Modulation of the G protein regulator phosducin by Ca2+/calmodulin-dependent protein kinase II phosphorylation and 14-3-3 protein binding

Phototransduction is a canonical G protein-mediated cascade of retinal photoreceptor cells that transforms photons into neural responses. Phosducin (Pd) is a G beta gamma -binding protein that is highly expressed in photoreceptors. Pd is phosphorylated in dark-adapted retina and is dephosphorylated in response to light. Dephosphorylated Pd binds G beta gamma with high affinity and inhibits the interaction of G beta gamma with G alpha or Other effecters, whereas phosphorylated Pd does not. These results have led to the hypothesis that Pd down-regulates the light response. Consequently, it is important to understand the mechanisms of regulation of Pd phosphorylation. We have previously shown that phosphorylation of Pd by cAMP-dependent protein kinase moderately inhibits its association with G beta gamma. In this study, we report that Pd was rapidly phosphorylated by Ca2+/calmodulin-dependent kinase II, resulting in 100-fold greater inhibition of G beta gamma binding than cAMP-dependent protein kinase phosphorylation, Furthermore, Pd phosphorylation by Ca2+/calmodulin-dependent kinase II at Ser-54 and Ser-73 led to binding of the phosphoserine-binding protein 14-3-3. Importantly, in vivo decreases in Ca2+. concentration blocked the interaction of Pd with 14-3-3, indicating that Ca2+ controls the phosphorylation state of Ser-54 and Ser-73 in vivo. These results are consistent with a role for Pd in Ca2+-dependent light adaptation processes in photoreceptor cells and also suggest other possible physiological functions.