Although it has been demonstrated that membrane proteins (MPs) require lipids to ensure their structural and functional integrity, details on how lipid-MP interactions regulate MPs are still unclear. Recently, we developed a concise method for quantitatively evaluating lipid-MP interactions and applied it to bacteriorhodopsin (bR), a halobacterial MP that forms trimers and acts as a light-driven proton pump. Consequently, we found that the halobacterial glycolipid, S-TGA-1, has the highest affinity for bR, among other lipids. In this study, we examined the effects of S-TGA-1 on bR via visible circular dichroism spectroscopy, flash photolysis, and proton influx measurement. The results showed that S-TGA-1 efficiently promotes trimer formation, photocycle, and proton pumping in bR. Our data also suggested that the bR photocycle is restored as a consequence of the trimerization induced by the lipid. This study demonstrates clearly that lipids specifically interacting with MPs can have significant impacts on MP structure and/or function. The methodology adopted in our studies can be applied to other MPs and will help elucidate the physiological functions of lipids in terms of lipid-MP interactions, thus accelerating "lipid chemical biology" studies.
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