PROTEIN-DEPENDENT TRANSLOCATION OF AMINOPHOSPHOLIPIDS AND ASYMMETRIC TRANSBILAYER DISTRIBUTION OF PHOSPHOLIPIDS IN THE PLASMA-MEMBRANE OF RAM SPERM CELLS

We have investigated the transbilayer movement of phospholipids in the plasma membrane of ram sperm cells using spin- and fluorescence-label ed lipid analogues. After incorporation into the outer leaflet, phosph atidylcholine (PC) and sphingomyelin (SM) moved slowly to the inner cy toplasmic leaflet, whereas phosphatidylserine (PS) and phosphatidyleth anolamine (PE) rapidly disappeared from the exoplasmic monolayer. Vari ation of the initial velocity of the relocation kinetics vs the amount of analogue incorporated into the membrane suggests a saturability of the transbilayer movement of aminophospholipids. ATP depletion or pre treatment with N-ethylmaleimide of ram sperm cells reduced the fast in ward motion of PS and PE, indicating a protein-mediated aminophospholi pid translocation. The results suggest for the plasma membrane of ram sperm cells the presence of an aminophospholipid translocase and an as ymmetric transversal lipid distribution with aminophospholipids prefer entially located in the inner leaflet and choline-containing phospholi pids in the outer leaflet. The relevance of the transversal segregatio n of phospholipids for membrane fusion processes occurring during fert ilization is discussed.