Regulation of M-phase promoting factor (MPF) in Nereis virens oocytes during meiotic maturation

Oocyte maturation involves the reinitiation of meiosis which is itself driven by the activation of M-phase promoting factor (MPF), We describe the regulation of MPF activity, particularly of the Cdk1 subunit, during the prophase to metaphase I transition in the oocytes of the polychaete Nereis virens. Cdk1/cyclin B complexes were purified from oocytes by affinity chromatography. The recovered protein samples were run on SDS PAGE and following transfer by Western blotting, the proteins were cross-reacted with antibodies. The phosphorylation state of Cdk1 subunit was identified by both observation of changes in electrophoretic mobility and crossreactivity with various antibodies. As with other organisms, entry into M-phase in N. virens oocytes was accompanied by the dephosphorylation of Cdk1 and phosphorylation of cyclin B and an increase in histone H1 kinase activity. In all other higher eukaryotes, the precursor of MPF is maintained inactive by the phosphorylation of both threonine 14 and tyrosine 15 on the Cdk1 subunit. Our results suggest that pre-MPF in N, virens oocytes contains tyrosine-only phosphorylated Cdk1. The only other organism in which this has been recorded is yeast.