Abstract
When suspensions of cattle retinal outer segments are incubated with Mg 2+ and [γ- 32P]ATP, the rhodopsin band, identified on sodium dodecyl sulfate-polyacrylamide gels, is labeled when the samples are incubated in bright white light, but not in darkness. Calf thymus histone added to the reaction mixture is labeled equally in darkness and in light. Cyclic AMP (cAMP) has no effect on the phosphorylation of either substrate. After washing in potassium alum, outer segment suspensions lose their ability to phosphorylate either rhodopsin or histone, even though alum treatment does not destroy the spectral properties of rhodopsin. Addition of a soluble kinase from beef skeletal muscle restores the ability to phosphorylate histone but not rhodopsin. These results suggest that the stimulation of rhodopsin phosphorylation by light is due to structural changes in the visual pigment protein rather than to direct light activation of the protein kinase. In addition, the results imply that rhodopsin phosphorylation may require a specific kinase located in retinal outer segments.
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