Properties of a second sensory receptor protein in Halobacterium halobium phototaxis

A second slow‐cycling retinylidene protein, in addition to slow‐cycling (sensory) rhodopsin (SR), can be bleached with hydroxylamine and regenerated with all‐trans retinal in photosensory signaling Halobacterium halobium membranes. Flash photolysis shows this protein undergoes a photochemical reaction cycle characterized by photoconversion of its ground state (λmax 480 nm) to a species with λmax ⩽ 360 nm, which thermally regenerates the 480‐nm species with a t½ of 260 msec at 25°C, under conditions in which SR photocycles at 650 msec in the same membranes. Mutants characterized with respect to their phototaxis behavior are identified which contain SR and the 480‐nm pigment, the latter ranging from undetectable to a concentration equal to that of SR. Receptor mutants lacking all phototaxis sensitivity lack both of the photochemically reactive proteins. The mutant properties contribute to an accumulation of behavioral and spectroscopic evidence that the 480‐nm pigment is a second sensory photoreceptor in H. halobium. NaDodSO4‐polyacrylamide gel electrophoresis of [3H]retinal‐labeled membrane proteins from the mutants indicates SR and the 480‐nm pigment contain distinct chromophoric polypeptides differing in their migration rates. The data implicate polypeptides of 25,000 Mr and 23,000 Mr as retinal‐binding polypeptides of SR and the 480‐nm protein, respectively.