Conformational changes in bacteriorhodopsin associated with protein-protein interactions: a functional alpha I-alpha II helix switch?

Fourier transform infrared spectra of bacteriorhodopsin samples were obtained in conditions in which the aggregation state of the protein (i.e., monomeric or trimeric) was modified by different treatments. Two approaches were followed: (1) renaturation of bacteriorhodopsin starting from bacterioopsin dissolved in SDS and (2) reconstitution of bacterioopsin in Halobacterium lipid liposomes at two different lipid/protein ratios. Concomitant with the gradual recovery of the native interactions between transmembrane helices, we observed clear and gradual changes in the infrared absorption spectra in the amide I band and also in the band at 1741 cm-1. These processes were found to be compatible with the two-state oligomerization model. The whole set of experiments shows that the band at 1665 cm-1 in the deconvoluted spectra appears only when monomers interact forming trimers, even when the lattice is not present. This implies that the trimeric organization of bacteriorhodopsin is responsible for the unique features described in the amide I of purple membrane. The spectroscopic changes detected can be attributed to changes in secondary structure compatible with the interconversion of alpha I and alpha II helices. However, the exact nature and functional relevance of these changes is still unknown.