Functional interaction of Puralpha with the Cdk2 moiety of cyclin A/Cdk2.

Puralpha is a sequence-specific single-stranded nucleic acid-binding protein and a member of the highly conserved Pur family. Puralpha has been shown to colocalize with cyclin A/Cdk2 and to coimmunoprecipitate with cyclin A during S-phase. Here we show that this interaction is mediated by a specific affinity of Puralpha for Cdk2. In pull-down assays GST-Puralpha efficiently binds Cdk2 and Cdk1, binds Cdk4 less efficiently, and does not display binding to Cdk6. Puralpha stimulates several-fold the phosphorylation in vitro of histone H1 by cyclin A/Cdk2, produced from baculovirus constructs. Double chromatin immunoprecipitation using antibodies to Cdk2 and Puralpha reveals that both proteins colocalize in HeLa cells to DNA segments upstream of the c-MYC gene. Pur family member Purgamma colocalizes with Cdk2 to a specific DNA segment in this region.