Growth Factor Receptor-Bound Protein 14 Undergoes Light-Dependent Intracellular Translocation in Rod Photoreceptors: Functional Role in Retinal Insulin Receptor Activation

Autor: Dustin T. Allen, Ammaji Rajala, Roger J. Daly, Vadim Y. Arshavsky, Raju V. S. Rajala, Masaki Tanito, Ekaterina S. Lobanova, Lowenna J Holt
Rok vydání: 2009
Předmět:
Zdroj: Biochemistry. 48:5563-5572
ISSN: 1520-4995
0006-2960
DOI: 10.1021/bi9000062
Popis: Phosphoinositide 3-kinase kinase (PI3K) is at the heart of one of the major signal transduction pathways (1–4). The signals mediated by this enzyme influence a wide variety of cellular functions, including cell growth, differentiation and survival, glucose metabolism and cytoskeletal organization. The PI3K is expressed in photoreceptor cells and is regulated through the light-induced tyrosine phosphorylation of the insulin receptor (IR) in vivo (5;6). We have reported that light-induced tyrosine phosphorylation of IR requires the photoactivation of rhodopsin, but not transducin signaling (7). We also found that photoreceptor-specific deletion of IR resulted in stress-induced photoreceptor degeneration, suggesting the importance of IR in the survival of photoreceptor neurons (8). The molecular mechanism behind the light-induced activation of retinal IR is not known. Retinal IR has a high basal level of autophosphorylation compared to liver IR (9) and retinal IR autophosphorylation is light-dependent (5). These observations led us to hypothesize that retinal IR phosphorylation could be modulated by soluble factor(s) in the retina. To identify the regulators of IR, yeast two-hybrid screening of a bovine retinal cDNA library with the cytoplasmic domain of retinal IR (10) identified growth factor receptor-bound protein 14 (Grb14) (11;12), which binds to various tyrosine kinase receptors including IR (13–16). The crystal structure of the tyrosine kinase domain in complex with the IR-interacting domain of Grb14 has been resolved and revealed that Grb14 acts as a pseudo-substrate inhibitor that binds in the peptide binding groove of the kinase, and thus functions as a selective inhibitor of insulin signaling (17). In vitro experiments have shown that Grb14 impairs the tyrosine kinase activity of the IR towards exogenous substrates and protects the tyrosine-phosphorylation from dephosphorylation by protein tyrosine phosphatase-1B (PTP1B) (18). In liver, Grb14 deletion resulted in decreased IR phosphorylation due to increased dephosphorylation of the IR by PTP1B (19). The precise functional role of Grb14 in the retina is not known. Here we report that Grb14 undergoes light-dependent intracellular redistribution upon illumination of rod photoreceptor cells. In the dark, Grb14 was found to occupy all subcellular compartments of the rod, except for the outer segment. Following thirty minutes of light exposure, Grb14 was found evenly distributed throughout the entire rod cell, including the outer segment. This process is triggered by the photoexcitation of rhodopsin but is not mediated by transducin signaling. Ablation of Grb14 in the retina resulted in the loss of light-dependent activation of retinal IR and our studies suggest Grb14 translocates to photoreceptor outer segments following photobleaching of rhodopsin and protects the IR phosphorylation in rod photoreceptors cells. These findings demonstrate that Grb14 can undergo subcellular redistribution upon illumination and suggest that rhodopsin may have additional previously uncharacterized signaling functions in photoreceptors.
Databáze: OpenAIRE
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