Oocytes from the Japanese clam Ruditapes philippinarum are naturally blocked at the prophase-I stage of meiosis. Following physiological activation by the neurohormone serotonin (5HT), oocytes undergo germinal vesicle breakdown (GVBD) and reach a second cell cycle arrest in metaphase-I. To identify the kinases activated during meiosis reinitiation, we used a phosphorylation assay following sodium dodecyl sulphate-polyacrylamide gel electrophoresis and in situ renaturation. A soluble 85-kDa serine/threonine kinase (PK85) was highly and consistently activated (up to 17-fold) within 5 minutes following addition of the hormone. This activation occurred 5 to 10 minutes before GVBD and only when 5HT concentration was sufficient to induce meiosis reinitiation. The calcium ionophore A23187 and NH4Cl, two compounds known to induce GVBD by increasing intracellular calcium concentration, also activate PK85. In crude oocyte extracts, the presence of beta-glycerophosphate, NaF, okadaic acid, calyculin A or microcystin, prevented inactivation of PK85, suggesting that it is activated by phosphorylation. Partial purification of PK85 followed by Western blotting showed that this kinase is related to the ribosomal S6 kinase pp90rsk. PK85 phosphorylates the peptides LRRASLG (kemptide) and PLARTLSVAGLPGGK (syntide-2), and to a lesser extent the synthetic polyamino acids poly(R3:S1) while myelin basic protein (MBP), histone III-S, casein, the peptides pEKRPSQRSKYL ((pGlu4)-MBP 4-14), GTFRASIRRLAARRR (NIMA kinase substrate), the protein kinase C (PKC) substrate LRTLRR and the synthetic polyaminoacids poly(R1:P1:T1) were poor substrates. 5HT-induced GVBD and PK85 activation are both inhibited by the phorbol ester 12-myristate 13-acetate (PMA) and this inhibition can be reversed by 5 microM of the bisindolyl-maleimide GF109203X, a potent PKC inhibitor. PMA inhibitory action appears to take place between 5HT binding to its receptor and the intracellular calcium surge since it has no effect on GVBD induced by calcium ionophore A23187 and thapsigargin. Taken together, these results suggest that serotonin-induced activation of PK85 occurs after the intracellular calcium surge in a PKC-independent pathway.