Bogomolni R A, Stoeckenius W, Szundi I, Perozo E, Olson K D, Spudich J L
Department of Chemistry and Biochemistry, University of California, Santa Cruz 95064.
Proc Natl Acad Sci U S A. 1994 Oct 11;91(21):10188-92. doi: 10.1073/pnas.91.21.10188.
Sensory rhodopsin I (sR-I) is a phototaxis receptor in halobacteria, which is closely related to the light-driven proton pump bacteriorhodopsin and the chloride pump halorhodopsin found in the same organisms. The three pigments undergo similar cyclic photoreactions, in spite of their different functions. In intact cells or isolated membranes sR-I is complexed with protein HtrI, the next link in the signal transduction chain, and does not function as an electrogenic ion pump. However, illumination of sR-I in membranes lacking HtrI causes pH changes in the medium, and its photoreaction kinetics become pH-dependent. We show here that in closed vesicles, near neutral pH it functions as an electrogenic proton pump capable of generating at least -80 mV transmembrane potential. The action spectrum shows a maximum 37 nm below the 587-nm absorption maximum of the native pigment. This apparent discrepancy occurs because the 587-nm form of HtrI-free sR-I exists in a pH-dependent equilibrium with a 550-nm absorbing species generated through deprotonation of one group with a pKa of 7.2, which we have tentatively identified as Asp-76. We interpret the results in terms of a general model for ion translocation by the bacterial rhodopsins.
感官视紫红质I（sR-I）是嗜盐菌中的一种趋光性受体，它与在同一生物体中发现的光驱动质子泵细菌视紫红质和氯离子泵嗜盐视紫红质密切相关。尽管这三种色素功能不同，但它们经历相似的循环光反应。在完整细胞或分离的膜中，sR-I与信号转导链中的下一个环节蛋白质HtrI结合，并不作为生电离子泵发挥作用。然而，在缺乏HtrI的膜中照射sR-I会导致介质中的pH值变化，并且其光反应动力学变得依赖于pH值。我们在此表明，在封闭囊泡中，接近中性pH时，它作为一种生电质子泵发挥作用，能够产生至少-80 mV的跨膜电位。作用光谱显示，其最大值比天然色素587 nm的吸收最大值低37 nm。出现这种明显差异的原因是，不含HtrI的sR-I的587 nm形式与通过pKa为7.2的一个基团去质子化产生的吸收550 nm的物种处于pH依赖的平衡状态，我们初步将其鉴定为Asp-76。我们根据细菌视紫红质离子转运的一般模型来解释这些结果。
