Important regulator of cell cycle progression. Inhibits the kinase activity of CDK2 bound to cyclin A, but has little inhibitory activity on CDK2 bound to SPDYA. Involved in G1 arrest. Potent inhibitor of cyclin E- and cyclin A-CDK2 complexes. Forms a complex with cyclin type D-CDK4 complexes and is involved in the assembly, stability, and modulation of CCND1-CDK4 complex activation. Acts either as an inhibitor or an activator of cyclin type D-CDK4 complexes depending on its phosphorylation state and/or stoichometry. Forms a ternary complex composed of CCNE1, CDK2 and CDKN1B. Interacts directly with CCNE1; the interaction is inhibited by CDK2-dependent phosphorylation on Thr-187. Interacts with COPS5, subunit of the COP9 signalosome complex; the interaction leads to CDKN1B degradation. Interacts with NUP50; the interaction leads to nuclear import and degradation of phosphorylated CDKN1B. Interacts with CCND1 and SNX6 (By similarity). Interacts (Thr-198-phosphorylated form) with 14-3-3 proteins, binds strongly YWHAQ, weakly YWHAE and YWHAH, but not YWHAB nor YWHAZ; the interaction with YWHAQ results in translocation to the cytoplasm. Interacts with AKT1 and LYN; the interactions lead to cytoplasmic mislocation, phosphorylation of CDKN1B and inhibition of cell cycle arrest. Forms a ternary complex with CCNA2 and CDK2; CDKN1B inhibits the kinase activity of CDK2 through conformational rearrangements. Interacts (unphosphorylated form) with CDK2. Forms a complex with CDK2 and SPDYA, but does not directly interact with SPDYA. Forms a ternary complex composed of cyclin D, CDK4 and CDKN1B. Interacts (phosphorylated on Tyr-88 and Tyr-89) with CDK4; the interaction is required for cyclin D and CDK4 complex assembly, induces nuclear translocation and activates the CDK4 kinase activity. Interacts with GRB2. Interacts with PIM1. Identified in a complex with SKP1, SKP2 and CKS1B. Interacts with UHMK1; the interaction leads to cytoplasmic mislocation, phosphorylation of CDKN1B and inhibition of cell cycle arrest. Interacts also with CDK1. Dephosphorylated on Thr-187 by PPM1H, leading to CDKN1B stability (By similarity). Nuclear and cytoplasmic in quiescent cells. AKT- or RSK-mediated phosphorylation on Thr-198, binds 14-3-3, translocates to the cytoplasm and promotes cell cycle progression. Mitogen-activated UHMK1 phosphorylation on Ser-10 also results in translocation to the cytoplasm and cell cycle progression. Phosphorylation on Ser-10 facilitates nuclear export. Translocates to the nucleus on phosphorylation of Tyr-88 and Tyr-89 (By similarity). Colocalizes at the endosome with SNX6; this leads to lysosomal degradation (By similarity). A peptide sequence containing only AA 28-79 retains substantial Kip1 cyclin A/CDK2 inhibitory activity. Phosphorylated; phosphorylation occurs on serine, threonine and tyrosine residues. Phosphorylation on Ser-10 is the major site of phosphorylation in resting cells, takes place at the G(0)-G(1) phase and leads to protein stability. Phosphorylation on other sites is greatly enhanced by mitogens, growth factors, cMYC and in certain cancer cell lines. The phosphorylated form found in the cytoplasm is inactivate. Phosphorylation on Thr-198 is required for interaction with 14-3-3 proteins. Phosphorylation on Thr-187, by CDK1 and CDK2 leads to protein ubiquitination and proteasomal degradation. Tyrosine phosphorylation promotes this process. Phosphorylation by PKB/AKT1 can be suppressed by LY294002, an inhibitor of the catalytic subunit of PI3K. Phosphorylation on Tyr-88 and Tyr-89 has no effect on binding CDK2, but is required for binding CDK4. Dephosphorylated on tyrosine residues by G-CSF (By similarity).Dephosphorylated on Thr-187 by PPM1H, leading to CDKN1B stability (By similarity). Ubiquitinated; in the cytoplasm by the KPC complex (composed of RNF123/KPC1 and UBAC1/KPC2) and, in the nucleus, by SCF(SKP2). The latter requires prior phosphorylation on Thr-187. Ubiquitinated; by a TRIM21-containing SCF(SKP2)-like complex; leads to its degradation (By similarity). Subject to degradation in the lysosome. Interaction with SNX6 promotes lysosomal degradation (By similarity). Belongs to the CDI family.