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C. W. Morgans, T. G. Wensel, R. L. Brown, R. M. Duvoisin; Gß5-RGS Complexes in Retinal ON-Bipolar Cell Dendrites. Invest. Ophthalmol. Vis. Sci. 2007;48(13):54.
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In photoreceptor outer segments (OS), activated rhodopsin is negatively coupled to a cation channel through the G-protein transducin, so that absorption of a photon by rhodopsin closes the channel and hyperpolarizes the cell. The metabotropic glutamate receptor of ON-bipolar cells (ON-BPCs), mGluR6, is similarly coupled to a cation channel through the G protein, Go. In both photoreceptor OS and ON-BPC dendrites, the slow intrinsic GTPase activity of the Gα subunit requires the action of a GTPase accelerating protein (GAP). In photoreceptor OS, the GAP activity is provided by the Gß5-RGS9 complex. We present evidence here for two similar complexes, Gß5-RGS11 and Gß5-RGS7, in ON-BPC dendrites, and propose that they accelerate the GTPase activity of Gαo.
Immunohistochemistry and western blots were performed as described previously (Morgans et al., 2006). A Zeiss LSM510 confocal microscope was used to visualize the labeling. Immunoprecipitations were done as described by Hu and Wensel (2004).
Bright, punctate Gß5 immunofluorescence was observed in the OPL of the mouse retina associated with both rod and cone terminals. Double labeling with pre-synaptic (ribeye) and post-synaptic (calbindin and mGluR6) markers revealed that Gß5 is localized to the tips of ON-BPC dendrites. Identical staining was observed for RGS11 and RGS7, GGL-domain containing RGS proteins that form tightly bound complexes with Gß5. Immunolabeling of acutely dissociated rod-bipolar cells for Gß5 and RGS11 confirmed their localization to ON-BPC dendrites. Furthermore, in a mGluR6-deficient mouse, RGS11 was diffusely distributed in the BPCs, suggesting that mGluR6 is necessary for proper dendritic localization of the Gß5-RGS complex. We also find that R9AP, a protein known to anchor the Gß5-RGS9 complex in photoreceptor OS, is co-localized with mGluR6 in the OPL. Comparison of R9AP-/- and wild-type retinas revealed a differential dependence of RGS7 and RGS11 on R9AP. In the R9AP-/- retina, RGS11 is absent from the OPL whereas the levels of RGS7 are increased.
The precise co-localization of the Gß5-RGS7 and Gß5-RGS11 complexes with mGluR6 in ON-BPC dendritic tips argues that they function specifically in the mGluR6 signal transduction pathway. They are likely to accelerate the GTPase activity of Gαo, thus accelerating the opening of ON-BPC cation channels and the depolarizing response to light.
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