SURFACE-POTENTIAL REGULATION OF PHOSPHOLIPID-COMPOSITION AND IN-OUT TRANSLOCATION IN YEAST

In yeast cells the anionic phospholipids, phosphatidylinositol and phosphatidylserine, determine to a large extent the magnitude of the negative surface charge density (sigma) [Cerbon, J. and Calderon, V. (1990) Biochim. Biophys. Acta 1028, 261-267]. We now report further findings. (a) When the yeast phi(out) was reduced by increasing the concentration of monovalent (C+) or divalent (C2+) cations in the culture medium, the relative amount of anionic phospholipids increased (45-52%). (b) For each such increment, a corresponding increase in the external surface charge density (a) was found, due to the translocation from the cytoplasmic side to the exoplasmic side of the plasma membrane. (c) These changes were reversed when the phi(out) was increased by reducing the concentration of cations in the culture medium. (d) When the phi(out) was reduced and phosphatidylserine decarboxylation or phosphatidylinositol degradation were inhibited, to measure synthesis of anionic phospholipids, a 1.4 times further increase in the anionic/zwitterionic phospholipid ratio occurred. As a consequence, a similar increase in the external surface charge (sigma) was found. (e) Under all the conditions studied, the percentage of anionic phospholipid at the external surface of the plasma membrane calculated from the a values was 2.3-3.0 times less than that in the cells, indicating that the asymmetric composition (more inside) was maintained. A model for the regulation of the anionic phospholipid composition of the yeast membranes is proposed.