A mutation directing an amino acid substitution in the conserved beta-hinge region of one of the human Cks isoforms, CksHs2, was constructed by site-directed mutagenesis. Replacement of glutamine for glutamate 63 (E63Q) was predicted to stabilize the beta-interchanged dimeric and hexameric assembly of CksHs2. However, such an effect was seen only at high, non-physiological pH. Three-dimensional structures of the E63Q hexameric mutant protein were determined to 2.6 A resolution in a P4(3)2(1)2 space group and 2.1 A in the C2 space group isostructural with wild-type, and both were shown to be virtually identical to the refined 1.7 A wild-type structure. Thus, the E63Q mutation did not alter the wild-type structure and assembly of CksHs2 but, surprisingly, disrupted the essential biological function of the protein and significantly reduced its ability to bind to cyclin-dependent kinases. The Kd of wild-type CksHs2 for CDK2 was 5.05 x 10(-8) M, whereas the affinity of the mutant protein for CDK2 was too low to allow a determination. These data, coupled with the observation that monomeric but not hexameric CksHs2 interacts with cyclin-dependent kinases, suggest that glutamine 63 is likely to be directly involved in cyclin-dependent kinase binding in vitro and in vivo.