Modulation of the G Protein Regulator Phosducin by Ca super(2+)/Calmodulin-dependent Protein Kinase II Phosphorylation and 14-3-3 Protein Binding

• Published in: The Journal of biological chemistry Vol. 276; no. 26; pp. 23805 - 23815
• Main Authors: Thulin, C D, Savage, J R, McLaughlin, J N, Truscott, S M, Old, WM, Ahn, NG, Resing, KA, Hamm, HE, Bitensky, M W, Willardson, B M
• Format: Journal Article
• Language: English
• Published: 29.06.2001
• Subjects: 14-3-3 protein; phosducin
• ISSN: 0021-9258

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 effectors, 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 Yssociation with G beta gamma . In this study, we report that Pd was rapidly phosphorylated by Ca super(2+)/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 Ca super(2+)/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 Ca super(2+) concentration blocked the interaction of Pd with 14-3-3, indicating that Ca super(2+) controls the phosphorylation state of Ser-54 and Ser-73 in vivo. These results are consistent with a role for Pd in Ca super(2+)-dependent light adaptation processes in photoreceptor cells and also suggest other possible physiological functions.