Regulatory component of the cyclin D1-CDK4 (DC) complex that phosphorylates and inhibits members of the retinoblastoma (RB) protein family including RB1 and regulates the cell-cycle during G(1)/S transition (PubMed:
1833066, PubMed:
1827756, PubMed:
8114739, PubMed:
8302605, PubMed:
19412162, PubMed:
33854235). Phosphorylation of RB1 allows dissociation of the transcription factor E2F from the RB/E2F complex and the subsequent transcription of E2F target genes which are responsible for the progression through the G(1) phase (PubMed:
1833066, PubMed:
1827756, PubMed:
8114739, PubMed:
8302605, PubMed:
19412162). Hypophosphorylates RB1 in early G(1) phase (PubMed:
1833066, PubMed:
1827756, PubMed:
8114739, PubMed:
8302605, PubMed:
19412162). Cyclin D-CDK4 complexes are major integrators of various mitogenenic and antimitogenic signals (PubMed:
1833066, PubMed:
1827756, PubMed:
8302605, PubMed:
19412162). Also substrate for SMAD3, phosphorylating SMAD3 in a cell-cycle-dependent manner and repressing its transcriptional activity (PubMed:
15241418). Component of the ternary complex, cyclin D1/CDK4/CDKN1B, required for nuclear translocation and activity of the cyclin D-CDK4 complex (PubMed:
9106657). Exhibits transcriptional corepressor activity with INSM1 on the NEUROD1 and INS promoters in a cell cycle-independent manner (PubMed:
16569215, PubMed:
18417529).
