Isolation and partial characterization of the plasma membrane of the sea urchin egg
The cell surface complex (Detering et al., 1977, J. Cell Biol. 75, 899- 914) of the sea urchin egg consists of two subcellular organelles: the plasma membrane, containing associated peripheral proteins and the vitelline layer, and the cortical vesicles. We have now developed a method of isolating the plasma membrane from this complex and have undertaken its biochemical characterization. Enzymatic assays of the cell surface complex revealed the presence of a plasma membrane marker enzyme, ouabain-sensitive Na+/K+ ATPase, as well as two cortical granule markers, proteoesterase and ovoperoxidase. After separation from the cortical vesicles and purification on a sucrose gradient, the purified plasma membranes are recovered as large sheets devoid of cortical vesicles. The purified plasma membranes are highly enriched in the Na+/K+ ATPase but contain only very low levels of the proteoesterase and ovoperoxidase. Ultrastructurally, the purified plasma membrane is characterized as large sheets containing a "fluffy" proteinaceous layer on the external surface, which probably represent peripheral proteins, including remnants of the vitelline layer. Extraction of these membranes with Kl removes these peripheral proteins and causes the membrane sheets to vesiculate. Polyacrylamide gel electrophoresis of the cell surface complex, plasma membranes, and Kl- extracted membranes indicates that the plasma membrane contains five to six major proteins species, as well as a large number of minor species, that are not extractable with Kl. The vitelline layer and other peripheral membrane components account for a large proportion of the membrane-associated protein and are represented by at least six to seven polypeptide components. The phospholipid composition of the Kl- extracted membranes is unique, being very rich in phosphatidylethanolamine and phosphatidylinositol. Cholesterol was found to be a major component of the plasma membrane. Before Kl extraction, the purified plasma membranes retain the same species- specific sperm binding property that is found in the intact egg. This observation indicates that the sperm receptor mechanisms remain functional in the isolated, cortical vesicle-free membrane preparation.