Effect of lipid composition on the transfer of sterols mediated by non-specific lipid transfer protein (sterol carrier protein2).

The rate of non-specific lipid transfer protein (nsLTP)-mediated exchange is independent of structure for dissimilar sterols: cholesterol, lanosterol, sitosterol and vitamin D-3. Conversely, the nsLTP-mediated exchange of cholesterol is markedly affected by the phospholipid composition of the donor liposome. Negatively charged phosphatidylglycerols strikingly increase cholesterol exchange in the presence of nsLTP while not altering the exchange in the absence of nsLTP. The presence of unsaturated acyl chains in the phospholipid enhances exchange. Sphingomyelin drastically decreases cholesterol exchange, as does di-O-alkylphosphatidylcholine. Decreased exchange produced by these substitutions can be reversed by addition of phosphatidylcholine. The presence of an acyl group and a negative charge in the phospholipid are critical for the nsLTP-mediated transfer of cholesterol. In addition to these studies on composition of the donor membrane, the charge on the membrane also appears critical. Maximal exchange rates accompany optimization of potential interaction of negatively charged surface and the basic nsLTP. The nsLTP also mediates an approximately equal rate of exchange of cholesterol and phosphatidylcholine. However, approaching equilibrium, only half of the phospholipid can be exchanged while there is exchange of about 90% of cholesterol. Thus, it appears that only the phospholipid in an outer membrane layer may be available whereas cholesterol is fully available. Therefore, in contrast to a 'carrier' model we suggest that nsLTP facilitates exchange by binding to the membranes, and binding is highly dependent upon lipid composition. Once bound, the protein functions as a bridge between membranes, thus, facilitating exchange.