Cloning and functional analusis of cell-cycle regulatory genes by complementation cloning.

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By west-western hybridization using an anti-cyclin G antibody, we cloned a cDNA encoding a novel association partner of cyclin G.The cDNA encodes a protein harboring a predicted N-terminal Ser/Thr protein kinase catalytic domain with a long C-terminal extension.Hence we named it GAK (cyclin G-associated kinase) . The C-terminal extension shares homology with tensin and auxilin, and contains a leucine zipper. By coimmunoprecipitation and western blotting, we detected and association between GAK and cyclin G in NRK cell extracts, and in extracts of COS cells expressing myc-tagged cyclin G.GAK was also found to coprecipitate with CDK5 and actin, and CDK5 is shown to be associated with cyclin G in NRK extracts. A direct interaction between GAK and cyclin G was detected using immobilized GST-GK and recombinant GST-free cyclin G for BIAcore analysis. Recombinant and immunoprecipitated GAK were found to have kinase activity towards histone H1, but this activity was not enhanced (or decreased) by the presence of cyclin G.In contrast to brain specific expression of auxilin, GAK was found to be ubiquitously expressed. In HeLa cells synchronized from S phase by the thymidine double block, RNA and protein levels of GAK and cyclin G oscillated, peaking at G1 phase, and they showed a similar pattern of oscillation. These results suggest that GAK is somehow involved in cell cycle regulation, forming a complex with cyclin G.