Molecular Mechanisms of Phototransduction in Visual Cells.

杆状外节是一种典型的将光子能量转化为电信号的生物传感器。这一机制由三个过程组成。第一个过程是由发色团的光激发引发的视紫红质的光化学反应，形成具有生理活性的中间体--变视紫红质II。第二个过程是由两步酶级联组成的信号放大过程。变视紫红质II催化将数百个GDP结合转导蛋白转化为GTP结合转导蛋白。该复合体进而激活cGMP-磷酸二酯酶，该酶能迅速水解胞内cGMP。其三是胞内cGMP减少引起受体电位的产生。通过讨论和合作，我们在这些过程中取得了以下进展：(1)视紫红质及其类似物的激光光谱和常规光谱表明，视紫红质的第一个光产物光紫质应该具有扭曲的反式…更多的发色团(T.Yoshizswa和M.Ito)对视紫红质的吸收光谱进行了理论分析，预测发色团的11-12双键将在一皮秒内发生旋转(T.Kakitani)。用X-射线衍射技术(M.Kataoka)估计了类维甲酸蛋白的三维构象。在头足类视紫红质的再生过程中，视紫红质参与了视黄醇结合蛋白的表达(R.Hara)。(2)生化分析表明，转导蛋白的亚基由两种不同的GTP结合活性组分组成(T.Akino)，cGMP-磷酸二酯酶的Km值在光照下增加约10倍(M.Murakami)，以及由GTP结合蛋白介导的磷脂酰肌醇的转换被光激活(T.Amakawa)。在遗传分析中，发现了一种视网膜特异性蛋白，称为Meka，它影响磷酸二酯酶的活性(N.Miki)。(3)250 kDa、66 kDa(T.Shinozawa)和57 kDa蛋白(T.Yoshizawa)被认为是质膜上cGMP敏感的阳离子通道。较少

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