Calorimetric and fluorescence depolarization studies on the lipid phase transition of bacteriorhodopsin--dimyristoylphosphatidylcholine vesicles

Abstract

The thermotropic lipid phase transition of dimyristoylphosphatidylcholine vesicles reconstituted with bacteriorhodopsin was investigated as a function of the lipid to protein ratio by means of differential scanning calorimetry and fluorescence depolarization of the embedded probe 1,6-diphenyl-1,3,5-hexatriene. Two attractive features of this system are that the lipid phase transition induces lipid-protein segregation and that the state of aggregation of the protein is known. Above the lipid phase transition and above molar lipid to protein ratios of about 100, bacteriorhodopsin is monomeric. Well below the phase transition, bacteriorhodopsin is aggregated in a hexagonal protein lattice. With increasing amounts of incorporated bacteriorhodopsin, the calorimetric transition broadens, and a second component develops at a temperature which is lower than that of the unperturbed transition. The latter transition was assigned to the disaggregation of the bacteriorhodpsin lattice which occurs 6-7 degrees C below the phase transition of the protein-free lipids according to previous measurements. The van't Hoff enthalpy of the phase transition, as determined from the fluorescence depolarization of diphenylhexatriene, is in surprisingly good agreement with that obtained from differential scanning calorimetry over a wide range of lipid to protein ratios. The differential scanning calorimetry data can be simulated on the basis of a model which takes the protein segregation and crystallization specifically into account. The essential feature of this model is that calorimetrically detectable lipid melting occurs in the temperature region of the protein crystallization, since for the disaggregation of the close packed bacteriorhodopsin lattice additional lipids of an intermediate chain conformation are required.