Molecular Mechanisms of Phototransduction in Visual Cells.

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Rod outer segments is a typical biosensor which can convert a photon energy into an electrical signal. This mechanism consists of three processes. The first process is a photochemical reaction of rhodopsin triggered by photo-excitation of the chromophore to form a physiologically active intermediate, metarhodopsin II. The second is a signal amplification composed of two steps of enzyme cascade. Metarhodopsin II catalyzes to convert several hundreds of GDP-binding transducin to those of GTP-binding one. This complex in turn activates cGMP-phosphodiesterase which rapidly hydrolyzes cytosolic cGMP. The third is the generation of receptor potential induced by the decrease of the cytosolic cGMP. Following progresses have been made in these processes by us keeping in contact with each other through discussion and collaboration.(1) Laser and conventional spectroscopies of rhodopsin and its analogues showed that photorhodopsin, the first photoproduct of rhodopsin should have a twisted trans ch … More romophore (T.Yoshizswa and M.Ito) Theoretical analysis of absorption spectrum of rhodopsin predicted that a rotation of 11-12 double bond of the chromophore would occur within one picosecond (T.Kakitani). Three-dimensional conformations of retinoid proteins were estimated with an X-ray diffraction technique (M.Kataoka). In the regeneration process of cephalopod rhodopsin, involvement of retinal-binding protein was shown (R.Hara). (2) Biochemical analyses revealed that -subunit of transducin consists of two components which are different in GTP-binding activity (T.Akino), that the Km value of cGMP-phosphodiesterase increases about 10-fold in the light (M.Murakami), and that phosphatidyl-inositol turnover mediated by a GTP-binding protein was activated by light (T.Amakawa). In genetical analysis, a retina-specific protein named MEKA was found, which affects the phosphodiesterase activity (N.Miki). All these observations suggest the involvement of fine regulation in this process.(3) 250kDa, 66kDa (T.Shinozawa) and 57kDa protein (T.Yoshizawa) were presented as the candidates of cGMP-sensitive cation channel in the plasma membrane. Less