Action of the p53 Effector, p21, on its Targets: Cyclin-cdk and PCNA

摘要

Cy or RXL motifs have been previously shown to be cyclin binding motifs found in a wide range of cyclin-cdk interacting proteins. For both cyclin A/cdk2 and cyclin E/cdk2, the presence of a Cy motif decreased the Km(sup peptide) 75 to 120-fold while the kcat remained unchanged. Changes in the length of the linker between the Cy motif and the phosphoacceptor serine suggest that both sites are engaged simultaneously to the cyclin and the cdk, respectively. PS100, a peptide containing a Cy motif, acts as a competitive inhibitor of cyclin/cdk complexes with a 15-fold lower Ki for cyclin E/cdk2 than for cyclin A/cdk2. These results provide kinetic proof that a Cy motif located at a minimal distance from the SPXK is essential for optimal phosphorylation by cdks and suggest that small chemicals that mimic the Cy motif would be specific inhibitors of substrate recognition by cyclin-dependent kinases. In addition, a detailed mutagenesis approach to define what sequence serves as a Cy motif reveals that RXL is neither necessary nor sufficient to act as a Cy motif. Instead, the results suggest that a hydrophobic molecule that spans the length of 3-4 peptide bonds would be a suitable Cy-mimetic inhibitor of cdk.