The 33K/BETA-Gamma Transducin Complex of Mammalian Rod Photopeceptors

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This research project investigates a protein complex that is found only in retinal photoreceptors. The complex is composed of a phosphoprotein of 33,000 (33K) daltons and two proteins (beta and gamma transducin) which have been shown to participate in the visual transduction process. Association of beta-gamma transducin with the 33K protein and the ability of light to trigger the dephosphorylation of 33K suggests that the 33K/T beta-gamma complex may act to modulate the phototransduction process, particularly in mammalian photoreceptors. The PI will focus on the light-modulated phosphorylation/dephosphorylation cycle of the 33K protein. Phosphorylation of the 33K protein may modify interactions between the 33K protein and the beta-gamma subunits of transducin or it could alter the functional activities of the 33K/T beta-gamma complex. These studies are significant in a broader sense because transducin belongs to a family of homologous GTP-binding proteins (G-proteins) that are involved in signal transduction in a variety of tissues. In each tissue, a receptor for an extracellular signal, e.g., light, hormone or neurotransmitter, is coupled to a G-protein which regulates cellular function. It is still unclear how G-proteins are down-regulated, but proteins which may be analogous to the 33K/T-beta-gamma complex seem to be involved. Understanding the mechanisms which regulate the visual process is a major goal of this project, and each increment of accomplishment provides insight into how phosphoproteins interact with the family of G- proteins and, thereby, influence the many cellular functions which G-proteins mediate.