CDKN1B

CDKN1B encodes p27Kip1 (p27), a 198-residue, largely intrinsically disordered member of the Cip/Kip family of cyclin-dependent kinase regulators (with p21/CDKN1A and p57/CDKN1C). It is not an enzyme: it acts stoichiometrically, through a bipartite N-terminal kinase-inhibitory (CDI) domain spanning roughly residues 25-93, whose subdomain 1 docks into the MRAIL groove of the cyclin box and whose subdomain 2 then folds onto the CDK, rearranging its N-terminal lobe and inserting a 3(10) helix into the catalytic cleft as an ATP mimic. The principal consequence is potent inhibition of cyclin E-CDK2 and cyclin A-CDK2 in the nucleus, which keeps retinoblastoma-family proteins hypophosphorylated, holds E2F-dependent transcription in check, and restrains the G1/S transition; p27 is the CKI through which antimitogenic inputs such as TGF-beta signalling and cell-cell contact impose quiescence, and it is maximal in quiescent cells and early G1. Toward cyclin D-CDK4/6 its role is different and context-dependent: p27 promotes assembly of the ternary complex, escorts it to the nucleus, and depending on abundance and tyrosine phosphorylation may inhibit or permit CDK4 activity, so the protein is better described as a CDK regulator with a dominant inhibitory mode than as a universal inhibitor. Its activity is set largely post-translationally: phosphorylation of Thr187 by cyclin-CDK2 creates the phosphodegron read by CKS1 and the SCF(SKP2) ubiquitin ligase, giving the switch-like positive feedback that destroys p27 at G1/S, while Ser10 phosphorylation (by UHMK1/KIS) drives CRM1/Ran-dependent nuclear export, Thr157/Thr198 phosphorylation by AKT and RSK promotes 14-3-3 binding and cytoplasmic retention, Tyr74/Tyr88 phosphorylation by Src-family and ABL kinases relieves CDK inhibition, and cytoplasmic p27 is also ubiquitinated by the KPC (RNF123-UBAC1) complex. The cytoplasmic pool has functions independent of CDK inhibition, binding RhoA to dampen RhoA-ROCK signalling and binding the microtubule-destabilising protein stathmin, thereby influencing actin and microtubule dynamics, cell migration and invasion. p27 is a haploinsufficient tumour suppressor: its protein levels are reduced, largely by enhanced proteasomal degradation, in many epithelial cancers, and germline loss-of-function variants cause multiple endocrine neoplasia type 4 (MEN4).

Disorders

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