Protein redistribution in model membranes: clearing of M13 coat protein from calcium-induced gel-phase regions in phosphatidylserine/phosphatidylcholine multilamellar vesicles.

A model system of M13 bacteriophage coat protein reconstituted into multilamellar vesicles composed of phosphatidylserine (PS) and phosphatidylcholine (PC) is used to examine protein redistribution in membranes in the presence of Ca2+. The reconstitution procedure is analyzed by using protease digestion and gel permeation chromatography of radioactively labeled coat protein and is found to incorporate coat protein into lipid vesicles predominantly in the in vivo orientation and without aggregation. Quenching of protein tryptophanyl fluorescence by spin-labeled PC is used to determine the local lipid environment of the coat protein in binary lipid mixtures. The distribution of coat protein between fluid liquid-crystal (LC) and Ca2+-induced gel (G) phases in PS/PC multilayers, expressed as a concentration ratio RLC/G, is found to be 25 +/- 5 in favor of the fluid phase, indicating significant clearing of membrane protein from Ca2+-induced gel-phase regions.