Transducin Activation State Controls Its Light-dependent Translocation in Rod Photoreceptors

Nikolai O Artemyev,Desheng Chen,Vasily Kerov,Ching-Kang Chen,Yu-Jiun Chen,Mustapha Moussaif

doi:10.1074/jbc.m508849200

Nikolai O Artemyev, Desheng Chen + Show 4 more

Open Access

https://doi.org/10.1074/jbc.m508849200

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Abstract

Light-dependent redistribution of transducin between the rod outer segments (OS) and other photoreceptor compartments including the inner segments (IS) and synaptic terminals (ST) is recognized as a critical contributing factor to light and dark adaptation. The mechanisms of light-induced transducin translocation to the IS/ST and its return to the OS during dark adaptation are not well understood. We have probed these mechanisms by examining light-dependent localizations of the transducin-alpha subunit (Gtalpha)in mice lacking the photoreceptor GAP-protein RGS9, or expressing the GTPase-deficient mutant GtalphaQ200L. An illumination threshold for the Gtalpha movement out of the OS is lower in the RGS9 knockout mice, indicating that the fast inactivation of transducin in the wild-type mice limits its translocation to the IS/ST. Transgenic GtalphaQ200L mice have significantly diminished levels of proteins involved in cGMP metabolism in rods, most notably the PDE6 catalytic subunits, and severely reduced sensitivity to light. Similarly to the native Gtalpha, the GtalphaQ200L mutant is localized to the IS/ST compartment in light-adapted transgenic mice. However, the return of GtalphaQ200L to the OS during dark adaptation is markedly slower than normal. Thus, the light-dependent translocations of transducin are controlled by the GTP-hydrolysis on Gtalpha, and apparently, do not require Gtalpha interaction with RGS9 and PDE6.

Highlights

The rhodopsin-kinase mediated phosphorylation and the binding of arrestin to phosphorylated R* (6 – 8)
We have examined the mechanism of transducin movement using the RGS9 knock-out [11] and GTPase-deficient Gt␣Q200L mouse models
If the activated Gt␣GTP moves by simple diffusion, the rate of its translocation to the inner segments (IS)/synaptic terminals (ST) must be limited by the fast inactivation of Gt␣ by the RGS9 GAP complex

Summary

Introduction

The rhodopsin-kinase mediated phosphorylation and the binding of arrestin to phosphorylated R* (6 – 8). The estimates of Gt␣ content in the OS of the light- and dark-adapted wild type and RGS9Ϫ/Ϫ mice by immunofluorescence are similar to those obtained previously by a more quantitative procedure of tangential retina microsectioning combined with immunoblotting (Fig. 2B) [29].

Results

Conclusion