The protein p16 (INK4) primarily inhibits:
INK4 family represents dedicated inhibitors of CDK4 and CDK6 distinguishing them from Cip/Kip family that target cyclin E-CDK2 and cyclin A-CDK2. p16INK4a product of CDKN2A locus contains four ankyrin repeats forming elongated structure that binds CDK6 opposite cyclin-binding interface, inducing distortion of ATP-binding pocket and preventing association with cyclin D. Since cyclin D-CDK4/6 is earliest kinase phosphorylating Rb in response to mitogens, its inhibition maintains Rb in hypophosphorylated active repressor state bound to E2F, arresting cell before restriction point. Genetic inactivation through homozygous deletion, nonsense mutation, or promoter CpG hypermethylation occurs at high frequency in melanoma familial syndromes, pancreatic ductal adenocarcinoma, glioblastoma multiforme, and esophageal cancers, leading to uncontrolled Rb phosphorylation. Clinically, pharmacologic mimics of p16 action including palbociclib, ribociclib, and abemaciclib selectively inhibit CDK4/6, restoring Rb function and inducing G1 arrest in estrogen receptor-positive breast cancer, demonstrating translational exploitation. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.
Ref: Sherr & Roberts, Genes & Dev 1999, INK4 Family Biology. Alberts 7th ed., Chapter 20.