Localised MPF activation and mitotic phosphorylation in fertilised Xenopus eggs.

In amphibians such as Xenopus laevis, the initial polarity of the oocyte is established during oogenesis to produce animal and vegetal hemispheres with extensive structural and compositional differences. The animal-vegetal polarity ot the egg is reinforced by "Surtace Contraction Waves" (SCWs), occurring immediately prior to each cleavage division (Hara et aI., 1980; Sawai, 1982). These are instrumental in concentrating germ plasm at the vegetal pole (Savage and Danilchik, 1993). We hypothesised that the SCWs, and other animal-vegetal asymmetries in egg organisation, may result from localised activation of maturation promoting factor (MPF) in the animal hemisphere, where the mitotic spindle is positioned. MPF is a complex of active Cdc2 kinase and cyclin B, and is a universal activator of mitosis that can activate autonomously in egg cytoplasm. Mitotic phosphorylation, provoked directly or independently by MPF activation, affects the organisation of the cell by regulating, for instance, microtubule dynamics, myosin activity, microtubule-severing activity and microtubule motor protein activity. We have examined the spatial regulation of MPF activation in Xenopus eggs at first mitosis and assessed the distribution of mitotic phosphorylation using the monoclonal antibody MPM-2 (Davis et aI., 1983). MPM-2 reactive proteins include spindle proteins, centrosomal proteins and microtubule binding proteins, as well the phosphatase Cdc25 (Kuang et aI., 1994) which enhances MPF activation in a positive feedback loop.