PHYSICAL DETERMINANTS OF INTERMEMBRANE PROTEIN TRANSFER

Intermembrane protein transfer between erythrocytes and phospholipid v esicles was examined under a variety of conditions to investigate phys ical factors governing this process. Human erythrocytes were incubated with sonicated dimyristoylphosphatidylcholine vesicles containing tra ce [C-14]dipalmitoylphosphatidylcholine. Protein-vesicle complexes wer e separated from cells and from membrane fragments by density gradient centrifugation. The yield of isolated protein vesicles was determined from the C-14-vesicle marker; protein compositions were analyzed by S DS-polyacrylamide gel electrophoresis. Enzymatic removal of portions o f the cytoplasmic or exoplasmic domains of cell membrane proteins had little effect on the extent of protein transfer. Membrane additives su ch as cholate produced a 2-fold increase in protein-vesicle yield. The selectivity of protein transfer from erythrocytes was influenced by t he lipid composition of recipient vesicles: inclusion of cholesterol i ncreased band 3 content while the presence of anionic phospholipids re duced transfer. Proteins transferred from P-32-labeled cells differed in specific radioactivity from bulk cell proteins: glycophorin, highly phosphorylated in the cell membrane, showed no detectable labeling in the corresponding protein-vesicle band. These observations suggest th at cell-to-vesicle protein transfer is insensitive to bulk steric and electrostatic properties of cell membranes, but enhanced by membrane d efects. Recipient membrane composition influences the selectivity of t ransferred proteins and may reveal subtle differences in the membrane association of protein subpopulations.