Binding of HTm4 to cyclin-dependent kinase (Cdk)-associated phosphatase (KAP)·Cdk2·cyclin A complex enhances the phosphatase activity of KAP, dissociates cyclin A, and facilitates KAP dephosphorylation of Cdk2

Cyclin-dependent kinase 2 (cdk2) activation requires phosphorylation of Thr¹⁶⁰ and dissociation from cyclin A. The T-loop of cdk2 contains a regulatory phosphorylation site at Thr¹⁶⁰. An interaction between cdc-associated phosphatase (KAP) and cdk2 compromises the interaction between cdk2 and cyclin A, which permits access of KAP, a Thr¹⁶⁰-directed phosphatase, to its substrate, cdk2. We have reported that KAP is bound and activated by a nuclear membrane protein, HTm4. Here, we present in vitro data showing the direct interaction between the HTm4 C terminus and KAP Tyr ¹⁴¹. We show that this interaction not only facilitates access of KAP to Thr¹⁶⁰ and accelerates KAP kinetics, but also forces exclusion of cyclin A from the KAP·cdk2 complex.