DEFECTIVE FOLDING OF MUTANT P16(INK4) PROTEINS ENCODED BY TUMOR-DERIVED ALLELES

p16(INK4) is a specific cyclin D-dependent kinase inhibitor and a mult iple tumor suppressor. Inactivation of p16 is frequent in both primary tumors and tumor-derived cell lines. We describe here the conformatio nal properties and oligomerization state of seven mutant p16 proteins; all of them are deficient in function. Four of the seven proteins sho w significantly disrupted secondary structure and backbone folding. Th e other three adopt partially folded, molten globule-like conformation s. These proteins have near-native levels of secondary structure, but lack the ability to undergo a cooperative thermal transition and are s ubstantially less resistant to proteolysis than is wild type p16. At l ow concentrations, two of the seven proteins are monomers, three exhib it an apparent molecular weight between the value of a monomer and a d imer, and the other two aggregate significantly. Our results strongly suggest that defective protein folding and/or aggregation is a common mechanism for inactivation of p16.