A COMPARISON OF THE EFFICIENCY OF G-PROTEIN ACTIVATION BY LIGAND-FREEAND LIGHT-ACTIVATED FORMS OF RHODOPSIN

Activation of the photoreceptor G protein transducin (G(t)) by opsin, the ligand-free form of rhodopsin, was measured using rod outer segmen t membranes with densities of opsin and G(t) similar to those found in rod cells. When GTP gamma S was used as the activating nucleotide, op sin catalyzed transducin activation with an exponential time course wi th a rate constant k(act) on the order of 2 x 10(-3) s(-1). Comparison under these conditions to activation by flash-generated metarhodopsin II (MII) revealed that opsin-and R-catalyzed activation showed simil ar kinetics when Mil was present at a surface density similar to 10(-6 ) lower than that of opsin. Thus, in contrast to some previous reports , we find that the catalytic potency of opsin is only similar to 10(-6 ) that of MII. In the presence of residual retinaldehyde-derived speci es present in membranes treated with hydroxylamine after bleaching, th e apparent k(act) observed was much higher than that for opsin, sugges ting a possible explanation for previous reports of more efficient act ivation by opsin. These results are important for considering the poss ible role of opsin in the diverse phenomena in which it has been sugge sted to play a key role, such as bleaching desensitization and retinal degeneration induced by continuous light or vitamin A deprivation.