INCA1

INCA1 (Inhibitor of CDK Interacting with Cyclin A1) is a nuclear cell-cycle regulator that restrains CDK2 activity and proliferation (PMID:15159402, PMID:21540187). It was identified as a direct interaction partner and phosphorylation substrate of the cyclin A1-CDK2 complex, binding through a novel cyclin-binding domain that is also required for its inhibitory function (PMID:15159402, PMID:21540187). Genetic loss of Inca1 elevates CDK2 activity and increases the S-phase fraction in fibroblasts, establishing INCA1 as a bona fide CDK inhibitor in vivo, with its expression suppressed by mitogenic and oncogenic signals and induced upon cell-cycle arrest (PMID:21540187). INCA1 acts as an obligate co-factor for the antiproliferative and pro-apoptotic activities of ING5, which require INCA1 to suppress proliferation and enhance Fas-induced apoptosis (PMID:21750715); its inhibitory output is antagonized by the zinc finger protein HZF1 (ZNF16), whose binding relieves CDK2 inhibition and promotes S-to-G2/M transition (PMID:21874239). INCA1 functions as a terminal effector of growth-suppressive programs, being transcriptionally induced by ATF5 downstream of TMEM11-METTL1-mediated m7G methylation to restrain cardiomyocyte proliferation (PMID:37286744). Functionally, Inca1 is selectively required to maintain leukemia-initiating cells in multiple murine AML models while remaining largely dispensable for normal hematopoiesis (PMID:25525809).

1. 2004 High

   Establishing that an uncharacterized nuclear protein physically engages and is phosphorylated by cyclin A1-CDK2 defined INCA1's first molecular link to the cell-cycle machinery.

   Evidence Yeast triple-hybrid screen, GST pull-down, co-immunoprecipitation and in vitro kinase assay

   PMID:15159402

   Open questions at the time

   • Whether INCA1 phosphorylation by CDK2 alters its activity or stability was not determined
   • Functional consequence of the interaction for cell-cycle progression not yet tested
   • No structural basis for the interaction
2. 2011 High

   Genetic knockout and domain mapping converted INCA1 from a CDK2-associated substrate into a defined CDK inhibitor with a novel cyclin-binding domain, and tied its expression to mitogenic/arrest signals.

   Evidence Inca1-/- mouse model, MEF cell-cycle analysis, CDK2 kinase assay, domain deletion mutagenesis and retroviral overexpression

   PMID:21540187

   Open questions at the time

   • Mechanism by which mitogenic/oncogenic signals suppress INCA1 transcription not resolved
   • Mild knockout phenotype leaves open functional redundancy with other CDK inhibitors
3. 2011 High

   Epistasis in knockout MEFs showed INCA1 is an obligate co-factor for ING5, explaining how a CDK inhibitor connects to a chromatin/apoptosis regulator.

   Evidence Yeast two-hybrid, Inca1-/- MEFs, retroviral ING5 overexpression, proliferation, S-phase and apoptosis assays

   PMID:21750715

   Open questions at the time

   • Molecular basis for how INCA1 enables ING5 activity is unknown
   • Whether INCA1-ING5 cooperation requires CDK2 inhibition not addressed
4. 2011 Medium

   Identification of HZF1 (ZNF16) as an antagonist revealed a counter-regulatory layer that relieves INCA1-mediated CDK2 inhibition to drive cell-cycle progression.

   Evidence Yeast two-hybrid, reciprocal co-immunoprecipitation, CDK2 activity assay and cell-cycle analysis in K562 cells with lentiviral overexpression

   PMID:21874239

   Open questions at the time

   • Single-lab study without orthogonal in vivo validation
   • Whether HZF1 displaces INCA1 from cyclin A1-CDK2 or acts indirectly not resolved
5. 2008 Medium

   A cytoplasmic interaction with testis-specific RSB-66 hinted at a context-dependent localization and partner set beyond the nuclear CDK axis.

   Evidence Yeast two-hybrid, GST pull-down, co-immunoprecipitation, immunofluorescence co-localization and interaction-residue mutagenesis in HeLa cells

   PMID:18756329

   Open questions at the time

   • Functional consequence of the cytoplasmic INCA1-RSB-66 interaction unknown
   • Physiological relevance in testis not demonstrated
6. 2014 High

   In vivo leukemia models defined a selective requirement for INCA1 in leukemia-initiating cells, distinguishing its role in malignant versus normal stem cell maintenance.

   Evidence Inca1-/- mice with AML1-ETO9a, MLL-AF9 and c-myc/BCL2 leukemia induction and bone marrow transplantation assays

   PMID:25525809

   Open questions at the time

   • Molecular pathway linking INCA1 to leukemia-initiating cell maintenance not defined
   • Whether the leukemia phenotype depends on CDK2 inhibition is unresolved
7. 2023 Medium

   Placing INCA1 as the terminal effector of a TMEM11-METTL1-ATF5 axis explained how an upstream m7G methylation program transcriptionally controls INCA1 to suppress proliferation.

   Evidence TMEM11 overexpression/deletion in cardiomyocytes, m7G methylation assay, ATF5 reporter and proliferation assays

   PMID:37286744

   Open questions at the time

   • Direct ATF5 binding at the INCA1 promoter not structurally mapped
   • Generalizability of the axis beyond cardiomyocytes untested
8. 2017 Low

   Observation of INCA1 induction during CREB3L4-depletion-induced G2/M arrest reinforced its association with cell-cycle arrest responses.

   Evidence siRNA knockdown of CREB3L4, Western blot and cell-cycle analysis in LNCaP prostate cancer cells

   PMID:28338058

   Open questions at the time

   • INCA1 measured only as part of a protein panel with no direct functional follow-up
   • Causal contribution of INCA1 to the arrest phenotype not tested

• How INCA1's CDK-inhibitory activity mechanistically drives leukemia-initiating cell maintenance, and how its multiple partners (ING5, HZF1, RSB-66) are integrated, remains unresolved.
• No structural model of INCA1 or its cyclin-binding domain
• Mechanistic link between CDK2 inhibition and leukemia-initiating cell survival undefined
• How nuclear CDK regulation and cytoplasmic RSB-66 interaction relate is unknown