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R.V. Rajala, M. Tanito, J.–X. Ma, J. Lem, A. Rajala; Light–Dependent Trafficking of Growth Factor Receptor–Bound Protein 14 Requires the Photobleaching of Rhodopsin but Not Transducin Signaling . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1092.
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We previously demonstrated light–induced tyrosine phosphorylation of the retinal insulin receptor (IR), independent of insulin secretion, and localized the light effect to photoreceptor neurons. These results suggest that there exists a cross talk between phototransduction and other signal transduction pathways. This cross talk phenomenon has been shown in other G–protein–coupled receptors (GPCRs) as many tyrosine kinase cascades are regulated by GPCRs. These studies suggest that rhodopsin might also initiate other signaling pathways. The idea that rhodopsin can initiate non–canonical signaling pathways is further supported by studies on the translocation of visual arrestin.
We used yeast two–hybrid assay of protein–protein interaction to identify binding partners to the IR from a bovine retinal cDNA library. Rod outer segments (ROS) were prepared from the light– and dark–adapted wild type, Talpha, and Rpe65 knockout mice. Immunohistochemistry was performed on light– and dark–adapted mouse retina sections.
Yeast two–hybrid screening identified growth factor receptor–bound protein 14 (Grb14) as an IR binding partner. Grb14 is an inhibitor of protein tyrosine phosphatase 1b, which specifically dephosphorylates the IR. We found more Grb14 bound to light–adapted ROS membranes than to dark–adapted membranes. Immunohistochemical studies indicate that Grb14 is localized in the inner segment in the dark and in ROS in the light. In Talpha knockout mice, Grb14 trafficking to ROS is similar to wild–type, whereas in Rpe65 knockout mice we observed only ∼10% of Grb14 bound to both light– and dark–adapted ROS. These results suggest that rhodopsin photobleaching is required for Grb14 trafficking and proper localization to ROS, but transducin signaling is not.
These studies show that rhodopsin can initiate signaling pathways other than visual transduction, and support our hypothesis that signaling proteins not directly involved in phototransduction could undergo subcellular redistribution upon illumination.
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