PAK1 assays, p27 localization by fluorescence microscopy","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP, functional mutant analysis, multiple orthogonal methods establishing pathway order","pmids":["28287395"],"is_preprint":false},{"year":2018,"finding":"p27(Kip1) stabilizes α-tubulin acetyltransferase 1 (ATAT1), thereby promoting α-tubulin acetylation and proper axonal transport in cortical neurons. Suppression of the p27 ortholog dacapo in Drosophila disrupts axonal transport in vivo, causing locomotor defects.","method":"In vitro cortical neuron assays, Drosophila dacapo knockdown, ATAT1 stability assays, vesicle/organelle transport imaging","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mechanistic link to ATAT1 stability established in vitro and validated in Drosophila in vivo; single lab","pmids":["29791853"],"is_preprint":false},{"year":2016,"finding":"PPM1G is a phosphatase that directly dephosphorylates p27(Kip1) at T198 in cells and in vitro. PPM1G interacts with p27, and its ectopic expression enhances p27 protein stability and delays G1-to-S cell cycle progression. PPM1G inhibits the interaction of p27 with 14-3-3θ, a chaperone that facilitates nuclear export. Knockdown of PPM1G accelerates p27 degradation and promotes cytoplasmic p27 localization.","method":"In vitro phosphatase assay, co-immunoprecipitation, knockdown/overexpression cell cycle analysis, subcellular fractionation","journal":"American journal of cancer research","confidence":"Medium","confidence_rationale":"Tier 1–2 / Moderate — in vitro phosphatase assay plus multiple cellular assays, single lab","pmids":["27822412"],"is_preprint":false},{"year":2002,"finding":"PGP9.5 (UCH-L1) interacts with JAB1 and forms a heteromeric complex containing p27(Kip1) in the nucleus of lung cancer cells. Under serum re-stimulation, nuclear translocation of both PGP9.5 and JAB1 coincides with reduced nuclear p27(Kip1) levels, suggesting PGP9.5 contributes to p27(Kip1) degradation via JAB1.","method":"Yeast two-hybrid, co-immunoprecipitation in vitro and in vivo, immunofluorescence co-localization","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — co-IP and co-localization confirmed interaction but degradation mechanism not directly reconstituted; single lab","pmids":["12082530"],"is_preprint":false},{"year":2006,"finding":"Focal adhesion kinase (FAK) controls p27(Kip1) stability through Skp2-dependent mechanisms. Dominant-negative FAK (FRNK) impairs Skp2 mRNA expression, elevating p27 levels by reducing its degradation. Skp2-dependent degradation of p27 is required for endothelial cell proliferation and angiogenesis.","method":"Dominant-negative FAK expression, siRNA against Skp2 and p27, flow cytometry, DNA synthesis assay","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with siRNA knockdowns establishing pathway order; single lab","pmids":["16705171"],"is_preprint":false},{"year":2009,"finding":"AP-1 transcription factors (Jun/Fos heterodimers) mediate mitogen-induced transcriptional repression of the p27(Kip1) gene. An AP-1 element at position -469 in the human p27(Kip1) promoter is required for mitogen-induced down-regulation. Both MAPK and PI3K pathways mediate this transcriptional repression.","method":"Promoter-luciferase reporter assay, site-directed mutagenesis of AP-1 element, chromatin immunoprecipitation, enforced expression of Jun/Fos","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP demonstrating direct promoter binding, mutagenesis, and kinase pathway inhibitors; single lab","pmids":["19959471"],"is_preprint":false},{"year":2017,"finding":"FLT3 and oncogenic FLT3-ITD directly bind and phosphorylate p27(Kip1) at tyrosine residue 88, inactivating its CDK-inhibitory function and promoting cell cycle progression in acute myeloid leukemia. FLT3 inhibition with AC220 reduces p27 Y88 phosphorylation and causes cell cycle arrest. Src family kinases also phosphorylate p27 Y88 in some FLT3-ITD-positive AML cells.","method":"Direct binding assay, in vitro phosphorylation, phospho-specific antibodies, FLT3 inhibitor treatment, primary patient samples","journal":"Haematologica","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — direct in vitro phosphorylation, validated in patient samples with inhibitor treatment, replicated across multiple AML contexts","pmids":["28522571"],"is_preprint":false},{"year":2013,"finding":"p21(Cip1) and p27(Kip1) bind cyclin E, cyclin A, and CDK2 in postnatal cardiomyocytes and are required for cardiomyocyte cell cycle exit at G1 phase after birth. Knockout of p21 and p27 in mice results in failure of cardiomyocyte cell cycle exit at G1 and endoreplication.","method":"Co-immunoprecipitation of CDK complexes, p21/p27 knockout mice, flow cytometry of cardiomyocyte cell cycle distribution","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockout with defined cellular phenotype plus biochemical co-IP; single lab","pmids":["24380855"],"is_preprint":false},{"year":2011,"finding":"T198 modification of p27(Kip1) controls protein stability and cell motility through distinct mechanisms. The steric presence of threonine at position 198 (independent of phosphorylation) controls p27 stability via proteasome-dependent degradation through conformational changes in the disordered C-terminus, not through Skp2 binding. T198 phosphorylation specifically promotes p27/stathmin interaction and regulates cell motility.","method":"T198 point mutants, proteasome inhibitor assays, co-immunoprecipitation with Skp2 and stathmin, motility assays","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple mutant forms with biochemical assays distinguishing two mechanisms; single lab","pmids":["21423803"],"is_preprint":false},{"year":2016,"finding":"CSN6 interacts with p27(Kip1) and facilitates ubiquitin-mediated proteasomal degradation of p27(Kip1) via COP1, an E3 ubiquitin ligase. COP1 overexpression causes cytoplasmic redistribution of p27, accelerating its degradation. CSN6-mediated p27 degradation depends on p27 nuclear export.","method":"Co-immunoprecipitation, siRNA knockdown, overexpression studies, subcellular fractionation, ubiquitination assay","journal":"Cell cycle (Georgetown, Tex.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP and ubiquitination assay establishing mechanism; single lab","pmids":["25945542"],"is_preprint":false},{"year":2014,"finding":"Stathmin is a relevant p27(Kip1) binding partner in vivo, and genetic co-ablation of stathmin rescues most hyper-proliferative phenotypes of p27(Kip1) null mice (increased body/organ weight, retinal outgrowth, pituitary adenomas). The cooperativity of p27 and stathmin controls early G1-to-S phase transition, linked at the molecular level to decreased CDK4/6 kinase activity in double-knockout mice.","method":"Double-knockout mouse model, in vivo proliferation analysis, CDK kinase activity assays, gene expression profiling","journal":"Cell cycle (Georgetown, Tex.)","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with double-knockout rescuing multiple p27-null phenotypes, molecular mechanism identified; multiple orthogonal methods","pmids":["25486569"],"is_preprint":false},{"year":2019,"finding":"BAP31 (B-cell receptor-associated protein 31) interacts with p27kip1 and promotes its proteasomal degradation. An intrabody (VH-D1) against BAP31 inhibits p27kip1 proteasome degradation by blocking the BAP31-p27kip1 interaction, resulting in reduced gastric cancer cell growth and tumor xenograft growth.","method":"Co-immunoprecipitation, intrabody expression, proteasome inhibitor assay, xenograft tumor model","journal":"International journal of cancer","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — co-IP and intrabody blocking demonstrating functional interaction; in vivo xenograft validation; single lab","pmids":["30338855"],"is_preprint":false},{"year":2016,"finding":"Nucleostemin (NS) promotes nucleolar polyubiquitylation of p27kip1, leading to p27 inactivation during hepatocellular carcinoma progression. p27 shows nucleolar distribution and interacts with NS; depletion of NS inhibits nucleolar polyubiquitylation of p27 and promotes p27 binding to CDK2-Cyclin E, inhibiting CDK2 activity and causing cell cycle arrest.","method":"Subcellular fractionation, co-immunoprecipitation, ubiquitination assay, siRNA knockdown, CDK2 kinase assay","journal":"Cancer letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP, ubiquitination assay, and CDK2 activity assay establishing pathway; single lab","pmids":["27998760"],"is_preprint":false}],"current_model":"p27(Kip1)/CDKN1B is an intrinsically disordered CDK inhibitor that arrests the cell cycle at G1/S by binding and inhibiting cyclin/CDK complexes (particularly cyclin E–CDK2 and cyclin D–CDK4/6); its activity, stability, and subcellular localization are regulated by a cascade of post-translational modifications—including phosphorylation at S10 by hKIS (promoting nuclear export), at T187 by CDKs (triggering SKP2-dependent proteasomal degradation), at Y88/Y74 by non-receptor tyrosine kinases such as Abl/Src and FLT3 (partially reactivating CDK4), and at T198 by Pim-1 and other kinases (controlling cytoplasmic stability and stathmin interaction)—while cytoplasmic p27 exerts CDK-independent oncogenic functions by modulating RhoA, stathmin, Cortactin/PAK1, and Akt to promote cell migration and invasion, and nuclear p27 additionally acts as a transcriptional co-repressor (e.g., repressing Sox2 via p130-E2F4-SIN3A) and regulates cytokinesis by inhibiting citron kinase downstream of RhoA."},"narrative":{"mechanistic_narrative":"p27(Kip1)/CDKN1B is a CDK inhibitor that enforces cell cycle arrest by binding cyclin/CDK complexes, and it functions as a tumor suppressor whose loss drives hyperproliferation, pituitary tumors, and gigantism [PMID:15950907, PMID:25486569]. Its inhibitory activity is set by the abundance and stoichiometry of its targets: Myc-induced cyclins D1/D2 sequester p27 to release cyclin E–CDK2, while pRb drives posttranscriptional accumulation of cyclin E-bound p27 to enforce senescence [PMID:10508164, PMID:11340156]. p27 inhibits cyclin D–CDK4 with nanomolar potency, and this inhibition is relieved through sequential tyrosine phosphorylation—Abl phosphorylates Tyr88, enabling Src-mediated Tyr74 modification that partially reactivates CDK4 within ternary complexes—a mechanism co-opted oncogenically by FLT3/FLT3-ITD, which phosphorylates Tyr88 to inactivate p27 in AML [PMID:22276948, PMID:28522571]. p27 abundance is controlled at every level: transcriptionally it is induced by the Ah receptor and by BRCA1/FOXA1, and repressed by Myc, AP-1, and MLL-AF4 [PMID:10398686, PMID:16331276, PMID:11313917, PMID:19959471, PMID:16169901]; its mRNA is stabilized by QKI RNA-binding proteins [PMID:15568022]; and its protein stability is governed by phosphorylation (CK2α', and a steric/phospho switch at T198 reversed by the phosphatase PPM1G) and by degradation routes including Skp2 downstream of FAK, COP1/CSN6, BAP31, nucleolar polyubiquitylation by nucleostemin, and selective autophagy via the SQSTM1/p62 receptor [PMID:18311148, PMID:21423803, PMID:27822412, PMID:16705171, PMID:25945542, PMID:30338855, PMID:27998760, PMID:26569626]. Beyond cyclin/CDK inhibition, p27 arrests the cycle independently of CDK2 and executes CDK-independent functions: cytoplasmic p27 promotes motility, invasion, and survival by downregulating RhoA, modulating Akt stability, and bridging Cortactin to PAK1 at invadopodia, and it controls cytokinesis by binding citron kinase to block its activation by RhoA [PMID:15950907, PMID:16489017, PMID:28287395, PMID:22293177]. Nuclear p27 acts as a transcriptional co-repressor, associating with the Sox2 SRR2 enhancer in a p130-E2F4-SIN3A complex [PMID:23217425]. p27 also cooperates with p57 to retain Hsc70/cyclin D1 in the cytoplasm to maintain hematopoietic stem cell quiescence, and stabilizes ATAT1 to support tubulin acetylation and axonal transport [PMID:21885020, PMID:29791853].","teleology":[{"year":1999,"claim":"Establishing how mitogenic Myc overcomes p27 inhibition showed that cyclin D sequestration—not D-CDK kinase activity—redistributes CKIs to free cyclin E-CDK2, defining a stoichiometric titration model of cell cycle entry.","evidence":"Cyclin D1/D2 knockout MEFs with kinase-dead complementation and co-IP","pmids":["10508164"],"confidence":"High","gaps":["Does not quantify endogenous p27 redistribution kinetics","Does not address other CKI targets"]},{"year":1999,"claim":"Linking p27 to an environmental signaling receptor, AhR was shown to directly induce p27 transcription to mediate dioxin-induced growth arrest, broadening p27 regulation beyond intrinsic mitogen pathways.","evidence":"Antisense p27 and Kip1-knockout mouse tissue with transcription assays","pmids":["10398686"],"confidence":"High","gaps":["Direct AhR binding to the p27 promoter not mapped","Tissue specificity beyond thymus unaddressed"]},{"year":2001,"claim":"Identifying transcriptional repressors of p27 explained mitogen-driven downregulation: Myc represses the p27 promoter via box II and an Inr element with Max, while pRb conversely drives posttranscriptional accumulation of cyclin E-bound p27 to enforce senescence.","evidence":"Promoter-luciferase/deletion analysis and pRb reintroduction with antisense knockdown","pmids":["11313917","11340156"],"confidence":"Medium","gaps":["Repression mechanism is reporter-based, not endogenous chromatin","pRb-to-p27 posttranscriptional link not molecularly defined"]},{"year":2002,"claim":"Defining the kinase for S10 phosphorylation, hKIS was shown to bind p27's C-terminus and phosphorylate S10 to promote nuclear export, establishing localization control as a regulatory layer.","evidence":"In vitro kinase assay, co-IP, and siRNA depletion with cell cycle analysis","pmids":["12093740"],"confidence":"High","gaps":["Structural basis of C-terminal binding unresolved","Crosstalk with degradation pathways not addressed"]},{"year":2004,"claim":"Demonstrating mRNA-level control, QKI RNA-binding proteins were found to bind and stabilize p27 mRNA to drive oligodendrocyte differentiation, adding post-transcriptional regulation to the p27 abundance circuit.","evidence":"RNA-IP, retroviral QKI expression, and mRNA stability assays in Qkv mutant mice","pmids":["15568022"],"confidence":"High","gaps":["QKI binding site on p27 mRNA not mapped","Generality beyond oligodendrocytes unknown"]},{"year":2005,"claim":"Resolving whether p27's tumor-suppressive arrest required CDK2, genetic epistasis showed p27 inhibits the cycle and suppresses tumors independently of CDK2, decoupling p27 function from its canonical target.","evidence":"Cdk2-null MEFs, ectopic expression, and p27/Cdk2 double-knockout tumor analysis","pmids":["15950907"],"confidence":"High","gaps":["Identity of the CDK2-independent effector not defined here","Does not exclude CDK1/other CDK redundancy"]},{"year":2006,"claim":"Uncovering CDK-independent oncogenic activity, cytoplasmic p27 was shown to downregulate RhoA, increase motility, and stabilize Akt to promote survival and tumorigenicity, reframing mislocalized p27 as oncogenic.","evidence":"p27deltaNLS mutant, RNAi, Akt turnover assays, and glioma xenografts","pmids":["16489017"],"confidence":"Medium","gaps":["Direct molecular link between p27 and Akt stability unresolved","Single cell-type/xenograft model"]},{"year":2006,"claim":"Two transcriptional and stability inputs were mapped: BRCA1/FOXA1 synergistically activate the p27 promoter, while FAK sustains Skp2-dependent p27 degradation to permit endothelial proliferation and angiogenesis.","evidence":"Reporter assays, EMSA, and dominant-negative FAK with Skp2/p27 siRNA","pmids":["16331276","16705171"],"confidence":"Medium","gaps":["Single-lab reporter/EMSA data for the activation arm","FAK-Skp2 link is indirect (mRNA level)"]},{"year":2008,"claim":"Identifying a tissue-specific stability circuit, CK2α' was shown to phosphorylate p27 for degradation in angiotensin II-stimulated cardiomyocytes, with unphosphorylated p27 inhibiting CK2α' in a feedback loop controlling hypertrophy.","evidence":"In vitro kinase assay, co-IP, proteasome inhibition, and CK2α' manipulation in cardiomyocytes","pmids":["18311148"],"confidence":"High","gaps":["CK2α' target residue on p27 not specified","Relationship to other degradation routes unclear"]},{"year":2011,"claim":"Two stem/quiescence and stability mechanisms were defined: p27 and p57 cooperatively retain Hsc70/cyclin D1 in the cytoplasm to maintain HSC quiescence, while a steric T198 switch controls proteasomal stability and T198 phosphorylation governs stathmin binding and motility.","evidence":"Conditional double-knockout mice with fractionation/transplantation, and T198 mutants with proteasome and binding assays","pmids":["21885020","21423803"],"confidence":"High","gaps":["Mechanism of Hsc70/cyclin D1 cytoplasmic retention by p27 incomplete","T198 conformational model based on mutants, not structure"]},{"year":2012,"claim":"CDK-independent and biochemical mechanisms were established: p27 binds citron kinase to block RhoA activation during cytokinesis, associates with the Sox2 SRR2 enhancer in a p130-E2F4-SIN3A repressor complex, and integrates sequential Abl/Src tyrosine phosphorylation to partially reactivate CDK4.","evidence":"p27CK- knock-in mice, ChIP with Sox2-haploinsufficiency epistasis, and in vitro reconstitution with ITC","pmids":["22293177","23217425","22276948"],"confidence":"High","gaps":["In vivo prevalence of Y88/Y74 dual phosphorylation not quantified","Sox2 co-repressor assembly order incompletely defined"]},{"year":2013,"claim":"Extending the arrest function to terminal differentiation, p21/p27 were shown to bind cyclin E/A-CDK2 and be required for postnatal cardiomyocyte G1 cell cycle exit, with loss causing endoreplication.","evidence":"Co-IP of CDK complexes and p21/p27 knockout cardiomyocyte cell cycle profiling","pmids":["24380855"],"confidence":"Medium","gaps":["Relative contribution of p27 vs p21 not separated","Single-lab study"]},{"year":2014,"claim":"Validating stathmin as a physiological partner, co-ablation of stathmin rescued most p27-null hyperproliferative phenotypes via restored CDK4/6 activity, establishing the p27-stathmin axis in G1/S control in vivo.","evidence":"p27/stathmin double-knockout mice with proliferation and CDK kinase assays","pmids":["25486569"],"confidence":"High","gaps":["Molecular link between p27-stathmin and CDK4/6 activity indirect","Cytoplasmic vs nuclear contribution not separated"]},{"year":2016,"claim":"Multiple degradation and stability regulators were defined: CSN6/COP1 and nucleostemin (nucleolar polyubiquitylation) target p27 for proteasomal turnover, while PPM1G dephosphorylates T198 to stabilize p27 and block 14-3-3θ-mediated export.","evidence":"Co-IP, ubiquitination and phosphatase assays, fractionation, and CDK2 activity assays","pmids":["25945542","27998760","27822412"],"confidence":"Medium","gaps":["E3 ligase hierarchy among COP1/Skp2/nucleostemin unresolved","Single-lab biochemistry for each route"]},{"year":2017,"claim":"Disease-relevant and cytoplasmic mechanisms were established: FLT3/FLT3-ITD directly phosphorylates p27 Y88 to inactivate it in AML, and cytoplasmic p27 bridges Cortactin to PAK1 to limit invadopodia turnover during invasion.","evidence":"In vitro phosphorylation with phospho-antibodies and inhibitor in patient AML samples; reciprocal co-IP with Cortactin phospho-mutants","pmids":["28522571","28287395"],"confidence":"High","gaps":["Contribution of Y88 phosphorylation to clinical FLT3-ITD outcomes not quantified","Cortactin/PAK1 axis tested in limited cell contexts"]},{"year":2018,"claim":"A neuronal cytoskeletal role was uncovered: p27 stabilizes ATAT1 to promote α-tubulin acetylation and axonal transport, with Drosophila dacapo loss disrupting transport, extending p27 function to cytoskeletal regulation.","evidence":"Cortical neuron assays, ATAT1 stability measurements, and Drosophila dacapo knockdown","pmids":["29791853"],"confidence":"Medium","gaps":["Mechanism of ATAT1 stabilization by p27 not defined","Mammalian in vivo confirmation limited"]},{"year":2019,"claim":"An additional degradation partner, BAP31, was shown to promote p27 proteasomal degradation, with an anti-BAP31 intrabody stabilizing p27 and suppressing gastric tumor growth.","evidence":"Co-IP, intrabody blocking, proteasome inhibition, and xenografts","pmids":["30338855"],"confidence":"Medium","gaps":["E3 ligase mediating BAP31-dependent degradation unidentified","Single-lab and tumor-model specific"]},{"year":null,"claim":"How the many parallel degradation routes (Skp2, COP1/CSN6, BAP31, nucleostemin, autophagy/p62) and modification switches are integrated to set p27 levels and localization in a given cell context, and the structural basis of its disordered-domain conformational switching, remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified hierarchy of competing degradation pathways","No structural model linking T198/Y88 modifications to conformation","Context-dependence of nuclear vs cytoplasmic function not predictable"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,8,15,26]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[10]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[1,4]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,10,19]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[4,7,16]},{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[28]}],"pathway":[{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[1,5,8,23,26]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[2,10,13,21]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[20,24,25,27,28]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[7,16,22]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[11]}],"complexes":["cyclin E-CDK2","cyclin D-CDK4","p130-E2F4-SIN3A repressor complex"],"partners":["CDK4","STATHMIN","CITRON KINASE","CTTN","PAK1","SQSTM1","ATAT1","SKP2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P46527","full_name":"Cyclin-dependent kinase inhibitor 1B","aliases":["Cyclin-dependent kinase inhibitor p27","p27Kip1"],"length_aa":198,"mass_kda":22.1,"function":"Important regulator of cell cycle progression. Inhibits the kinase activity of CDK2 bound to cyclin A, but has little inhibitory activity on CDK2 bound to SPDYA (PubMed:28666995). Involved in G1 arrest. Potent inhibitor of cyclin E- and cyclin A-CDK2 complexes. Forms a complex with cyclin type D-CDK4 complexes and is involved in the assembly, stability, and modulation of CCND1-CDK4 complex activation. Acts either as an inhibitor or an activator of cyclin type D-CDK4 complexes depending on its phosphorylation state and/or stoichometry","subcellular_location":"Nucleus; Cytoplasm; Endosome","url":"https://www.uniprot.org/uniprotkb/P46527/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CDKN1B","classification":"Not Classified","n_dependent_lines":13,"n_total_lines":1208,"dependency_fraction":0.01076158940397351},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CDK2","stoichiometry":0.2},{"gene":"SLC9A3R1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/CDKN1B","total_profiled":1310},"omim":[{"mim_id":"620226","title":"UBIQUITIN-SPECIFIC PEPTIDASE 37; USP37","url":"https://www.omim.org/entry/620226"},{"mim_id":"618686","title":"TEKTIN 5; TEKT5","url":"https://www.omim.org/entry/618686"},{"mim_id":"618069","title":"CCR4-NOT TRANSCRIPTION COMPLEX, SUBUNIT 6-LIKE; CNOT6L","url":"https://www.omim.org/entry/618069"},{"mim_id":"617819","title":"RAL GUANINE NUCLEOTIDE EXCHANGE FACTOR WITH PH DOMAIN AND SH3 DOMAIN-BINDING MOTIF 2; RALGPS2","url":"https://www.omim.org/entry/617819"},{"mim_id":"616016","title":"PROTEIN PHOSPHATASE, MAGNESIUM/MANGANESE-DEPENDENT, 1H; PPM1H","url":"https://www.omim.org/entry/616016"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Centrosome","reliability":"Supported"},{"location":"Primary cilium","reliability":"Additional"},{"location":"Primary cilium transition zone","reliability":"Additional"},{"location":"Basal body","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CDKN1B"},"hgnc":{"alias_symbol":["KIP1","P27KIP1"],"prev_symbol":[]},"alphafold":{"accession":"P46527","domains":[{"cath_id":"4.10.365.10","chopping":"37-87","consensus_level":"high","plddt":94.1843,"start":37,"end":87}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P46527","model_url":"https://alphafold.ebi.ac.uk/files/AF-P46527-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P46527-F1-predicted_aligned_error_v6.png","plddt_mean":69.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CDKN1B","jax_strain_url":"https://www.jax.org/strain/search?query=CDKN1B"},"sequence":{"accession":"P46527","fasta_url":"https://rest.uniprot.org/uniprotkb/P46527.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P46527/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P46527"}},"corpus_meta":[{"pmid":"17627278","id":"PMC_17627278","title":"Regulation of the 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and enhances growth arrest.\",\n      \"method\": \"In vitro kinase assay, co-immunoprecipitation, siRNA knockdown, cell cycle analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — in vitro kinase assay plus in vivo validation, siRNA epistasis, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"12093740\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Cyclins D1 and D2, induced by Myc, sequester p27(Kip1) and p21(Cip1) into cyclin D-CDK complexes, thereby releasing cyclin E-CDK2 from inhibition and promoting cell cycle progression. Primary fibroblasts from cyclin D1(-/-) or cyclin D2(-/-) mice do not proliferate in response to Myc. A kinase-defective cyclin D1 mutant that still binds CKIs restores Myc sensitivity, demonstrating that the sequestration function—not kinase activity—is required.\",\n      \"method\": \"Genetic knockout mouse embryo fibroblasts, retroviral complementation with kinase-dead mutants, biochemical co-immunoprecipitation\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — genetic epistasis with multiple knockouts, biochemical validation, mutant complementation\",\n      \"pmids\": [\"10508164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"c-Myc transcriptionally represses the p27(Kip1) gene. The repression requires c-Myc homology box II, maps to a region -20 to +20 of the p27 promoter that contains a putative Inr element, and is facilitated by Max. c-Myc represses p27 promoter activity in multiple cell types including B cells, T cells, smooth muscle cells, and breast cancer cells.\",\n      \"method\": \"Promoter-luciferase reporter assays, co-transfection of c-Myc expression vectors, deletion/mutation analysis, Max co-transfection\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reporter assays with mutagenesis in multiple cell types, single lab\",\n      \"pmids\": [\"11313917\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The Ah receptor (AhR) directly induces p27(Kip1) transcription without requiring ongoing protein synthesis or cell proliferation, and this induction mediates dioxin-induced suppression of cell proliferation. p27(Kip1) antisense-expressing cells are resistant to dioxin-mediated growth arrest, and thymuses from Kip1-deficient mice are largely resistant to dioxin-induced proliferation inhibition.\",\n      \"method\": \"Antisense expression, Kip1-knockout mouse tissue culture, transcription assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic rescue with antisense and knockout, multiple orthogonal approaches\",\n      \"pmids\": [\"10398686\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"p27(Kip1) and p57(Kip2) cooperatively maintain hematopoietic stem cell (HSC) quiescence by retaining Hsc70/cyclin D1 complex in the cytoplasm. Combined deficiency of p57 and p27 in HSCs allows nuclear import of the Hsc70/cyclin D1 complex with concomitant Rb phosphorylation, causing severe defects in HSC quiescence and reduced transplantation capacity.\",\n      \"method\": \"Conditional double knockout mice, co-immunoprecipitation of Hsc70 with p57/p27, subcellular fractionation, transplantation assay\",\n      \"journal\": \"Cell stem cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic double-knockout with defined molecular mechanism (co-IP, fractionation, rescue), multiple orthogonal methods\",\n      \"pmids\": [\"21885020\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Retinoblastoma protein (pRb) causes posttranscriptional accumulation of p27(KIP1) specifically bound to cyclin E, decreasing cyclin E-associated kinase activity, and this is required for pRb-mediated senescence. Ablation of p27(KIP1) expression by antisense reversibly abrogates pRb-induced cell cycle arrest and senescence.\",\n      \"method\": \"pRb reintroduction into RB(-/-) cells, western blot, kinase assays, antisense knockdown of p27\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — gain-of-function and loss-of-function with biochemical readouts, single lab\",\n      \"pmids\": [\"11340156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Quaking (QKI) RNA binding proteins bind and stabilize p27(Kip1) mRNA, leading to increased p27(Kip1) protein accumulation in oligodendrocytes, thereby promoting cell cycle arrest and oligodendrocyte differentiation. QKI isoforms absent from Qk(v) mutant mice oligodendrocytes are required for this effect.\",\n      \"method\": \"RNA immunoprecipitation (mRNA binding assay), retroviral expression of QKI isoforms, mRNA stability assay, in vivo retroviral injection\",\n      \"journal\": \"Nature neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct mRNA binding demonstrated, in vitro and in vivo validation with mutant mouse model\",\n      \"pmids\": [\"15568022\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Cytoplasmic p27(Kip1) promotes cell motility, survival, and tumorigenicity through modulation of Akt stability, independently of its cell cycle inhibitory function. A p27 mutant restricted to the cytoplasm (p27deltaNLS) downregulates RhoA and increases motility. RNA interference of cytoplasmic p27 in U87 glioma cells inhibits motility and survival and reduces Akt levels and tumorigenicity in vivo.\",\n      \"method\": \"Nuclear localization signal deletion mutant, RNAi knockdown, Akt turnover assays, xenograft tumor model\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutant construct plus RNAi with in vivo validation, single lab\",\n      \"pmids\": [\"16489017\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"p27(Kip1) and p21(Cip1) inhibit cell cycle progression and tumor suppression independently of CDK2; ectopic expression of p27(Kip1) efficiently inhibits cell cycle progression in Cdk2(-/-) fibroblasts, and loss of p27(Kip1) confers similar proliferative advantages regardless of Cdk2 status. Ablation of Cdk2 does not suppress phenotypic defects including pituitary tumors in p27(Kip1) null mice.\",\n      \"method\": \"Cdk2(-/-) mouse embryo fibroblasts, ectopic expression, double knockout mice, tumor analysis\",\n      \"journal\": \"Cancer cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — genetic epistasis in multiple cell and tissue contexts with replicated approaches\",\n      \"pmids\": [\"15950907\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"p27(Kip1) controls cytokinesis through a CDK-independent mechanism by binding to and regulating citron kinase (citron-K), a Rho effector. p27 and citron-K colocalize at the contractile ring and mid-body during telophase and cytokinesis; p27 prevents citron-K interaction with its activator RhoA. A p27 mutant unable to bind or inhibit cyclin-CDK complexes (p27CK-) causes multinucleation and cytokinesis defects, rescued by expression of the minimal citron-K p27-binding domain.\",\n      \"method\": \"Co-immunoprecipitation, co-localization studies, rescue experiments with domain mutants, p27(CK-) knock-in mice\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro binding, in vivo co-IP, genetic rescue with specific mutants, multiple orthogonal methods in single study\",\n      \"pmids\": [\"22293177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"p27 directly associates with the SRR2 enhancer of the Sox2 gene together with a p130-E2F4-SIN3A repressive complex to repress Sox2 transcription during differentiation. Sox2 haploinsufficiency genetically rescues gigantism, pituitary hyperplasia, pituitary tumors, and retinal defects characteristic of p27 null mice.\",\n      \"method\": \"ChIP assay, genetic epistasis (p27-null × Sox2 haploinsufficient mice), iPSC reprogramming without ectopic Sox2\",\n      \"journal\": \"Cell stem cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — ChIP demonstrating direct chromatin association, genetic epistasis in vivo, multiple orthogonal methods\",\n      \"pmids\": [\"23217425\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Autophagy selectively degrades p27(Kip1) (CDKN1B) after TCR stimulation to allow T lymphocyte proliferation. p27(Kip1) forms polymers that associate physiologically with the autophagy receptor SQSTM1/p62. Genetic deletion of one allele of CDKN1B in autophagy-deficient T cells restores proliferative capability.\",\n      \"method\": \"Atg7-deficient T cells, adoptive transfer model, co-immunoprecipitation with SQSTM1/p62, genetic rescue by CDKN1B haploinsufficiency\",\n      \"journal\": \"Autophagy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — co-IP for direct interaction, genetic epistasis rescue, in vivo infection model, multiple orthogonal methods\",\n      \"pmids\": [\"26569626\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Protein kinase CK2-alpha' phosphorylates p27(Kip1) in response to angiotensin II (a cardiac growth factor), leading to proteasomal degradation of p27 in cardiomyocytes. Unphosphorylated p27 potently inhibits CK2-alpha', forming a regulatory feedback loop in differentiated cardiomyocytes that controls cardiac hypertrophic growth.\",\n      \"method\": \"In vitro kinase assay, co-immunoprecipitation, proteasome inhibitor experiments, CK2-alpha' knockdown/overexpression in cardiomyocytes\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro kinase assay plus in vivo validation, bidirectional regulation demonstrated with multiple methods\",\n      \"pmids\": [\"18311148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"BRCA1 and FOXA1 cooperatively activate p27(Kip1) promoter transcription. FOXA1 binds a specific site within the BRCA1-responsive element of the p27(Kip1) promoter (-545 to -511), and co-expression of BRCA1 and FOXA1 synergistically activates the promoter. BRCA1 increases FOXA1 protein stability. BRCA1 siRNA reduces both FOXA1 expression and p27(Kip1) promoter activity.\",\n      \"method\": \"Promoter-luciferase reporter assay, EMSA with antibody supershift, siRNA knockdown, co-transfection, protein half-life assay\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — EMSA demonstrating direct DNA binding, reporter assays with mutagenesis, siRNA validation; single lab\",\n      \"pmids\": [\"16331276\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"MLL-AF4 fusion protein directly binds the CDKN1B (p27kip1) promoter and regulates its transcriptional activity in a cell-type-dependent manner (activating or repressing). Chromatin immunoprecipitation (ChIP) confirmed MLL-AF4 binding to the CDKN1B promoter in MLL-AF4 and MLL-AF9 leukemia cell lines.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), luciferase reporter gene assay, inducible MLL-AF4 cell lines\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP demonstrating direct promoter binding, reporter assay validation, replicated in multiple cell lines but single lab\",\n      \"pmids\": [\"16169901\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"p27(Kip1) integrates sequential phosphorylation signals from non-receptor tyrosine kinases (NRTKs) to regulate CDK4 activity and cell cycle entry. Abl phosphorylates p27 on Tyr88, facilitating a second modification on Tyr74 by Src; doubly tyrosine-phosphorylated p27 causes partial reactivation of Cdk4 within ternary complexes. Unmodified p27 inhibits Cdk4/cyclin D with IC50 ~1 nM.\",\n      \"method\": \"In vitro kinase assay, isothermal titration calorimetry, phosphorylation-specific analysis, Cdk4 activity reconstitution\",\n      \"journal\": \"ACS chemical biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with kinetic measurements, sequential phosphorylation mechanism established biochemically, single lab\",\n      \"pmids\": [\"22276948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Cytoplasmic p27(Kip1) promotes cell invasion by binding to Cortactin and facilitating its interaction with PAK1. p27 localizes to invadopodia and limits their number and stability. PAK1-mediated phosphorylation of Cortactin promotes invadopodia turnover, and expression of Cortactin mutants at PAK-targeted phosphorylation sites abolishes p27's effects on invadopodia dynamics.\",\n      \"method\": \"Co-immunoprecipitation, invadopodium formation assays, Cortactin phospho-mutants, PAK1 assays, p27 localization by fluorescence microscopy\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP, functional mutant analysis, multiple orthogonal methods establishing pathway order\",\n      \"pmids\": [\"28287395\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"p27(Kip1) stabilizes α-tubulin acetyltransferase 1 (ATAT1), thereby promoting α-tubulin acetylation and proper axonal transport in cortical neurons. Suppression of the p27 ortholog dacapo in Drosophila disrupts axonal transport in vivo, causing locomotor defects.\",\n      \"method\": \"In vitro cortical neuron assays, Drosophila dacapo knockdown, ATAT1 stability assays, vesicle/organelle transport imaging\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mechanistic link to ATAT1 stability established in vitro and validated in Drosophila in vivo; single lab\",\n      \"pmids\": [\"29791853\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PPM1G is a phosphatase that directly dephosphorylates p27(Kip1) at T198 in cells and in vitro. PPM1G interacts with p27, and its ectopic expression enhances p27 protein stability and delays G1-to-S cell cycle progression. PPM1G inhibits the interaction of p27 with 14-3-3θ, a chaperone that facilitates nuclear export. Knockdown of PPM1G accelerates p27 degradation and promotes cytoplasmic p27 localization.\",\n      \"method\": \"In vitro phosphatase assay, co-immunoprecipitation, knockdown/overexpression cell cycle analysis, subcellular fractionation\",\n      \"journal\": \"American journal of cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — in vitro phosphatase assay plus multiple cellular assays, single lab\",\n      \"pmids\": [\"27822412\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PGP9.5 (UCH-L1) interacts with JAB1 and forms a heteromeric complex containing p27(Kip1) in the nucleus of lung cancer cells. Under serum re-stimulation, nuclear translocation of both PGP9.5 and JAB1 coincides with reduced nuclear p27(Kip1) levels, suggesting PGP9.5 contributes to p27(Kip1) degradation via JAB1.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation in vitro and in vivo, immunofluorescence co-localization\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — co-IP and co-localization confirmed interaction but degradation mechanism not directly reconstituted; single lab\",\n      \"pmids\": [\"12082530\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Focal adhesion kinase (FAK) controls p27(Kip1) stability through Skp2-dependent mechanisms. Dominant-negative FAK (FRNK) impairs Skp2 mRNA expression, elevating p27 levels by reducing its degradation. Skp2-dependent degradation of p27 is required for endothelial cell proliferation and angiogenesis.\",\n      \"method\": \"Dominant-negative FAK expression, siRNA against Skp2 and p27, flow cytometry, DNA synthesis assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with siRNA knockdowns establishing pathway order; single lab\",\n      \"pmids\": [\"16705171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"AP-1 transcription factors (Jun/Fos heterodimers) mediate mitogen-induced transcriptional repression of the p27(Kip1) gene. An AP-1 element at position -469 in the human p27(Kip1) promoter is required for mitogen-induced down-regulation. Both MAPK and PI3K pathways mediate this transcriptional repression.\",\n      \"method\": \"Promoter-luciferase reporter assay, site-directed mutagenesis of AP-1 element, chromatin immunoprecipitation, enforced expression of Jun/Fos\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP demonstrating direct promoter binding, mutagenesis, and kinase pathway inhibitors; single lab\",\n      \"pmids\": [\"19959471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"FLT3 and oncogenic FLT3-ITD directly bind and phosphorylate p27(Kip1) at tyrosine residue 88, inactivating its CDK-inhibitory function and promoting cell cycle progression in acute myeloid leukemia. FLT3 inhibition with AC220 reduces p27 Y88 phosphorylation and causes cell cycle arrest. Src family kinases also phosphorylate p27 Y88 in some FLT3-ITD-positive AML cells.\",\n      \"method\": \"Direct binding assay, in vitro phosphorylation, phospho-specific antibodies, FLT3 inhibitor treatment, primary patient samples\",\n      \"journal\": \"Haematologica\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — direct in vitro phosphorylation, validated in patient samples with inhibitor treatment, replicated across multiple AML contexts\",\n      \"pmids\": [\"28522571\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"p21(Cip1) and p27(Kip1) bind cyclin E, cyclin A, and CDK2 in postnatal cardiomyocytes and are required for cardiomyocyte cell cycle exit at G1 phase after birth. Knockout of p21 and p27 in mice results in failure of cardiomyocyte cell cycle exit at G1 and endoreplication.\",\n      \"method\": \"Co-immunoprecipitation of CDK complexes, p21/p27 knockout mice, flow cytometry of cardiomyocyte cell cycle distribution\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout with defined cellular phenotype plus biochemical co-IP; single lab\",\n      \"pmids\": [\"24380855\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"T198 modification of p27(Kip1) controls protein stability and cell motility through distinct mechanisms. The steric presence of threonine at position 198 (independent of phosphorylation) controls p27 stability via proteasome-dependent degradation through conformational changes in the disordered C-terminus, not through Skp2 binding. T198 phosphorylation specifically promotes p27/stathmin interaction and regulates cell motility.\",\n      \"method\": \"T198 point mutants, proteasome inhibitor assays, co-immunoprecipitation with Skp2 and stathmin, motility assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple mutant forms with biochemical assays distinguishing two mechanisms; single lab\",\n      \"pmids\": [\"21423803\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CSN6 interacts with p27(Kip1) and facilitates ubiquitin-mediated proteasomal degradation of p27(Kip1) via COP1, an E3 ubiquitin ligase. COP1 overexpression causes cytoplasmic redistribution of p27, accelerating its degradation. CSN6-mediated p27 degradation depends on p27 nuclear export.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, overexpression studies, subcellular fractionation, ubiquitination assay\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP and ubiquitination assay establishing mechanism; single lab\",\n      \"pmids\": [\"25945542\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Stathmin is a relevant p27(Kip1) binding partner in vivo, and genetic co-ablation of stathmin rescues most hyper-proliferative phenotypes of p27(Kip1) null mice (increased body/organ weight, retinal outgrowth, pituitary adenomas). The cooperativity of p27 and stathmin controls early G1-to-S phase transition, linked at the molecular level to decreased CDK4/6 kinase activity in double-knockout mice.\",\n      \"method\": \"Double-knockout mouse model, in vivo proliferation analysis, CDK kinase activity assays, gene expression profiling\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with double-knockout rescuing multiple p27-null phenotypes, molecular mechanism identified; multiple orthogonal methods\",\n      \"pmids\": [\"25486569\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"BAP31 (B-cell receptor-associated protein 31) interacts with p27kip1 and promotes its proteasomal degradation. An intrabody (VH-D1) against BAP31 inhibits p27kip1 proteasome degradation by blocking the BAP31-p27kip1 interaction, resulting in reduced gastric cancer cell growth and tumor xenograft growth.\",\n      \"method\": \"Co-immunoprecipitation, intrabody expression, proteasome inhibitor assay, xenograft tumor model\",\n      \"journal\": \"International journal of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — co-IP and intrabody blocking demonstrating functional interaction; in vivo xenograft validation; single lab\",\n      \"pmids\": [\"30338855\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Nucleostemin (NS) promotes nucleolar polyubiquitylation of p27kip1, leading to p27 inactivation during hepatocellular carcinoma progression. p27 shows nucleolar distribution and interacts with NS; depletion of NS inhibits nucleolar polyubiquitylation of p27 and promotes p27 binding to CDK2-Cyclin E, inhibiting CDK2 activity and causing cell cycle arrest.\",\n      \"method\": \"Subcellular fractionation, co-immunoprecipitation, ubiquitination assay, siRNA knockdown, CDK2 kinase assay\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP, ubiquitination assay, and CDK2 activity assay establishing pathway; single lab\",\n      \"pmids\": [\"27998760\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"p27(Kip1)/CDKN1B is an intrinsically disordered CDK inhibitor that arrests the cell cycle at G1/S by binding and inhibiting cyclin/CDK complexes (particularly cyclin E–CDK2 and cyclin D–CDK4/6); its activity, stability, and subcellular localization are regulated by a cascade of post-translational modifications—including phosphorylation at S10 by hKIS (promoting nuclear export), at T187 by CDKs (triggering SKP2-dependent proteasomal degradation), at Y88/Y74 by non-receptor tyrosine kinases such as Abl/Src and FLT3 (partially reactivating CDK4), and at T198 by Pim-1 and other kinases (controlling cytoplasmic stability and stathmin interaction)—while cytoplasmic p27 exerts CDK-independent oncogenic functions by modulating RhoA, stathmin, Cortactin/PAK1, and Akt to promote cell migration and invasion, and nuclear p27 additionally acts as a transcriptional co-repressor (e.g., repressing Sox2 via p130-E2F4-SIN3A) and regulates cytokinesis by inhibiting citron kinase downstream of RhoA.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"p27(Kip1)/CDKN1B is a CDK inhibitor that enforces cell cycle arrest by binding cyclin/CDK complexes, and it functions as a tumor suppressor whose loss drives hyperproliferation, pituitary tumors, and gigantism [#8, #26]. Its inhibitory activity is set by the abundance and stoichiometry of its targets: Myc-induced cyclins D1/D2 sequester p27 to release cyclin E\\u2013CDK2, while pRb drives posttranscriptional accumulation of cyclin E-bound p27 to enforce senescence [#1, #5]. p27 inhibits cyclin D\\u2013CDK4 with nanomolar potency, and this inhibition is relieved through sequential tyrosine phosphorylation\\u2014Abl phosphorylates Tyr88, enabling Src-mediated Tyr74 modification that partially reactivates CDK4 within ternary complexes\\u2014a mechanism co-opted oncogenically by FLT3/FLT3-ITD, which phosphorylates Tyr88 to inactivate p27 in AML [#15, #22]. p27 abundance is controlled at every level: transcriptionally it is induced by the Ah receptor and by BRCA1/FOXA1, and repressed by Myc, AP-1, and MLL-AF4 [#3, #13, #2, #21, #14]; its mRNA is stabilized by QKI RNA-binding proteins [#6]; and its protein stability is governed by phosphorylation (CK2\\u03b1', and a steric/phospho switch at T198 reversed by the phosphatase PPM1G) and by degradation routes including Skp2 downstream of FAK, COP1/CSN6, BAP31, nucleolar polyubiquitylation by nucleostemin, and selective autophagy via the SQSTM1/p62 receptor [#12, #24, #18, #20, #25, #27, #28, #11]. Beyond cyclin/CDK inhibition, p27 arrests the cycle independently of CDK2 and executes CDK-independent functions: cytoplasmic p27 promotes motility, invasion, and survival by downregulating RhoA, modulating Akt stability, and bridging Cortactin to PAK1 at invadopodia, and it controls cytokinesis by binding citron kinase to block its activation by RhoA [#8, #7, #16, #9]. Nuclear p27 acts as a transcriptional co-repressor, associating with the Sox2 SRR2 enhancer in a p130-E2F4-SIN3A complex [#10]. p27 also cooperates with p57 to retain Hsc70/cyclin D1 in the cytoplasm to maintain hematopoietic stem cell quiescence, and stabilizes ATAT1 to support tubulin acetylation and axonal transport [#4, #17].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Establishing how mitogenic Myc overcomes p27 inhibition showed that cyclin D sequestration\\u2014not D-CDK kinase activity\\u2014redistributes CKIs to free cyclin E-CDK2, defining a stoichiometric titration model of cell cycle entry.\",\n      \"evidence\": \"Cyclin D1/D2 knockout MEFs with kinase-dead complementation and co-IP\",\n      \"pmids\": [\"10508164\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not quantify endogenous p27 redistribution kinetics\", \"Does not address other CKI targets\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Linking p27 to an environmental signaling receptor, AhR was shown to directly induce p27 transcription to mediate dioxin-induced growth arrest, broadening p27 regulation beyond intrinsic mitogen pathways.\",\n      \"evidence\": \"Antisense p27 and Kip1-knockout mouse tissue with transcription assays\",\n      \"pmids\": [\"10398686\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct AhR binding to the p27 promoter not mapped\", \"Tissue specificity beyond thymus unaddressed\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identifying transcriptional repressors of p27 explained mitogen-driven downregulation: Myc represses the p27 promoter via box II and an Inr element with Max, while pRb conversely drives posttranscriptional accumulation of cyclin E-bound p27 to enforce senescence.\",\n      \"evidence\": \"Promoter-luciferase/deletion analysis and pRb reintroduction with antisense knockdown\",\n      \"pmids\": [\"11313917\", \"11340156\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Repression mechanism is reporter-based, not endogenous chromatin\", \"pRb-to-p27 posttranscriptional link not molecularly defined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Defining the kinase for S10 phosphorylation, hKIS was shown to bind p27's C-terminus and phosphorylate S10 to promote nuclear export, establishing localization control as a regulatory layer.\",\n      \"evidence\": \"In vitro kinase assay, co-IP, and siRNA depletion with cell cycle analysis\",\n      \"pmids\": [\"12093740\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of C-terminal binding unresolved\", \"Crosstalk with degradation pathways not addressed\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstrating mRNA-level control, QKI RNA-binding proteins were found to bind and stabilize p27 mRNA to drive oligodendrocyte differentiation, adding post-transcriptional regulation to the p27 abundance circuit.\",\n      \"evidence\": \"RNA-IP, retroviral QKI expression, and mRNA stability assays in Qkv mutant mice\",\n      \"pmids\": [\"15568022\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"QKI binding site on p27 mRNA not mapped\", \"Generality beyond oligodendrocytes unknown\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Resolving whether p27's tumor-suppressive arrest required CDK2, genetic epistasis showed p27 inhibits the cycle and suppresses tumors independently of CDK2, decoupling p27 function from its canonical target.\",\n      \"evidence\": \"Cdk2-null MEFs, ectopic expression, and p27/Cdk2 double-knockout tumor analysis\",\n      \"pmids\": [\"15950907\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the CDK2-independent effector not defined here\", \"Does not exclude CDK1/other CDK redundancy\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Uncovering CDK-independent oncogenic activity, cytoplasmic p27 was shown to downregulate RhoA, increase motility, and stabilize Akt to promote survival and tumorigenicity, reframing mislocalized p27 as oncogenic.\",\n      \"evidence\": \"p27deltaNLS mutant, RNAi, Akt turnover assays, and glioma xenografts\",\n      \"pmids\": [\"16489017\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct molecular link between p27 and Akt stability unresolved\", \"Single cell-type/xenograft model\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Two transcriptional and stability inputs were mapped: BRCA1/FOXA1 synergistically activate the p27 promoter, while FAK sustains Skp2-dependent p27 degradation to permit endothelial proliferation and angiogenesis.\",\n      \"evidence\": \"Reporter assays, EMSA, and dominant-negative FAK with Skp2/p27 siRNA\",\n      \"pmids\": [\"16331276\", \"16705171\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab reporter/EMSA data for the activation arm\", \"FAK-Skp2 link is indirect (mRNA level)\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identifying a tissue-specific stability circuit, CK2\\u03b1' was shown to phosphorylate p27 for degradation in angiotensin II-stimulated cardiomyocytes, with unphosphorylated p27 inhibiting CK2\\u03b1' in a feedback loop controlling hypertrophy.\",\n      \"evidence\": \"In vitro kinase assay, co-IP, proteasome inhibition, and CK2\\u03b1' manipulation in cardiomyocytes\",\n      \"pmids\": [\"18311148\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"CK2\\u03b1' target residue on p27 not specified\", \"Relationship to other degradation routes unclear\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Two stem/quiescence and stability mechanisms were defined: p27 and p57 cooperatively retain Hsc70/cyclin D1 in the cytoplasm to maintain HSC quiescence, while a steric T198 switch controls proteasomal stability and T198 phosphorylation governs stathmin binding and motility.\",\n      \"evidence\": \"Conditional double-knockout mice with fractionation/transplantation, and T198 mutants with proteasome and binding assays\",\n      \"pmids\": [\"21885020\", \"21423803\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of Hsc70/cyclin D1 cytoplasmic retention by p27 incomplete\", \"T198 conformational model based on mutants, not structure\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"CDK-independent and biochemical mechanisms were established: p27 binds citron kinase to block RhoA activation during cytokinesis, associates with the Sox2 SRR2 enhancer in a p130-E2F4-SIN3A repressor complex, and integrates sequential Abl/Src tyrosine phosphorylation to partially reactivate CDK4.\",\n      \"evidence\": \"p27CK- knock-in mice, ChIP with Sox2-haploinsufficiency epistasis, and in vitro reconstitution with ITC\",\n      \"pmids\": [\"22293177\", \"23217425\", \"22276948\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo prevalence of Y88/Y74 dual phosphorylation not quantified\", \"Sox2 co-repressor assembly order incompletely defined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Extending the arrest function to terminal differentiation, p21/p27 were shown to bind cyclin E/A-CDK2 and be required for postnatal cardiomyocyte G1 cell cycle exit, with loss causing endoreplication.\",\n      \"evidence\": \"Co-IP of CDK complexes and p21/p27 knockout cardiomyocyte cell cycle profiling\",\n      \"pmids\": [\"24380855\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Relative contribution of p27 vs p21 not separated\", \"Single-lab study\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Validating stathmin as a physiological partner, co-ablation of stathmin rescued most p27-null hyperproliferative phenotypes via restored CDK4/6 activity, establishing the p27-stathmin axis in G1/S control in vivo.\",\n      \"evidence\": \"p27/stathmin double-knockout mice with proliferation and CDK kinase assays\",\n      \"pmids\": [\"25486569\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular link between p27-stathmin and CDK4/6 activity indirect\", \"Cytoplasmic vs nuclear contribution not separated\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Multiple degradation and stability regulators were defined: CSN6/COP1 and nucleostemin (nucleolar polyubiquitylation) target p27 for proteasomal turnover, while PPM1G dephosphorylates T198 to stabilize p27 and block 14-3-3\\u03b8-mediated export.\",\n      \"evidence\": \"Co-IP, ubiquitination and phosphatase assays, fractionation, and CDK2 activity assays\",\n      \"pmids\": [\"25945542\", \"27998760\", \"27822412\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"E3 ligase hierarchy among COP1/Skp2/nucleostemin unresolved\", \"Single-lab biochemistry for each route\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Disease-relevant and cytoplasmic mechanisms were established: FLT3/FLT3-ITD directly phosphorylates p27 Y88 to inactivate it in AML, and cytoplasmic p27 bridges Cortactin to PAK1 to limit invadopodia turnover during invasion.\",\n      \"evidence\": \"In vitro phosphorylation with phospho-antibodies and inhibitor in patient AML samples; reciprocal co-IP with Cortactin phospho-mutants\",\n      \"pmids\": [\"28522571\", \"28287395\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Contribution of Y88 phosphorylation to clinical FLT3-ITD outcomes not quantified\", \"Cortactin/PAK1 axis tested in limited cell contexts\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"A neuronal cytoskeletal role was uncovered: p27 stabilizes ATAT1 to promote \\u03b1-tubulin acetylation and axonal transport, with Drosophila dacapo loss disrupting transport, extending p27 function to cytoskeletal regulation.\",\n      \"evidence\": \"Cortical neuron assays, ATAT1 stability measurements, and Drosophila dacapo knockdown\",\n      \"pmids\": [\"29791853\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of ATAT1 stabilization by p27 not defined\", \"Mammalian in vivo confirmation limited\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"An additional degradation partner, BAP31, was shown to promote p27 proteasomal degradation, with an anti-BAP31 intrabody stabilizing p27 and suppressing gastric tumor growth.\",\n      \"evidence\": \"Co-IP, intrabody blocking, proteasome inhibition, and xenografts\",\n      \"pmids\": [\"30338855\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"E3 ligase mediating BAP31-dependent degradation unidentified\", \"Single-lab and tumor-model specific\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the many parallel degradation routes (Skp2, COP1/CSN6, BAP31, nucleostemin, autophagy/p62) and modification switches are integrated to set p27 levels and localization in a given cell context, and the structural basis of its disordered-domain conformational switching, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified hierarchy of competing degradation pathways\", \"No structural model linking T198/Y88 modifications to conformation\", \"Context-dependence of nuclear vs cytoplasmic function not predictable\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 8, 15, 26]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [1, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 10, 19]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [4, 7, 16]},\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [28]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [1, 5, 8, 23, 26]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [2, 10, 13, 21]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [20, 24, 25, 27, 28]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [7, 16, 22]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [11]}\n    ],\n    \"complexes\": [\"cyclin E-CDK2\", \"cyclin D-CDK4\", \"p130-E2F4-SIN3A repressor complex\"],\n    \"partners\": [\"CDK4\", \"stathmin\", \"CITRON kinase\", \"CTTN\", \"PAK1\", \"SQSTM1\", \"ATAT1\", \"SKP2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":4,"faith_total":4,"faith_pct":100.0}}