suc1+ encodes an essential cell cycle regulator of the fission yeast Schizosaccharomyces pombe. Its product, a 13-kDa protein, interacts with the Cdc2 protein kinase. Both positive and negative effects on cell cycle progression have been attributed to Suc1. To date, the exact mechanisms and the physiological role of the interaction between Suc1 and Cdc2 remain unclear. Here we have studied the molecular basis of this association. We show that Cdc2 can bind Suc1 or its mammalian homolog directly in the absence of any additional protein component. Using an alanine scanning mutagenesis method, we analyzed the interaction between Cdc2 and Suc1. We show that the integrity of several domains on the Cdc2 protein, including sites directly involved in catalytic activity, is required for binding to Suc1. Furthermore, Cdc2 mutant proteins unable to bind Suc1 (but able to bind cyclins) are nonfunctional when overexpressed in S. pombe, indicating that a specific interaction with Suc1 is required for Cdc2 function.