The p16 protein has been identified as a tumor suppressor that functions by inhibiting the cyclin-dependent kinases CDK4 and CDK6. Deletions or point mutations in the p16 gene have been found in a number cancers, emphasizing its importance in regulating cell cycle progression. Inhibition by p16 occurs through protein-protein interactions with its targets. This is not surprising, since p16 is thought to contain ankyrin-like repeats, motifs implicated in protein-protein interactions. Our goal was to identify structural characteristics of p16 not only as an important step towards understanding CDK4 inhibition but also to explore the role of ankyrin repeats in the p16 structure, as no detailed structure of any protein containing these motifs has been reported. We have expressed, refolded, and purified p16 from E. coli and have shown it to be functionally active by specific binding to CDK4. Analytical ultracentrifugation has shown that p16 weakly self-associates to form dimers with a Kd = 270 microM. The CD spectrum indicates that the protein is composed of 33% alpha-helix, 22% beta-sheet, 19% beta-turn, and 27% other (which includes aromatic and random coil contributions). Further CD experiments suggest that p16 exhibits low structural stability with a delta G of -2.3 kcal/mol. This weak stability is a consequence of a highly dynamic structure as measured by ANS-binding, NMR hydrogen-deuterium exchange, and fluorescence. It is possible that a well-defined tertiary structure is imparted upon the binding of p16 to CDK4.