Regulation of Photo-Chemical Signal Transduction in Photoreceptor by Cdk5 and Phosphatase

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We have investigated the physiological importance of the phosphorylation of an inhibitory subunit of the cGMP-phosphodiesterse (PDE6) in the recovery process of phototransduction in vertebrate photoreceptors. Using in vitro and in situ experimental system, we have clarified that 1) the protein kinase involved in the phosphorylation of the inhibitory subunit of PDE6, i.e. Pγ, is a cyclin-dependent protein kinase 5 in the photoreceptor cells. 2) the phosphorylation occurs when the substrate interacts with the α sub unit of G protein, i.e. transducin. That means the phosphorylation occurs when the PDE is activated. 3) Phosphodiesterase, presumably PP2A, is always in its active state, and is ready for rapid dephosphorylation of Pγ. The importance of our findings is as follows: 1) we clarified a function of Cdk5 in photoreceptor cells, 2) our results revealed that the inactivation of transducin (Gα_t) is not completed with the GTP hydrolysis on Gα_t, but the phosphorylation of Pγ complexed with Gα_t is required. Phosphorylation and dephosphorylation of Pγ by Cdk5 and a phosphatase seem to have important roles in the formation of very sharp pulse response to a weak photosignal.