The 9-methyl group of 11-cis retinal is important in the efficient formation of the active conformation of rhodopsin, Meta II. Here, Tyr191 rhodopsin mutants were generated because of its proximity to that methyl group in the dark structure. If photoactivation results in movement of the 9-methyl group toward Tyr191, the steric interactions involved with activation and/or deactivation might not be as tightly coupled in mutant proteins with smaller amino acids at that position. Tyr191 mutations have no effect on the dark pigment. However, after photobleaching, the lifetime of Meta II is shorter; Meta II decays quickly into two inactive species: (1) a Meta III or Meta III-like species and (2) opsin and free retinal. The Meta III-like fraction maintains the covalent Schiff base linkage of the chromophore much longer than the wild type. On the other hand, the fast chromophore release is similar to cone pigments. Taken together, the data suggest that the role of the 9-methyl group after photoisomerization is not only to form Meta II efficiently, but also to maintain its active conformation and allow for the timely hydrolysis of the Schiff base. Perturbation of this interaction effects changes in the hydrolysis of the Schiff base and for the case of the Y191A mutation the folded structure of the protein after photobleaching.
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Vol. 82 • No. 6