Xenopus laevis Oocytes as a Model for Studying the Activation of Intracellular Kinases

As indicated in the previous chapter, Xenopus laevis oocytes are arrested at the G2/M border of the first meiotic prophase. After progesterone treatment, a complex process of intracellular signals is activated which ends in the maturation of the oocytes, a process reported by the Breakdown of the Germinal Vesicle (GVBD). Among the critical signals turned on, activation of MPF from inactive stores takes place prior to observance of GVBD. This activation is necessary and sufficient to induce maturation. The injection of active MPF induces precocious GVBD (Gotoh et al., 1991) and the microinjection of pl3suc protein that specifically binds to cdc2 protein (the kinase component of MPF complex) inhibits its kinase activity blocking the progesterone-induced maturation (Dunphy and Newport, 1989). There are two peaks of MPF activity, the first coincides approximately with metaphase I and needs synthesis of c-Mos protein for induction of activity (Sagata et al, 1989). The second peak appears in metaphase II and needs the resynthesis of cyclin (the second component of MPF complex) which is destroyed at the end of the first mitosis (Murray and Kirschner, 1989; Murray et al., 1989). Thus, MPF is a complex made of cyclin B and cdc2 proteins, which activation leads to a burst of protein phosphorylation 30–60 min prior to GVBD. This pleiotropic effect leads to activation of important molecules characterized as esential steps in the signal transduction pathways of several inducers of oocyte maturation.