MOLECULAR MECHANISMS IN VISUAL TRANSDUCTION

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Our long-term objectives are to understand the molecular mechanisms of visual transduction and adaptation. This proposal is focused on the molecular mechanism of rhodopsin action. After rhodopsin absorbs a photon it proceeds through a series of spectroscopically defined conformation states. One or more of these conformational states (R*) catalyzes the exchange of GTP for GDP in a disk associated G-protein resulting in the activation of the latter. Similar, if not identical , informational transducing steps occur as a consequences of the interaction of many hormones and drugs with their receptors. The proposal is largely concerned with defining the molecular changes in rhodopsin which enable activated rhodopsin R* to be reached. A major interest here is to establish a structural link between R* and the spectroscopically defined intermediates. This work will be of relevance both to our understanding of vision and the nature of drug-receptor interactions. The fact that Schiff base of rhodopsin can be protonated is of immense importance in the functioning of this protein. The protonated Schiff base of the chromophore, interacting with negatively charge amino acids at the active-site, is thought to be of primary importance in wavelength regulation and energization of rhodopsin. The ultimate deprotonation of the Schiff base is assumed also to be important in the formation of metarhodopsin II, the presumed spectroscopic signature of R*. Along these lines, the active-site lysine of opsin must also be deprotonated to form a Schiff base with 11-cis-retinal. The experiments described here are designed to prove the role of charge and charge movement in the functioning of rhodopsin and, to a much lesser extent, bacteriorhodopsin. The approaches used here are bio-organic chemical and biochemical in nature and will involve modification of the protein along with retinal analog studies. By introducing chemical probes, such as a methyl group, at the active-site lysine we will determine if a full positive charge on this lysine is required for photochemical energy storage, wavelength regulation and the formation of R*. Furthermore the pK of the active-site lysine of rhodopsin will be determined. The amino acid counterions which critically interact with the chromophore will be identified by structural studies designed to use the active-site lysine of opsin to direct a pseudo cross-linking reagent to these counterions. Finally, novel retinal analogs which form pigments with opsin will be used to probe the mechanism of energy storage.