| Property | Summary | Key details / examples | Evidence / citations |
|---|---|---|---|
| Identity verified | Human **CDK2** encodes cyclin-dependent kinase 2, a CMGC-family **serine/threonine protein kinase** that functions in cell-cycle control | Matches UniProt P24941 description: cyclin-dependent kinase 2; activity centered on G1/S and S-phase progression; functions with cyclin partners rather than as a constitutively active kinase (pqac-00000000, pqac-00000001) | (pqac-00000000, pqac-00000001) |
| Protein structure and domains | CDK2 has the canonical bilobed protein kinase fold, including the **ATP-binding site**, **PSTAIRE helix**, and **activation/T-loop** that must adopt an active conformation | Cyclin binding reorganizes the kinase conformation; the catalytic site becomes fully competent after activating phosphorylation of **Thr160** in the T-loop by CAK; recent structural analyses emphasize conformational tuning of catalytic efficiency in CDK2 versus CDK4 (pqac-00000004, pqac-00000010) | (pqac-00000004, pqac-00000010) |
| Enzymatic function | CDK2 catalyzes **ATP-dependent phosphorylation of serine/threonine residues** on protein substrates | Primary biochemical role is phosphotransfer to substrate S/T residues, especially in cell-cycle regulators and chromatin-associated proteins; phosphorylation controls substrate activity, stability, or protein-protein interactions during G1/S and S phase (pqac-00000003, pqac-00000008) | (pqac-00000003, pqac-00000008) |
| Catalytic mechanism | CDK2 is activated in a multistep manner requiring **cyclin binding** and **T-loop phosphorylation** | Cyclin E or cyclin A binds CDK2 through the cyclin box/PSTAIRE interface, inducing conformational changes that expose the catalytic site; CAK then phosphorylates **Thr160**, stabilizing the active kinase state (pqac-00000010, pqac-00000000) | (pqac-00000010, pqac-00000000) |
| Cyclin partners | Principal activating partners are **cyclin E1/E2** and **cyclin A** | **Cyclin E–CDK2** predominates in late G1/G1-S transition; **cyclin A–CDK2** acts later in S phase and into G2-associated functions; cyclin E is the main G1 activator and cyclin A sustains later interphase CDK2 activity (pqac-00000003, pqac-00000008, pqac-00000011) | (pqac-00000003, pqac-00000008, pqac-00000011) |
| Cell-cycle phases of activity | CDK2 activity peaks across **late G1, G1/S transition, and S phase**, with cyclin-specific phase usage | Cyclin E/CDK2 promotes **S-phase entry**; cyclin A/CDK2 coordinates **S-phase progression** and later interphase events; newer work also indicates CDK2 activity in interphase depends on continued upstream mitogen/CDK4/6 support more than older irreversible models predicted (pqac-00000005, pqac-00000012) | (pqac-00000005, pqac-00000012) |
| Core pathway role | Central effector in the **CDK4/6–Rb–E2F–cyclin E/A–CDK2** pathway controlling commitment to DNA replication | CDK4/6 initiates Rb inactivation and E2F activation; rising CDK2 activity then drives stronger Rb phosphorylation, promotes E2F-dependent transcription, and helps coordinate commitment with G1/S transition (pqac-00000005, pqac-00000011, pqac-00000009) | (pqac-00000005, pqac-00000011, pqac-00000009) |
| Substrate specificity motif | CDK2 is a **proline-directed kinase** with preference for **S/T-P** phosphoacceptor motifs | In the in situ nuclear substrate screen, 156/166 AS-CDK2-specific thiophosphopeptides (93%) contained at least one **S/T-P** site; kinome-wide profiling further shows specificity is shaped by both positive residue preference and negative selectivity around the phosphosite (pqac-00000007, pqac-00000002) | (pqac-00000007, pqac-00000002) |
| Cyclin-mediated substrate recognition | Substrate docking is influenced by cyclin features in addition to the catalytic pocket | Cyclin E contains substrate-interaction surfaces including **MRAIL** and **VDCLE** regions that help engage RLX-containing proteins and pocket proteins such as RB family members, contributing to substrate selection beyond the kinase active site alone (pqac-00000010) | (pqac-00000010) |
| Major validated substrate: RB1/Rb | **Retinoblastoma protein (Rb)** is a canonical CDK2 substrate and a key mediator of CDK2-driven cell-cycle progression | Cyclin E/CDK2 contributes to late G1 **hyperphosphorylation** of Rb, disrupting Rb–E2F repression and promoting transcription of S-phase genes; reported sites include **T373, S795, S807/S811** among others (pqac-00000006, pqac-00000005, pqac-00000009) | (pqac-00000006, pqac-00000005, pqac-00000009) |
| Other validated substrates: CKI/transcription regulators | CDK2 phosphorylates regulators that reinforce proliferation and transcriptional output | Examples summarized in recent reviews include **p27KIP1**, **E2F5**, and **CBP/p300**, linking CDK2 to CKI regulation and transcriptional control (pqac-00000010) | (pqac-00000010) |
| Other validated substrates: replication / histone control | CDK2 directly regulates DNA replication and histone gene expression machinery | Reported substrates include **CDC6**, **NPAT**, and **HIRA**; these support replication origin function and histone biosynthesis needed for S phase (pqac-00000010, pqac-00000008) | (pqac-00000010, pqac-00000008) |
| Other validated substrates: centrosome cycle | CDK2 phosphorylates centrosome-associated proteins important for duplication and cell-cycle coordination | Validated examples include **NPM**, **CP110**, and **MPS1**, supporting centrosome duplication and broader cell-cycle execution (pqac-00000010) | (pqac-00000010) |
| Nuclear substrate landscape | Proteomics identified a broad nuclear CDK2 substrate network beyond classic cell-cycle proteins | An in situ phosphorylation study identified **117 candidate nuclear substrates**, with ~**43%** already known CDK substrates; validated novel targets included **LSD1, DOT1L, and Rad54**, extending CDK2 function into chromatin, DNA repair, and transcription-linked regulation (pqac-00000003, pqac-00000007) | (pqac-00000003, pqac-00000007) |
| Subcellular localization | CDK2 functions **primarily in the nucleus**, where it phosphorylates Rb and many chromatin-associated substrates | Nuclear context was important for recovering physiological substrates in isolated nuclei; cyclin E is also described as mainly nuclear; nuclear localization aligns with roles in Rb/E2F control, replication, and histone transcription (pqac-00000003, pqac-00000010) | (pqac-00000003, pqac-00000010) |
| Cytoplasmic/extranuclear function | A subset of CDK2 activity also occurs outside the nucleus in later interphase | At the **S/G2 transition**, **cyclin A2–CDK2** appears in the cytoplasm and can activate **PLK1** through Bora phosphorylation, indicating regulated compartment switching for specific downstream outputs (pqac-00000010) | (pqac-00000010) |
| Positive regulation | CDK2 is positively regulated by **cyclin accumulation**, **CAK**, mitogen-driven upstream signaling, and Rb/E2F feedback | Mitogenic signaling induces cyclin D then E2F, leading to cyclin E accumulation and CDK2 activation; **CAK** phosphorylation of Thr160 is required for full activity; increasing CDK2 activity helps establish the Rb/E2F positive-feedback module before DNA replication (pqac-00000005, pqac-00000010, pqac-00000011) | (pqac-00000005, pqac-00000010, pqac-00000011) |
| Negative regulation | CDK2 is restrained by **p21CIP1**, **p27KIP1**, inhibitory phosphorylation pathways, and loss of mitogenic support | Cip/Kip proteins inhibit cyclin-CDK2 complexes and limit CAK access/substrate engagement; broader CDK control also involves inhibitory phosphorylation and phosphatase circuits; recent work shows CDK2 activity can collapse when mitogen/CDK4/6 support is lost, even outside G1 (pqac-00000000, pqac-00000010, pqac-00000012) | (pqac-00000000, pqac-00000010, pqac-00000012) |
| Current expert view | Modern models treat CDK2 as a **phase-specific amplifier and executor** of G1/S commitment rather than a simple redundant kinase | Reviews and recent single-cell studies support a model in which CDK2 integrates upstream mitogenic state, Rb/E2F feedback, and replication-entry machinery; it is especially important for the **timing and threshold** of commitment to DNA replication (pqac-00000011, pqac-00000005, pqac-00000012, pqac-00000013) | (pqac-00000011, pqac-00000005, pqac-00000012, pqac-00000013) |


*Table: This table summarizes the core structural, enzymatic, pathway, localization, and regulatory properties of human CDK2, with emphasis on experimentally supported functions and recent mechanistic interpretations. It is useful as a compact functional annotation reference grounded in the cited literature.*