A MAP KINASE-DEPENDENT SPINDLE ASSEMBLY CHECKPOINT IN XENOPUS EGG EXTRACTS

Like early Xenopus embryos, extracts made from Xenopus eggs lack the c ell cycle checkpoint that keeps anaphase from occurring before spindle assembly is complete. At very high densities of sperm nuclei, however , microtubule depolymerization arrests the extracts in mitosis. The ar rested extracts have high levels of maturation-promoting factor activi ty, fail to degrade cyclin B, and contain activated ERK2/mitogen-activ ated protein (MAP) kinase. The addition of the purified MAP kinase-spe cific phosphatase MKP-1 demonstrates that MAP kinase activity is requi red for both the establishment and maintenance of the mitotic arrest i nduced by spindle depolymerization. Increased calcium concentrations, which release unfertilized frog eggs from their natural arrest in meta phase of meiosis II, have no effect on the mitotic arrest.