TRANSBILAYER PHOSPHATIDYLETHANOLAMINE MOVEMENTS IN THE YEAST PLASMA-MEMBRANE - EVIDENCE FOR A PROTEIN-MEDIATED, ENERGY-DEPENDENT MECHANISM

Aminophospholipid movements in the plasma membrane of higher eukaryoti c cells seem to be regulated by an ATP-drpendent. protein-mediated pro cess, To examine whether similar mechanisms exist in yeast cells, we h ave analysed phosphatidylethanolamine (PtdEtn) distributions in Saccha romyces cerevisiae (A184D) cells under a variety of conditions, with t rinitrobenzenesulfonic acid and fluorescamine as the external membrane probes. The levels of external PtdEtn in the intact cells were reduce d to about 50% by pretreatment of the cells with inhibitors of mitocho ndrial ATP synthesis, ATPase inhibitors or protein-sulfhydryl-group-mo difying reagents, or by depletion of the cells of ATP by metabolic sta rvation. The levels of external PtdEtn could be restored to normal by repletion of the energy-depleted cells with ATP. Furthermore, treatmen t of the energy-depleted cells with sulfhydryl-modyfying reagents did not cause further reduction in the external PtdEtn levels but decrease d the accessibility of PtdEtn to fluorescamine after restoration of th e cellular ATP levels to normal in these cells. These results demonstr ate an involvement of an ATP-dependent, protein-mediated process(es) i n the regulation of the PtdEtn distribution across the plasma-membrane bilayer of yeast cells. The results are discussed with regard to poss ible models that can generate and maintain the transbilayer phospholip id asymmetry in the yeast plasma membrane.