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Volume 271, Number 32, Issue of August 9, 1996 pp. 19232-19237
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Regulation of the Kinetics of Phosducin Phosphorylation in Retinal Rods

(Received for publication, September 13, 1995, and in revised form, April 16, 1996)

From the Biophysics Group, Physics Division, Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545

Phosducin (Pd) is a widely expressed phosphoprotein that regulates G-protein (G) signaling. Unphosphorylated Pd binds to G subunits and blocks their interaction with G. This binding sequesters G and inhibits both receptor-mediated activation of G and direct interactions between G and effector enzymes. When phosphorylated by cAMP-dependent protein kinase, Pd does not affect these functions of G. To further understand the role of Pd in regulating G-protein signaling in retinal rod photoreceptor cells, we have measured the abundance of Pd in rods and examined factors that control the rate of Pd phosphorylation. Pd is expressed at a copy number comparable to that for the rod G-protein, transducin (G_t). The ratio of rhodopsin (Rho) to Pd is 15.5 ± 3.5 to 1. The rate of Pd phosphorylation in rod outer segment preparations was dependent on [cAMP]. K1/2 for cAMP was 0.56 ± 0.09 µM, and the maximal rate of phosphorylation was ~500 pmol PO₄ incorporated/min/nmol Rho. In the presence of G_t this rate was decreased ~50-fold. From these data, one can estimate a t1/2 of ~3 min for the rephosphorylation of Pd in rods during the recovery period after a light response. This relatively slow rephosphorylation of the Pd·G_t complex may provide a period of molecular memory in which sensitivity to further light stimuli is reduced as a result of sequestration of G_t by Pd.

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