Lipid Modulation of Dynamics of a Seven-Helical Transmembrane Protein, Proteorhodopsin
Membrane proteins which comprise approximately a third of all proteins are classified for their roles in specific cell signalling, catalysis of metabolic reactions and transport of ions and molecules. One specific membrane protein, called proteorhodopsin (PR) belongs to the family of microbial rhodopsins and functions as a light-driven proton pump. Its lysine residue (Lys231) on helix G forms a Schiff base (C=N) with retinal, its chromophore which photo-isomerizes from the all-trans to the 13-cis form. Photo-isomerization initiates a photocycle, with distinct intermediates (K, M, N, and O). This study tries to emphasize the importance of interactions occurring between the membrane bilayer and PR by examining the kinetics of its photocycle and structure of the retinal chromophore using time-resolved spectroscopy in the visible range and static Raman spectroscopy. Some of the parameters of the membrane that were found to be important include protein to lipid ratio, bilayer thickness, bilayer fluidity and surface charge. The main conclusion is that PR has a very fast photocycle in negatively charged membranes, but a slower photocycle in positively charged ones, as well as in more rigid, thicker membranes. These slower cycles can originate from 1) suppression of conformational changes by the rigid bilayer or dehydration; 2) lack of available protons due to surface charge and 3) impeded isomerization.