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Phosducin Regulates the Expression of Transducin ß Subunits in Rod Photoreceptors and Does Not Contribute to Phototransduction Adaptation

¹ Center for Neuroscience and Department of Ophthalmology and Vision Science, University of California, Davis, Davis, CA 95618
² Department of Ophthalmology and Department of Biochemistry, West Virginia University School of Medicine and West Virginia University Eye Institute, Morgantown, WV 26506
³ Department of Ophthalmology and Department of Pharmacology, Duke University, Durham, NC 27710

Correspondence to M.E. Burns: meburns{at}ucdavis.edu

For over a decade, phosducin's interaction with the ß subunits of the G protein, transducin, has been thought to contribute to light adaptation by dynamically controlling the amount of transducin heterotrimer available for activation by photoexcited rhodopsin. In this study we directly tested this hypothesis by characterizing the dark- and light-adapted response properties of phosducin knockout (Pd–/–) rods. Pd–/– rods were notably less sensitive to light than wild-type (WT) rods. The gain of transduction, as measured by the amplification constant using the Lamb-Pugh model of activation, was 32% lower in Pd–/– rods than in WT rods. This reduced amplification correlated with a 36% reduction in the level of transducin ß-subunit expression, and thus available heterotrimer in Pd–/– rods. However, commonly studied forms of light adaptation were normal in the absence of phosducin. Thus, phosducin does not appear to contribute to adaptation mechanisms of the outer segment by dynamically controlling heterotrimer availability, but rather is necessary for maintaining normal transducin expression and therefore normal flash sensitivity in rods.

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