Concerted Roles of Cyclin A, cdc25+ Mitotic Inducer, and Type 2A Phosphatase in Activating the Cyclin B/cdc2 Protein Kinase at the G2/M Phase Transition

Cells in M phase contain a dominant factor, called M-phase-promoting factor or maturation-promoting factor (MPF), which is able upon transfer to drive G2 cells into M phase (Masui and Markert 1971; Kishimoto et al. 1982) or to induce prematurely some cytological features of M phase in G1 cells (Johnson and Rao 1970). Purification of MPF activity from vertebrate and invertebrate oocytes at meiotic metaphase led to isolation of a single component, identified as cdc2 kinase. It corresponds to the major M-phase-specific H1 histone kinase, not only in oocytes and early embryos, but also in yeast (for review, see Dorée 1990; Nurse 1990). In starfish at first meiotic metaphase, cdc2 kinase can be purified as a heterodimer comprising one molecule of a single cyclin-B species and one molecule of cdc2 (Labbé et al. 1989b). In Xenopus oocytes at second meiotic metaphase, purified MPF was found to contain both cdc2 and two...