Association of the major ganglioside in sea urchin eggs with yolk lipoproteins

Abstract

The predominant ganglioside in sea urchin eggs, M5 (NeuGcα2-6Glcβ1-1'Cer), exists mainly in the cytoplasm before and after fertilization. Using immunofluorescence microscopy, M5 ganglioside has been shown to localize in the network of cortical endoplasmic reticulum and unidentified vesicles in the eggs. In this study, the subcellular localization of M5 ganglioside in unfertilized eggs was further examined by immunoelectronmicroscopy using preembedding and postembedding labeling techniques. In order to prevent lipid extraction during dehydration and embedding, fixed eggs were subjected to rapid-freezing and freeze-substitution, and then embedded in hydrophilic resin. When ultrathin sections were treated with the anti-M5 ganglioside monoclonal antibody, the labeling of M5 ganglioside localized in the endoplasmic reticula, yolk granules, and microvilli. M5 ganglioside in yolk granules was not restricted to the membrane, suggesting that M5 ganglioside associated with soluble proteins or components in yolk granules. Indeed, gel filtration of the soluble fraction of the yolk lysate showed that M5 ganglioside was coeluted with yolk lipoproteins which were one of the main components in yolk granules. Moreover, M5 ganglioside was extracted from the electrophoretically purified yolk lipoproteins. These results demonstrate that M5 ganglioside in yolk granules associate with yolk lipoproteins. These data suggest that much of M5 ganglioside may be taken up into growing oocytes during oogenesis and transported from yolk granules to other cellular components during embryogenesis. M5 ganglioside associating with yolk lipoproteins in yolk granules may be a significant stored material to be utilized for early embryogenesis.