HSP70-2 IS REQUIRED FOR CDC2-KINASE-ACTIVITY IN MEIOSIS-I OF MOUSE SPERMATOCYTES

Cyclin B-dependent CDC2 kinase activity has a key role in triggering t he G(2)/M-phase transition during the mitotic and meiotic cell cycles, The Hsp70-2 gene is expressed only in spermatogenic cells at a signif icant level. In Hsp70-2 gene knock-out (Hsp70-2(-/-)) mice, primary sp ermatocytes fail to complete meiosis I, suggesting a link between HSP7 0-2 heat-shock protein and CDC2 kinase activity during this phase of s permatogenesis, Members of the HSP70 protein family are molecular chap erones that mediate protein de novo folding, translocation and multime r assembly, This study used immunoprecipitation-coupled western blot a nd in vitro reconstitution experiments to show that HSP70-2 interacts with CDC2 in the mouse testis, appears to be a molecular chaperone for CDC2, and is required for CDC2/cyclin B1 complex formation, Previous studies reported that most CDC2 kinase activity in the mouse testis is present in pachytene spermatocytes. Although CDC2 kinase activity for histone H1 was present in the testis of wild-type mice, it was nearly absent from the testis of Hsp70-2(-/-) mice, probably due to defectiv e CDC2/cyclin B1 complex formation, Furthermore, addition of HSP70-2 t o freshly prepared extracts of testis from Hsp 70-2(-/-) mice not only restored CDC2/cyclin B1 complex formation but also reconstituted CDC2 kinase activity in vitro, It appears that one cause of failure to com plete meiosis I during spermatogenesis in Hsp70-2(-/-) mice is disrupt ion of CDC2/cyclin B1 assembly in pachytene spermatocytes, thereby pre venting development of the CDC2 kinase activity required to trigger G( 2)/M-phase transition, These studies provide novel in vivo evidence fo r a link between an HSP70 molecular chaperone and CDC2 kinase activity essential for the meiotic cell cycle in spermatogenesis.