Structure and thermal stability of monomeric bacteriorhodopsin in mixed pospholipid/detergent micelles

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Abstract
Thermal unfolding experiments on bacteriorhodopsin in mixed Phospholipid/detergent micelles were performed. Bacteriorhodopsin was extracted from the purple membrane in a denatured state and then renatured in the micellar system. The purpose of this study was to compare the changes, if any, in the structure and stability of a membrane protein that has folded in a nonnative environment with results obtained on the native system, i.e., the purple membrane. The purple membrane crystalline lattice is an added factor that may influence the structural stability of bacteriorhodopsin. Micelles containing bacteriorhodopsin are uniformly sized disks 105 ± 13 Å in diameter (by electron microscopy) and have an estimated molecular mass of 210 kDa (by gel filtration HPLC). The near-UV CD spectra (which is indicative of tertiary structure) for micellar bacteriorhodopsin and the purple membrane are very similar. In the visible CD region of retinal absorption, the double band seen in the spectrum of the purple membrane is replaced with a broad positive band for micellar bacteriorhodopsin, indicating that in micelles, bacteriorhodopsin is monomeric. The plot of denaturational temperature vs. pH for micellar bacteriorhodopsin is displaced downward on the temperature axis, illustrating the lower thermal stability of micellar bacteriorhodopsin when compared to the purple membrane at the same pH. Even though micellar bacteriorhodopsin is less stable, similar changes in response to pH and temperature are seen in the visible absorption spectra of micellar bacteriorhodopsin and the purple membrane. This demonstrates that changes in the protonation state or temperature have a similar affect on the local environment of the chromophore and the protein conformation. We conclude that the tertiary structure of the bacteriorhodopsin monomer is essentially the same in micelles and the purple membrane. On the other hand, in the synthetic mixed micelle system, the packing between the nonnative amphiphiles and bacteriorhodopsin is probably not optimal, protein-protein interactions have been lost, and thehelical packing may be looser because the crystalline lattice is absent. Itis likely that a combination of these effects leads to the decreased stability of micellar bacteriorhodopsin.