BIOCHEMICAL-CHARACTERIZATION OF P16(INK4)-CONTAINING AND P18-CONTAINING COMPLEXES IN HUMAN CELL-LINES

The regulation of the D-type cyclin-dependent kinase (CDK4 and CDK6) a ctivity appears to be the key step in the progression of eukaryotic ce lls through the G(1) cell cycle phase, One of the mechanisms involved in this process is the binding of some small proteic inhibitors, with a molecular mass ranging between 14 and 20 kDa, to these CDKs, We have evaluated the amount of two such inhibitors, namely p16(INK4) and p18 , in normal and transformed cells, as well as the biochemical features of the macromolecular complexes containing these proteins, The result s obtained indicated that Ii) p18 gene expression, unlike p16(INK4) ge ne, is not regulated by pRb status, (ii) no evident relationship exist s between the expression of p16(INK4) and p18 genes, (iii) significant amounts of the two proteins are not bound to CDKs but occur as free m olecules, (iv) each inhibitor forms a complex with the CDK protein wit h a 1:1 stoichiometry, and Iv) a competition exists between cyclin D a nd the inhibitor protein toward the CDK protein resulting in the absen ce of detectable cellular free kinase. Moreover, employing the human n ative partially purified p16(INK4) or the pure recombinant protein, we have been able to demonstrate in vitro the dissociation of CDR4-cycli n D1 complex and the formation of CDK4-p16INK4 bimolecular complex, Ou r findings suggest that during the cell division cycle the members of the p16(INK4) protein family and cyclin Ds compete for binding to CDK4 /CDK6 and that their quantitative ratio is essential for G(1) --> S tr ansition.