The cdc25 M-phase inducer: An unconventional protein phosphatase

Abstract

Jonathan B. A. Millar and Paul Russell Departments of Molecular and Cell Biology The Scripps Research Institute La Jolla, California 92037 The molecular events involved in regulating the initiation of nuclear division have been the subject of intensive in- vestigation in recent years. A milestone in these studies was the discovery that nuclear division was brought about through the rapid activation of mitosis promoting factor (MPF)-a seryl/threonyl protein kinase consisting of a 34 kd catalytic subunit, p34cdc2, and a 45-60 kd B-type cyclin. Subsequent efforts have focused on the processes regu- lating MPF activation. The subject of this review is recent advancesestablishing that cdc25, a universally conserved M-phase inducer, is a protein phosphatase that directly dephosphorylates and activates MPF. The Phosphatase That Activates MPF Studies of MPF activation have tended to concentrate on rapid posttranslational events, in particular protein phos- phorylation and dephosphorylation. That MPF activity might be inhibited by protein phosphorylation was pre- dicted by studies in fission yeast showing that the wee7 gene product, which functions as an inhibitor of mitosis, is a protein kinase (Russell and Nurse, 1987). This predic- tion was confirmed by the finding that phosphorylation of Tyr-15 of the fission yeast ~34~~“~ subunit negatively regulates MPF (Gould and Nurse, 1989). Activation of the complex at the G$M transition is coincident with tyrosyl dephosphorylation of p34-, indicating that this dephos- phorylation event is a critical step in the initiation of M-phase. More recent studies have shown that phosphory- lation of asecond site, Thr-167, is required for MPF activity in fission yeast, although there is no evidence indicating that it is involved in regulating the timing of M-phase (Gould et al., 1991). The first clue linking cdc25 to mitotic control was pro- vided by Fantes (1979), who found that the GP cell cycle arrest caused by c&25- mutations in fission yeast is sup- pressed by inactivation of weel. Later experiments ex- plained this observation by showing that cdc25 gene prod- uct is a dose-dependent inducer of mitosis that functions in opposition to the products of wee7 and mikl, the latter gene being a redundant homolog of wee7 (Russell and Nurse, 1986; Lundgren et al., 1991). A logical hypothesis following from these results was that cdc25 should be in- volved in promoting the dephosphorylation pathway that counteracts the weellmikl kinases (Figure 1). Observa- tions that inactivation of cdc25 results in a late-G* cell cycle arrest in which p34cdC2 becomes maximally phosphorylated on Tyr-15 (Gould and Nurse, 1969) and that cdc25 function is not required in strains expressing mutant cdc2 having nonphosphorylatable phenylalanyl at position 15 or in strains expressing the human T-cell phosphotyrosyl phos- phatase (PTPase) were fully consistent with the idea that