Defining the substrate specificity of cdk4 kinase-cyclin D1 complex.

cdk4 kinase-cyclin D1 complex (cdk4/D1) does not phosphorylate all of the sites within retinoblastoma protein (Rb) equally. Comparison of five phosphorylation sites within the 15 kDa C domain of Rb indicates that Ser795 is the preferred site of phosphorylation by cdk4/D1. A series of experiments has been performed to determine the properties of this site that direct preferential phosphorylation. For cdk4/D1, the preferred amino acid at the third position C-terminal to the phosphorylated serine/threonine is arginine. Substitution of other amino acids, including a conservative change to lysine, has dramatic effects on the rates of phosphorylation. This information has been used to mutate less favorable sites in Rb, converting them to sites that are now preferentially phosphorylated by cdk4/D1. A conserved site at Ser842 in the related pocket protein p107 is also preferentially phosphorylated by cdk4/D1. Although Rb and p107 differ significantly in sequence, the Rb Ser795 site can replace the p107 Ser842 site without affecting the rate of phosphorylation. These results suggest that although a determinant of specificity resides in the sequences surrounding the phosphorylated site, the structural context of the site is also a critical parameter of specificity.