On Membrane Phospholipids and Protein-lipid Association

Membranes of blood cells have an asymmetric bilayer distribution of phospholipid classes. In both erythrocytes and platelet plasma membranes the choline containing phospholipids (sphingomyelin and phosphatidylcholine) are preferentially located in the outer half of the membrane, whereas a large fraction of phosphatidylethanolamine and nearly all phosphatidylserine are confined to the cytoplasmic surface. The rationale of this phospholipid topology appears to be related to the function of phospholipids in blood coagulation. An abnormal phospholipid distribution in sickle cells appears to be accompanied by a clot-promoting activity. Native phosphatidylcholine molecules of erythrocytes can be replaced by defined synthetic species during incubation of intact cells with artificial phospholipid bilayers in the presence of a specific phosphatidylcholine transfer protein. All of the phosphatidylcholine present in the outer layer can be replaced by (1-palmitoyl-2-oleoyl)phosphatidylcholine without causing significant changes in membrane stability and permeability. However, an induced increase of disaturated species or introduction of dipoly-unsaturated phosphatidylcholine molecules modified drastically various membrane properties. This approach appears to give new information about the relation between chemical structure and membrane function of phospholipids. Earlier studies indicated that a phosphatidylcholine transfer protein contains a specific phospholipid binding site. The primary structure of the protein was elucidated and based on the predicted secondary structural elements a tentative folding model of the protein was proposed. Experiments with photoactivable phospholipid analogs allowed to identify the amino acid residues of the lipid binding site and gave information about the localization of the phosphatidylcholine molecule within the transfer protein. PHOSPHOLIPID ASYMMETRY OF BLOOD CELLS The erythrocyte membrane is probably one of the most widely studied biological membranes and has served in many respects as a stimulating model (Ref. 1). The total lipid content of the erythrocyte constitutes about 40% of its dry weight, and in molecular ratios consists of about equal parts of cholesterol (40%) and polar lipids, e.g. phospholipids (50%) and glycolipids (10%). The major phospholipids are sphingomyelin, phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine (Fig. 1). The sum of the two cholinecontaining phospholipids (sphingomyelin and phosphatidylcholine) is always 50-60% of the total phospholipid fraction, but their relative contents vary considerably in erythrocytes from different animal species (Ref. 2). The most extreme situation is found in erythrocytes from ruminants in which most, if not all, of the choline-phospholipids appears to be sphingomyelin. During the past decade several laboratories have established the topological distribution of these phospholipid classes in the erythrocyte membrane. It is generally accepted that in this membrane the lipids are arranged in a bimolecular leaflet to which peripheral proteins are attached via polar interactions and which is interrupted to allow hydrophobic interactions with integral membrane proteins embedded in or spanning the Filayer. The distribution