MAP AND CDC2 KINASE-ACTIVITIES AT GERMINAL VESICLE BREAKDOWN IN CHAETOPTERUS

In frog oocytes, activation of mitogen-activated protein kinase (MAPK, ERK) leads to activation of cdc2 and germinal vesicle breakdown (GVBD ). By contrast, in starfish, MAPK is activated after GVBD. Here we hav e examined the relative involvements of MAPK and cdc2 in GVBD of Chaet opterus oocytes. MAPK was rapidly tyrosine-phosphorylated and activate d (within 1-2 min) in response to exposure of the oocytes either to na tural seawater (the normal trigger of GVBD in this organism) or to the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate (T PA), which can also elicit GVBD. This response preceded the tyrosine d ephosphorylation and activation of cdc2 by several minutes. MAPK phosp horylation and activation were transient, lasting only until GVBD occu rred and the spindle migrated to the cortex. The enzyme was not phosph orylated again as a result of egg activation. These results are consis tent with the hypothesis that the activation of MAPK has a role in GVB D. However, PD 98059, a potent and selective inhibitor of MEK, the pro tein kinase that phosphorylates and activates MAPK, blocked the phosph orylation of MAPK but did not block GVBD, the dephosphorylation and ac tivation of cdc2, or spindle formation and migration. Oocytes that und erwent GVBD in PD 98059 could be fertilized and cleaved normally. Iono phore A23187, although it caused germinal vesicles to disappear and ca used transient phosphorylation of MAPK, did not cause dephosphorylatio n of cdc2, and therefore this disappearance is artifactual. These resu lts suggest that MAPK activation is neither obligatory nor sufficient for either GVBD or meiotic metaphase arrest in Chaetopterus and that a ctivation of MAPK and cdc2 occur on independent, parallel pathways. (C ) 1997 Academic Press.