Behaviour of bacteriorhodopsin incorporated into lipid vesicles after solubilization with different detergents

Incorporation of bacteriorhodopsin (BR), derived from the purple membrane by solubilization with detergents, into phosphatidylcholine-(PC-) vesicles resulted in a disperse distribution of the protein particles in the lipid bilayer as revealed by freeze-fracture electron microscopy. Rotational diffusion measurements showed a high rotational mobility when the system is kept above the lipid phase transition temperature (Tc). Below the Tc the particles are immobilized as observed by flash photolysis while in electron micrographs, segregation of the protein from the lipids and particle aggregation can be seen. In some cases a hexagonal lattice is observed. Solubilization of the purple membrane with LDAO (lauryl-dimethyl-amine oxide) followed by centrifugation through a sucrose density gradient resulted in BR depleted by 80% of bacterial lipids. Incorporation of this lipid-depleted BR into PC-vesicles did not reveal any significant differences in its mobility of its aggregation behaviour. Incorporation of cholesterol into the vesicle system resulted in a marked immobilization of BR. Freeze-fracture electron micrographs show typical segregation phenomena above the Tc of the pure lipid. In the sample with 30 mole% cholesterol, aggregation of the protein appears to be accompanied by lattice formation.