The cyclin-dependent kinase inhibitory domain of the yeast Sic1 protein is contained within the C-terminal 70 amino acids

By inhibiting the activity of Cdc28/Clb cyclin-dependent protein kinase (CDK) complexes, Sic1 prevents the premature initiation of S phase in the yeast Saccharomyces cerevisiae. By testing a series of Sic1 truncation mutants, we have mapped the minimal domain necessary for Cdc28/Clb inhibition in vivo to the C-terminal 70 amino acids of Sic1. Site-directed mutagenesis was used to show that a sequence that matches the zRxL motif found in mammalian CDK inhibitors is essential for Sic1 function. This motif is not found in the Schizosaccharomyces CDK inhibitor p25(rum1), which appears to be a structural and functional homolog of Sic1. Based on the mutational data and sequence comparisons, we argue that Sic1 and p25(rum1) are structurally distinct from the known mammalian CDK inhibitors, but may bind CDK complexes in a manner more closely resembling CDK substrates like the retinoblastoma and E2F proteins.