Purification and characterization of a cortical secretory vesicle membrane fraction

A membrane fraction has been prepared by sucrose density gradient fractionation of purified cortical secretory vesicles from the eggs of the sea urchin Strongylocentrotus purpuratus. The purified cortical vesicle membrane fraction has a phospholipid to protein ratio of 1.76 and exhibits a morphology typical of biological membranes as seen by electron microscopy. The protein composition of the purified membranes was analyzed by SDS-polyacrylamide gel electrophoresis and shown to be distinct from that of eggs, cell surface complex, cortical vesicles, fertilization product, and yolk platelets. Alkaline extraction (pH 11.0) of peripheral membrane proteins increased the phospholipid to protein ratio to 2.55 and removed several polypeptides. Immunoblot analysis of the isolated cortical vesicle membrane fraction revealed low levels of contamination with two major cortical vesicle content proteins. Fractions enriched in egg plasma membranes and yolk platelet membranes also have been isolated and compared with the cortical vesicle membranes by SDS-polyacrylamide gel electrophoresis. The protein compositions of the three membrane fractions were found to contain very little overlap, indicating that the cortical vesicle membrane preparation is relatively free of contamination from these likely noncortical vesicle sources of membrane. Both the plasma membrane and cortical vesicle membrane samples were found by immunoblotting to contain actin.