Duration and Amplitude of the Light-Induced cGMP Hydrolysis in Vertebrate Photoreceptors Are Regulated by Multiple Phosphorylation of Rhodopsin and by Arrestin Binding

The duration and amplitude of the light-induced cGMP hydrolysis in bovine rod outer segments were investigated using purified rhodopsin in nine different states of phosphorylation in a reconstituted system. Effects of varying amounts of arrestin at all states of rhodopsin phosphorylation were measured. The findings were the following: (1) At low bleaching levels, the activity of phosphodigesterase (PDE) depends strongly on the phosphorylation degree of the light-activated rhodopsin (R*), while at saturating light levels R* of all phosphorylation degrees activates PDE to the same extent. (2) The turnoff time for PDE is markedly shortened if R* is phosphorylated, independent of the number of phosphate groups incorporated into rhodopsin (P/R); i.e., the first phosphate which is bound to R* seems to be responsible for the shortened turnoff time. The lifetime of phosphorylated R* is shown to be dramatically reduced compared to that of unphosphorylated R*, as monitored by the ability of R* to activate PDE. (3) After activation with phosphorylated R*, addition of arrestin caused a further reduction of both the maximum activity and the turnoff time of PDE. Both effects were strongly dependent on (a) the phosphorylation degree of R*, (b) the concentration of arrestin, and (c) the concentration of R*. These results suggest that the light-induced phosphorylation of rhodopsin to different extents and the subsequent binding of arrestin are involved in the light adaptation and in the fine regulation of the light response in vertebrate photoreceptors.