Glycophorin, from the human erythrocyte membrane, has been isolated in pure form and reconstituted into unilamellar vesicles with bovine brain phosphatidylserine (PS). Fourier transform infrared spectroscopy has been used to monitor the protein conformation as well as the effect of protein on lipid order and melting. Glycophorin, at levels of 1 mol %, nearly abolishes the gel to liquid-crystal phase transition seen in pure PS vesicles between 8 and 16 °C by inducing significant disorder into the lipid gel phase. A transition of reduced magnitude remains between 14 and 22 °C in the lipid/protein complexes. Evidence is presented for specific interaction of glycophorin with the interfacial region of PS. In general, the effects on lipid melting produced by protein at the 1 mol % level are more pronounced than those noted in a previous study of glycophorin/phosphatidylcholine interactions [Mendelsohn, R., Dluhy, R. A., Taraschi, T., Cameron, D., & Mantsch, H. H. (1981) Biochemistry 20, 6699-6706], Two bands are observed for the protein amide I (C=0 stretching) mode. A main feature at 1653 cm-1 indicates that the bulk of the secondary structure is random coil or α-helical. A weaker shoulder at 1675 cm-1 suggests the occurrence of a small proportion of the /3-sheet form. The results confirm circular dichroism studies of Schulte & Marches! (1979) [Schulte, T. H., & Marchesi, V. T. (1979) Biochemistry 18, 275-280], Fourier transform infrared (FT-IR) studies of a ternary complex of PS/dipalmitoylphosphatidylcholine-d62 (DPPC-d62)/glycophorin indicate that the glycophorin preferentially interacts with the PS component. The melting of the DPPC-d62 and PS components may be separately monitored. DPPC-d62 has a reduced transition width and increased melting temperature in the ternary system (two lipids + protein) compared with the binary lipid mixture. The utility of deuterated phospholipids in FT-IR studies for monitoring the preferential partitioning of proteins in complex lipid environments is demonstrated.
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