Association of calmodulin with nuclear structures in starfish oocytes and its role in the resumption of meiosis.

Abstract

The resumption of meiosis in prophase-arrested starfish oocytes is induced by the hormone 1-methyladenine, which has been shown previously to induce a calcium transient in the nucleus which at this stage is called the germinal vesicle. This transient precedes the breakdown of the germinal vesicle (GVBD). Experiments were performed to establish whether nuclear calmodulin (CaM) was involved in the progression of the meiotic cycle. CaM antagonists, antibodies, and an inhibitory peptide corresponding to the CaM-binding domain of myosin-light-chain kinase have been injected into the nucleus of prophase-arrested starfish oocytes. The antagonists failed to affect the final response to 1-methyladenine, i.e. GVBD, although two antagonists delayed it, whereas the peptide inhibitor and the antibodies completely inhibited it. The antibodies suppressed the nuclear Ca2+ spikes that were shown by previous work to be induced by the photoreleasing of caged adenosine 3',5'-(cyclic)diphosphate ribose in the germinal vesicle. Immunofluorescence staining of isolated starfish oocyte nuclei with CaM antibodies showed CaM in the envelope and in the nucleolus. Immunogold labelling of oocytes revealed aggregates of CaM and of a 36-kDa protein, of the heterogeneous ribonucleoprotein particles (hnRNP), in electron-dense hnRNP in the nuclear matrix. 1-Methyladenine induced the disappearance of these hnRNP from the nucleoplasm and the translocation of CaM and the 36-kDa protein previously associated with them to the cytoplasm, prior to the breakdown of the nuclear envelope.