Identification of functional elements of p18(INK4C) essential for binding and inhibition of cyclin-dependent kinase (CDK)4 and CDK6

Members of the INK4 family of cyclin-depcndent kinase (CDK) inhibitors spec ifically bind and inhibit the G(1)-specific CDK molecules CDK4 and CDK6. On e of the INK4 molecules, p16, is also known as multiple tumor suppressor an d has been found to he mutated or deleted in various tumors and cell lines, We have previously identified p18 as a member of the INK4 family, To deter mine the molecular basis for the inhibitory function of p18, we introduced 11 missense mutations of conserved residues that were identified in p16 of cancer cell lines into p18, The effects of these mutations on the ability o f p18 to hind and inhibit CDK4 and CDK6 or to inhibit cell growth were dete rmined, Our results indicate that the third ankyrin repeat and the NH2-term inal portion of the fourth repeat constitute the essential element necessar y for the ability of p18 to bind and inhibit CDK4 and CDK6. Apart from this core interaction element, p18 seems to use additional, distinct residues t o differentially bind and inhibit CDK4 and CDK6, accounting for the known p enchant of p18 to preferentially interact with CDK6.