Members of the INK4 family of cyclin-dependent kinase (CDK) inhibitors 
specifically bind and inhibit the G1-specific CDK molecules CDK4 and CDK6. One 
of the INK4 molecules, p16, is also known as multiple tumor suppressor and has 
been found to be mutated or deleted in various tumors and cell lines. We have 
previously identified p18 as a member of the INK4 family. To determine 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 of p18 to bind and 
inhibit CDK4 and CDK6 or to inhibit cell growth were determined. Our results 
indicate that the third ankyrin repeat and the NH2-terminal portion of the 
fourth repeat constitute the essential element necessary 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 to differentially bind and inhibit 
CDK4 and CDK6, accounting for the known penchant of p18 to preferentially 
interact with CDK6.