Regulation of embryonic cell division by a Xenopus gastrula-specific protein kinase

We describe a novel protein kinase, Pk9.7, and its role in cell division in the Xenopus embryo. Pk9.7 is transcribed only during blastula and gastrula stages. Expression of Pk9.7 in Xenopus oocytes induces meiotic maturation, while overexpression in embryos blocks blastomere cleavage in a MAP kinase-independent fashion. In both Pk9.7-injected oocytes and mitotic cells of cleavage-blocked embryos, chromosomes appear detached from abnormal spindles, and in oocytes additional microtubule structures are formed, suggesting that one function of Pk9.7 is to regulate formation of, and chromosome attachment to, the spindle. Consistent with this, Pk9.7 co-immunoprecipitates tubulin and phosphorylates it in vitro. Pk9.7 expression coincides with the switch from maternal to zygotic control of the cell cycle, and with the switch from microtubule independence to microtubule dependence. Our results suggest that Pk9.7 plays a role in these processes.