Isolation and characterization of paracrystalline arrays of the plasma membrane of baker's yeast Saccharomyces cerevisiae.

Yeast plasma membranes were isolated from homogenized cells and analyzed by SDS-PAGE. Two glycoproteins of 160 000 and 240 000 molecular weight were found, both of which exhibited invertase activity (EC 3.2.1.26). By density gradient centrifugation a heavy membrane fraction which consisted of the glycoproteins and two hydrophobic proteins was isolated. Antibody labeling of protoplasts revealed a good correlation between the distribution of binding sites of the antibodies against the heavy fraction and the distribution of the intramembranous particles. The cytoplasmic surface of the yeast plasma membrane was visualized by freeze drying and subsequent platinum/carbon shadowing of membrane vesicles adsorbed to cationized glass and squirted with a hypotonic buffer stream. In contrast to the smooth exoplasmic surface the cytoplasmic surface showed paracrystalline arrays of particles which resembled in size, number and lattice constant the intramembranous particles. Removal of the adsorbed paracrystalline arrays and subsequent SDS-PAGE revealed the same protein pattern as the heavy membrane fraction. It can therefore be concluded that the glycoproteins which show invertase activity and the two hydrophobic proteins are the major components of the paracrystalline arrays. It is proposed that the glucose level of the nutrient medium influences the appearance and disappearance of the paracrystalline arrays, which consist mainly of invertase, because synthesis of invertase is inhibited by glucose levels higher than 1%.