Phosphorylation and Dephosphorylation of Frog Rod Outer Segment Membranes As Part of the Visual Process

The light-activated phosphorylation of rod outer segment membranes may be an intermediary process controlling photoreceptor responses. We have measured phosphorylation of the opsin moiety of rhodopsin in isolated frog retinas and in rod outer segment suspensions and have demonstrated a phosphorylation-dephosphorylation sequence in the suspensions. The results indicate that these reactions take place in vivo and may be physiologically relevant. Extraction of a protein kinase activity from rod outer segment membranes renders the membranes incapable of phosphorylation, but the light-activated reaction can be reconstituted by mixing the soluble extract and the "depleted" membranes. Illumination of the extract is without effect. Thus the light activation mechanism resides in the membranes. Regeneration of rhodopsin from opsin and 11-cis retinal does not influence the phosphorylation. Once activated, the reaction may use either rhodopsin or opsin as the substrate. Furthermore, 11-cis retinal regenerates rhodopsin from phosphorylated opsin without releasing bound phosphate. The isolated rod outer segment which contains regenerated rhodopsin thus differs from one that is dark adapted in that phosphate can remain bound and the phosphorylation reaction remains activated. Dark adaptation in vivo must include at least two membrane associated reactions beyond regeneration of rhodopsin's spectral properties: dephosphorylation, and the inactivation of the phosphorylation process.