Hyperphosphorylation of the N-terminal domain of cdc25 regulates activity toward cyclin B1/cdc2 but not cyclin A/cdk2

Cdc25 regulates entry into mitosis by regulating the activation of cyclin B/cdc2. In humans, at least two cdc25 isoforms have roles in controlling the G(2)/M transition. Here we show, using bacterially expressed recombinant proteins, that two cdc25B splice variants, cdc25B2 and cdc25B3, are capable of activating cyclin A/cdk2 and cyclin B/cdc2, but that mitotic hyperphosphorylation of these proteins increases their activity toward only cyclin B1/cdc2, Cdc25C has only very low activity in its unphosphorylated form, add following hyperphosphorylation it will efficiently catalyze the activation of only cyclin B/cdc2, This was reflected by the in vivo activity of the immunoprecipitated cdc25B and cdc25C from interphase and mitotic HeLa cells. The increased activity of the hyperphosphorylated cdc25s toward cyclin B1/cdc2 was in large part due to increased binding of this substrate. The substrate specificity, activities; and timing of the hyperphosphorylation of cdc25B and cdc25C during G(2) and M suggest that these two mitotic cdc25 isoforms are activated by different kinases and perform different functions during progression through G(2) into mitosis

Multiple splicing variants of cdc25B regulate G2/M progression

Progression through G2 phase into mitosis is regulated by the activation of the mitotic cyclin/cdk complexes, which are in turn activated cdc25B and cdc25C phosphatases. Here we report that alternate splicing produces at least five variants of cdc25B, although only cdc25B2 and cdc25B3 are detectable as proteins. Analysis of these two variants shows that cdc25B2 is expressed at lower levels relative to cdc25B3 in all cell lines tested, and the expression of both increased markedly during G2 and mitosis. Overexpression of the catalytically inactive version of either cdc25B variant produced a G2 arrest implicating both in regulating G2/M progression. (C) 1999 Academic Press