Figure 8.

Mos induces partial phosphorylation and activation of Plx1 independently of pre-MPF activation. (A) Groups of 35 oocytes were injected with either Xp9 mRNA or water and after overnight incubation were injected again with malE-mos protein. At the indicated times injected oocytes that did not have a normal white spot were transferred to dry ice. Uninjected oocytes (control) and malE-mos matured oocytes (GVBD) were also taken for comparison. Lysates were prepared from single oocytes, and half of this lysate was analyzed by immunoblotting with anti-Plx1, anti-p42^mpk1, and anti-p34^cdc2 antibodies. (B) Kinetics of maturation of the same oocytes as in A. (C) Lysates corresponding to half an oocyte (the second half of the lysates prepared from single oocytes that in A were used for immunblotting) were immunoprecipitated with anti-Plx1 antibodies, and their associated kinase activity was assayed using casein as an in vitro substrate. Each bar represents single oocytes that were uninjected (control), malE-mos injected and matured (GVBD), or injected with malE-mos either alone or plus Xp9 but had no GVBD (these corresponded to the oocytes that in A showed partial Plx1 phosphorylation but had no preMPF activation). The Plx1 activity taken as 100% in mature oocytes (GVBD) was usually 30- to 40-fold higher than in control oocytes.