Ser/Thr-kinase component of cyclin D-CDK4 (DC) complexes that phosphorylate and inhibit members of the retinoblastoma (RB) protein family including RB1 and regulate the cell-cycle during G(1)/S transition. Phosphorylation of RB1 allows dissociation of the transcription factor E2F from the RB/E2F complexes and the subsequent transcription of E2F target genes which are responsible for the progression through the G(1) phase. Hypophosphorylates RB1 in early G(1) phase. Cyclin D-CDK4 complexes are major integrators of various mitogenenic and antimitogenic signals. Also a substrate for SMAD3, phosphorylating SMAD3 in a cell-cycle-dependent manner and repressing its transcriptional activity. Component of the ternary complex, cyclin D/CDK4/CDKN1B, required for nuclear translocation and activity of the cyclin D-CDK4 complex (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Both phosphorylation at Thr-172 and binding of a D-type cyclin are necessary for enzymatic activity. Full activation of the cyclin-D-CDK4 complex appears to require other factors such as recruitment of the substrate via a substrate recruitment motif, and/or formation of the CDKN1B ternary complex. Inhibited by INK4 family members. In resting cells, the non-tyrosine-phosphorylated form of CDKN1B prevents phosphorylation at Thr-172 and inactivation, while, in proliferating cells, tyrosine phosphorylation of CDKN1B allows phosphorylation of Thr-172 of CDK4 and subsequennt activation. Component of the D-CDK4 complex, composed of CDK4 and some D-type G1 cyclin (CCND1, CCND2 or CCND3). Interacts directly in the complex with CCND1, CCND2 or CCND3. Interacts with SEI1 and ZNF655. Forms a ternary complex, cyclin D-CDK4-CDKN1B, involved in modulating CDK4 enzymatic activity. Interacts directly with CDKN1B (phosphorylated on 'Tyr-88' and 'Tyr-89'); the interaction allows assembly of the cyclin D-CDK4 complex, Thr-172 phosphorylation, nuclear translocation and enhances the cyclin D-CDK4 complex activity. CDK4 activity is either inhibited or enhanced depending on stoichiometry of complex. The non-tyrosine-phosphorylated form of CDKN1B prevents T-loop phosphorylation of CDK4 producing inactive CDK4. Interacts (unphosphorylated form) with CDK2. Also forms ternary complexes with CDKN1A or CDKN2A. Interacts directly with CDKN1A (via its N-terminal); the interaction promotes the assembly of the cyclin D-CDK4 complex, its nuclear translocation and promotes the cyclin D-dependent enzyme activity of CDK4. Interacts with FNIP1 and FNIP2. Cytoplasmic when non-complexed. Forms a cyclin D-CDK4 complex in the cytoplasm as cells progress through G(1) phase. The complex accumulates on the nuclear membrane and enters the nucleus on transition from G(1) to S phase. Also present in nucleoli and heterochromatin lumps. Colocalizes with RB1 after release into the nucleus (By similarity). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.