CALMODULIN-DEPENDENT PROTEIN KINASE-II MEDIATES INACTIVATION OF MPF AND CSF UPON FERTILIZATION OF XENOPUS EGGS

IN vertebrates, unfertilized eggs are arrested at second meiotic metap hase by a cytostatic factor (CSF)1, an essential component of which is the product of the c-mos proto-oncogene2. CSF prevents ubiquitin-depe ndent degradation of mitotic cyclins and thus inactivation of the M ph ase-promoting factor (MPF)3,4. Fertilization or parthenogenetic activa tion triggers a transient increase in the cytoplasmic free Ca2+ (revie wed in refs 5 and 6), inactivates both CSF and MPF, and releases eggs from meiotic metaphase arrest7-10. A calmodulin-dependent process is r equired for cyclin degradation to occur in cell-free extracts prepared from metaphase II-arrested eggs (CSF extracts) when the free Ca2+ con centration is transiently raised in the physiological micromolar range 10. Here we show that when a constitutively active mutant of calmoduli n-dependent protein kinase II (CaM K(II)) is added to a CSF extract, c yclin degradation and Cdc2 kinase inactivation occur even in the absen ce of Ca2+, and the extract loses its ability to cause metaphase arres t when transferred into embryos. Furthermore, specific inhibitors of C aM K(II) prevent cyclin degradation after calcium addition. Finally, t he direct microinjection of constitutively active CaM K(II) into unfer tilized eggs inactivates Cdc2 kinase and CSF, even in the absence of a Ca2+ transient. The target for Ca2+-calmodulin is thus CaM K(II).