Relationship between the reconstituted vesicle size and the transmembrane protein-to-lipid ratio

The lipid vesicles have been reconstituted with the transmembrane proteins, which are the trimer of the seven-transmembrane (7TM) proteins, by extracting the detergents from the protein/lipid/detergent mixture using detergent-adsorbent resin beads. It has been found that the size of the reconstituted vesicles significantly depends on the molar ratio of lipid to the protein (RL). 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1-palmitoyl-2-oleoyl-sn- glycero-3-phosphocholine (POPC) were used for the reconstitution. The size distribution of the reconstituted vesicles based on DMPC was narrowed by the insertion of 7TM proteins, suggesting that the protein insertion promotes the formation of unilamellar lipid bilayer rather than multilayered assemblies or undefined aggregates. In the case of POPC, the diameter of the reconstituted vesicles depended discreetly on the RL. Upon increase in the RL from 700 to 800, the average diameter jumped ca. three times larger. By assuming that this enlargement is driven by a phase separation between POPC and the 7TN proteins, the interaction energy between the inserted protein and the POPC bilayer was estimated.