INTERFACIAL PH MODULATION OF MEMBRANE-PROTEIN FUNCTION INVIVO - EFFECT OF ANIONIC PHOSPHOLIPIDS

In yeast cells, the magnitude of the membrane surface potential (psi) is determined to a large extent by the relative amount of anionic phospholipids (Cerbon and Calderon (1990) Biochim. Biophys. Acta 1028, 261-267). When a significant surface potential exists, the pH at the membrane surface (interfacial pH) will be different to that in the bulk suspending medium. We now report that: (1) In cells with higher psi (phosphatidylinositol-rich cells (PI-rich) and phosphatidylserine-rich cells (PS-rich) a 10-times lower proton concentration in the bulk was enough to achieve thc maximum transport activity of H+-linked transport systems when compared to normal cells. (2) When the psi was reduced by increasing thc concentration of cations in the medium, more protons were required to achieve maximum transport, that is, thc pH activity curves shifted downwards to a more acidic pH. (3) The magnitude of the downward pH shift was around 2.5-times higher for the more charged membranes. (4) Around 10-times more KCl than MgCl2 was necessary to give an equivalent pH shift, in agreement with their capacity to reduce the psi of artificial bilayers. The interfacial pH calculated from the values of psi indicates that it was 0.4 pH units lower in the anionic phospholipid rich cells as compared to normal cells. The results indicate that membrane surface potential may explain the complex relationship between pH, ionic strength and membrane protein function. Maximum transport activities were found for glutamate at interfacial pH of 4.2-4.8 and were inhibited at interfacial pH = 3.2-3.4, suggesting that surface groups of the carrier proteins with pK values in the region 3.8-4.2 (aspartyl and glutamyl) are involved in binding and/or release of charged substrates.