ASSOCIATION OF CALMODULIN WITH NUCLEAR-STRUCTURES IN STARFISH OOCYTESAND ITS ROLE IN THE RESUMPTION OF MEIOSIS

The resumption of meiosis in prophase-arrested starfish oocytes is ind uced by the hormone 1-methyladenine, which has been shown previously t o induce a calcium transient in the nucleus which at this stage is cal led the germinal vesicle. This transient precedes the breakdown of the germinal vesicle (GVBD), Experiments were performed to establish whet her nuclear calmodulin (CaM) was involved in the progression of the me iotic cycle. CaM antagonists, antibodies, and an inhibitory peptide co rresponding to the CaM-binding domain of myosin-light-chain kinase hav e been injected into the nucleus of prophase arrested starfish oocytes . The antagonists failed to affect the final response to 1-methyladeni ne, i.e. GVBD, although two antagonists delayed it, whereas the peptid e 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)dip hosphate ribose in the germinal vesicle. Immunofluorescence staining o f isolated starfish oocyte nuclei with CaM antibodies showed CaM in th e envelope and in the nucleolus. Immunogold labelling of oocytes revea led aggregates of CaM and of a 36-kDa protein, of the heterogeneous ri bonucleoprotein particles (hnRNP), in electron-dense hnRNP in the nucl ear matrix. 1-Methyladenine induced the disappearance of these hnRNP f rom the nucleoplasm and the translocation of CaM and tile 36-kDa prote in previously associated with them to the cytoplasm, prior to the brea kdown of the nuclear envelope.