Temperature Dependent Aggregation of Bacteriorhodopsin in Dipalmitoyl‐ and Dimyristoylphosphatidylcholine Vesicles

Abstract

AbstractBacteriorhodopsin has been incorporated into large unilamellar vesicles of dipalmitoyl‐ and dimyristoylphosphatidylcholine. The state of aggregation of bacteriorhodopsin in these vesicles was investigated using X‐ray diffraction, freeze fracture electron microscopy, circular dichroism and rotational diffusion measurements.At temperatures below the lipid phase transition, the X‐ray diffraction results show that bacteriorhodopsin crystallizes into patches with the same hexagonal lattice as is observed in the purple membrane. This hexagonal lattice is clearly visible in the electron micrographs. Measurements of the rotational diffusion of the protein, using the decay of flash‐induced dichroism, show that the protein is completely immobilized. In agreement with these results the circular dichroism spectra exhibit the typical exciton coupling bands of the native lattice.Above the phase transition, the lattice disaggregates and the protein molecules are monomeric provided the lipid to protein ratio is sufficiently high. In the X‐ray diffraction pictures both the protein and the lipid long‐range order are absent. The freeze fracture electron micrographs show that the bacteriorhodopsin particles are randomly distributed in the plane of the lipid bilayer. In harmony with these observations rotational diffusion measurements demonstrate that the bacteriorhodopsin molecules are free to rotate about an axis normal to the plane of the membrane. The rotational correlation time shows a marked dependence on the lipid to protein ratio. The absence of exciton coupling bands in the circular dichroism spectra indicates that specific bacteriorhodopsin aggregates are absent.It seems probable that the mechanism of bacteriorhodopsin crystallisation involves the growth of crystalline lipid areas from which the protein is excluded.