ENHANCEMENT OF OPSIN ACTIVITY BY ALL-TRANS-RETINAL

The rod cell photoreceptor apoprotein, opsin, activates the G-protein, transducin, although at a much reduced level than light-activated rho dopsin. The ability of all-trans-retinal to enhance opsin apoprotein a ctivity was investigated using a guanyl nucleotide exchange assay on t ransducin. All-trans-retinal enhanced opsin activity in a concentratio n-dependent manner. At high concentrations of all-trans-retinal, the a ctivity of the all-trans-retinal-opsin complex was comparable to that from an equimolar amount of metarhodopsin(II). However, in contrast to metarhodopsin(II), the active all-trans-retinalopsin complex did not require a stable Schiff base linkage between opsin and all-trans-retin al. The lack of a stable Schiff base and differences in activity at hi gh pH imply that opsin and all-trans-retinal. form a complex that is d istinct from metarhodopsin(II). The ability of all-trans-retinal to st imulate the transduction cascade may be a source of post-bleach noise in photoreceptors. (C) 1998 Academic Press Limited.