Circular-dichroism analyses of membrane proteins: examination of environmental effects on bacteriorhodopsin spectra

The secondary structure of bacteriorhodopsin is known from electron-diffraction studies, making bacteriorhodopsin a useful test system for analysing environmental influences on membrane proteins using c.d. spectroscopy. The conformational effects of detergent solubilization and incorporation into vesicles of various types were determined by comparison of the calculated secondary structures derived from c.d. spectra with the structure determined from diffraction studies. In addition, two modified forms of the native purple membrane, a shrunken form of the hexagonal lattice and an orthorhombic lattice form, were used to determine the effects of varying membrane fragment size and protein concentration within the membranes. The two different vesicle incorporation procedures yielded bacteriorhodopsin spectra which were nearly identical with each other and very close to the structure calculated from electron-diffraction studies. Solubilization of the native protein in the non-ionic detergent n-octyl glucoside, without subsequent vesicle incorporation, resulted in a significantly altered protein conformation. Organizing the protein in different membrane lattices produced even more apparent deviations from the secondary structure determined by diffraction studies, as a consequence of optical effects caused by the high protein concentrations in the lattices. These studies show the importance of maintaining a 'native' environment, and the influence of particle geometry in interpreting c.d. studies of membrane proteins.