The N-terminal helix of Xenopus cyclins A and B contributes to binding specificity of the cyclin-CDK complex

Mitotic cyclins A and B contain a conserved N-terminal helix upstream of th e cyclin box fold that contributes to a significant interface between cycli n and cyclin-dependent kinase (CDK). To address its contribution on cyclin- CDK interaction, we have constructed mutants in conserved residues of the N -terminal helix of Xenopus cyclins B2 and A1. The mutants showed altered bi nding affinities to Cdc2 and/or Cdk2, We also screened for mutations in the C-terminal lobe of CDK that exhibited different binding affinities for the cyclin-CDK complex. These mutations were at residues that interact with th e cyclin N-terminal helix motif. The cyclin N-terminal helix mutations have a significant effect on the interaction between the cyclin-CDK complex and specific substrates, Xenopus Cdc6 and Cdc25C, These results suggest that t he N-terminal helix of mitotic cyclins is required for specific interaction s with CDKs and that to interact with CDK, specific substrates Cdc6 and Cdc 25C require the CDK to be associated with a cyclin, The interaction between the cyclin N-terminal helix and the CDK C-terminal lobe may contribute to binding specificity of the cyclin-CDK complex.