Cortex and plasma membrane proteins of Xenopus laevis oocytes

The disposition of proteins in the plasma membrane and cortex of metaphase-arrested, shed oocytes of Xenopus laevis has been studied. Extraction at low ionic strengths or high pH reduces the number of identifiable bands seen on SDS-polyacrylamide gel electropherograms. After exposure to high pH the number of bands decreases approximately one half from about 25. Bands III (MW 115ooo) and VII2 (MW 27ooo) are most pronounced amongst the surviving bands. Band VII2 can be labeled by the non-penetrating, externally applied H2DIDS and is resistant to proteolytic attack by papain, pronase, chymotrypsin and trypsin. Band III is rapidly digested if the enzymes have access to the inner membrane surface. The behaviour of bands IV1 and VII2 suggests that they span the lipid bilayer. All extractable proteins, notably those with molecular weights above 120ooo can be easily digested by the proteolytic enzymes mentioned above, again provided that they are added to isolated membranes rather than to intact oocytes. Hence these proteins seem to be attached to the inner membrane surface. Bands V3-4 (MW approximately 55ooo) and VI1-4 (MW approximately 35ooo) are digested only slowly, if at all, by external proteases and can be labeled by external 3H2DIDS. This indicates that they are, in contrast to the majority of the other extractable proteins, exposed to the outer membrane surface.