Bacteriorhodopsin, either as purple membrane sheets or as detergent-solubilized protein, was found to incorporate spontaneously into both large unilamellar vesicles (LUVs) and sized multilamellar vesicles (MLVs) under either gel-phase or liquid-phase conditions. These results were obtained with LUVs of various lipid compositions, including dimyristoylphosphatidylcholine (DMPC), DMPC and cholesterol, dioleoylphosphatidylcholine (DOPC), and DOPC and cholesterol. The lipid to protein (L/P) ratio of all proteoliposomes arising from these preformed vesicles continually increased in the presence of protein-free vesicles. Under fluid-phase conditions, the mixing of LUVs of DMPC with proteoliposomes showed vesicle growth independent of lipid concentration, suggesting that growth was due to lipid transfer. However, under gel-phase conditions a more rapid, concentration-dependent increase in the L/P ratio of the proteoliposome was observed, suggesting that growth occurred by a mechanism other than lipid transfer from vesicles to proteoliposomes. The use of the proteoliposome as an artificial lipid-protein membrane model is discussed.
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