Selective requirement for Cdc25C protein synthesis during meiotic progression in porcine oocytes

Fundamental differences between meiosis and mitosis suggest that the shared central cell cycle machinery may be regulated differently during the two d ivision cycles. This paper focuses on unique features of Cdc25C protein fun ction during meiotic progression. We report on the existence of oocyte-spec ific CDC25C transcripts that differ from their somatic counterparts in the 3' untranslated region. While CDC25C mRNA levels remain constant in fully-g rown oocytes, corresponding protein levels increase progressively during ma turation to a maximum at metaphase II. Elevation of Cdc25C protein levels i n G2-oocytes by mRNA injection failed to increase MPF-kinase levels or to i nduce premature entry into M-phase, Likewise, antisense-induced arrest of t ranslation (translational arrest) had no effect on chromosome condensation, nucleolar disassembly, or nuclear membrane contraction. By contrast, trans lational arrest inhibited subsequent events including membrane disassembly and spindle formation. Neither up- nor down-regulation of Cdc25C synthesis after metaphase I plate formation influenced progression to metaphase II. H owever, translational arrest during metaphase resulted in incomplete chromo some decondensation and abnormal pronuclear membrane assembly after activat ion. We conclude that Cdc25 protein, translated from unique transcripts, is preferentially located in the oocyte nucleus and is essential for progress through late diakinesis, Subsequently, new synthesis of Cdc25C protein is required for the orderly transition from meiotic to mitotic cell division.