Regulation of phosducin phosphorylation in retinal rods by Ca2+/calmodulin-dependent adenylylcyclase

The phosphoprotein phosducin (Pd) regulates many guanine nucleotide binding protein (G protein)-linked signaling pathways. In visual signal transduction, unphosphorylated Pd blocks the interaction of light-activated rhodopsin with its G protein (G(t)) by binding to the beta gamma subunits of G(t) and preventing their association with the G(t) alpha subunit. When Pd is phosphorylated by cAMP-dependent protein kinase, it no longer inhibits G(t) subunit interactions. Thus, factors that determine the phosphorylation state of Pd in rod outer segments are important in controlling the number of G(t)s available for activation by rhodopsin. The cyclic nucleotide dependencies of the rate of Pd phosphorylation by endogenous cAMP-dependent protein kinase suggest that cAMP, and not cGMP, controls Pd phosphorylation. The synthesis of cAMP by adenylyl cyclase in rod outer segment preparations was found to be dependent on Ca2+ and calmodulin. The Ca2+ dependence was within the physiological range of Ca2+ concentrations in rods (K-1/2 = 230 +/- 9 nM) and was highly cooperative (n(app) = 3.6 +/- 0.5). Through its effect on adenylyl cyclase and cAMP-dependent protein kinase, physiologically high Ca2+ (1100 nM) was found to increase the rate of Pd phosphorylation 3-fold compared to the rate of phosphorylation at physiologically low Ca2+ (8 nM). No evidence for Pd phosphorylation by other Ca2+-dependent kinases was found. These results suggest that Ca2+ can regulate the light response at the level of G(t) activation through its effect on the phosphorylation state of Pd.