Spudich J L
Department of Microbiology and Molecular Genetics, University of Texas Medical School, Houston 77030, USA.
Mol Microbiol. 1998 Jun;28(6):1051-8. doi: 10.1046/j.1365-2958.1998.00859.x.
The archaeal rhodopsins are a family of seven-transmembrane-helix, visual pigment-like proteins found in Halobacterium salinarum and related halophilic Archaea. Two, bacteriorhodopsin (BR) and halorhodopsin (HR), are transport rhodopsins that carry out light-driven electrogenic translocation of protons and chloride, respectively, across the cell membrane. The other two, sensory rhodopsins I and II (SRI and SRII), are phototaxis receptors that send signals to tightly bound transducer proteins that in turn control a phosphorylation cascade modulating the cell's flagellar motors. Recent progress has cast light on how nature has modified the common design of these proteins to carry out their distinctly different functions: electrogenic ion transport and non-electrogenic signal transduction. A key shared mechanism between BR and SRII appears to be an interhelical salt bridge locked conformational switch that is released by photoisomerization of retinal. In BR disruption of the lock opens a cytoplasmic half-channel that ensures uptake of the transported proton from the cytoplasmic side of the membrane at a critical time in the pumping cycle. Transducer-free SRI uses the same mechanism to carry out light-driven proton transport, but interaction with its transducer blocks the cytoplasmic half-channel thereby interrupting the transport cycle. In SRI, transducer interaction also disrupts the salt bridge in the dark, poising the receptor in an intermediate conformation able to produce opposite signals depending on the colour of the stimulus light. A model for signalling is proposed in which the salt bridge-controlled half-channel is used to modulate interaction with the Htr proteins when the receptor signalling states are formed.
古菌视紫红质是一类具有七个跨膜螺旋的、类似于视觉色素的蛋白质家族，存在于盐生盐杆菌及相关嗜盐古菌中。其中两种，细菌视紫红质（BR）和嗜盐视紫红质（HR），是转运视紫红质，分别介导质子和氯离子通过细胞膜的光驱动电致转运。另外两种，感官视紫红质I和II（SRI和SRII），是趋光性受体，它们向紧密结合的转导蛋白发送信号，这些转导蛋白进而控制一个磷酸化级联反应，调节细胞的鞭毛马达。最近的研究进展揭示了自然界是如何改变这些蛋白质的共同结构以执行其截然不同的功能的：电致离子转运和非电致信号转导。BR和SRII之间一个关键的共同机制似乎是一个由螺旋间盐桥锁定的构象开关，它通过视黄醛的光异构化而释放。在BR中，锁定的破坏打开了一个胞质半通道，确保在泵浦循环的关键时期从膜的胞质侧摄取被转运的质子。无转导蛋白的SRI利用相同的机制进行光驱动的质子转运，但与转导蛋白的相互作用会阻断胞质半通道，从而中断转运循环。在SRI中，转导蛋白的相互作用在黑暗中也会破坏盐桥，使受体处于一种中间构象，根据刺激光的颜色能够产生相反的信号。本文提出了一种信号传导模型，即在形成受体信号状态时，盐桥控制的半通道用于调节与Htr蛋白的相互作用。
