Phosphodiesterase activation by photoexcited XXXX is quenched when @entity8695 is phosphorylated and binds the intrinsic 48-kDa protein of rod outer segments.

@entity6708 @entity3007 @entity6370 @entity8695 @entity855
Each photoexcited @entity8695 (R*) molecule catalyzes binding of @entity3007 to many copies of the @entity6370 -binding protein transducin, which, in its @entity3007 -binding form, then activates @entity6708 phosphodiesterase (PDEase). Subsequent deactivation of this light-activated enzyme cascade involves hydrolysis of the @entity3007 bound to transducin, as well as decay of the activating capacity of R*. We report here that deactivation of PDEase in rod outer segment suspensions is highly enhanced by addition of @entity855 and purified 48-kDa protein, which is an intrinsic rod outer segment protein that is soluble in the dark but binds to photolyzed @entity8695 that has been phosphorylated by @entity8695 kinase and @entity855 [K  hn, H., Hall, S.W. _ Wilden, U. (1984) FEBS Lett. 176, 473-478]. To analyze the mechanism by which @entity855 and 48-kDa protein deactivate PDEase, we used an @entity855 -free system consisting of thoroughly washed disk membranes, whose @entity8695 had been previously phosphorylated and chromophore-regenerated, and to which purified PDEase and transducin were reassociated. Such phosphorylated membranes exhibited a significantly lower (by a factor less than or equal to 5) light-induced PDEase-activating capacity than unphosphorylated controls. Addition of purified 48-kDa protein to phosphorylated membranes further suppressed their PDEase-activating capacity; suppression could be as high as 98% (as compared to unphosphorylated membranes), depending on the amount of 48-kDa protein and the flash intensity. Addition of @entity855 had little further effect. In contrast, PDEase activation or deactivation with unphosphorylated control membranes was not influenced by 48-kDa protein, even in the presence of @entity855 , provided @entity8695 kinase was absent. Our data suggest that 48-kDa protein binds to phosphorylated R* and thereby quenches its capacity to activate transducin and PDEase.
