Influence of pH on the MI-MII equilibrium of rhodopsin in recombinant membranes

Rhodopsin in native rod membranes and incorporated into egg phosphatidylcholine (egg PC) vesicles was studied at pH 5 and 7 at 28°C. Rhodopsin function, as monitored by the formation of metarhodopsin II (MII) from metarhodopsin I (MI) following an actinic flash, was found to be largely blocked in egg PC vesicles at pH 7. When the pH was lowered to 5, however, rhodopsin showed essentially equal activity in both native and egg PC membranes. This activity exceeded that found for rhodopsin in native membranes at pH 7. Phospholipid composition is thus shown to directly affect the MI ⇌ MII equilibrium, which in turn is linked to visual function.