Sudo Yuki, Spudich John L
Center for Membrane Biology, Department of Biochemistry and Molecular Biology, University of Texas Medical School, Houston, TX 77030, USA.
Proc Natl Acad Sci U S A. 2006 Oct 31;103(44):16129-34. doi: 10.1073/pnas.0607467103. Epub 2006 Oct 18.
In haloarchaea, light-driven ion transporters have been modified by evolution to produce sensory receptors that relay light signals to transducer proteins controlling motility behavior. The proton pump bacteriorhodopsin and the phototaxis receptor sensory rhodopsin II (SRII) differ by 74% of their residues, with nearly all conserved residues within the photoreactive retinal-binding pocket in the membrane-embedded center of the proteins. Here, we show that three residues in bacteriorhodopsin replaced by the corresponding residues in SRII enable bacteriorhodopsin to efficiently relay the retinal photoisomerization signal to the SRII integral membrane transducer (HtrII) and induce robust phototaxis responses. A single replacement (Ala-215-Thr), bridging the retinal and the membrane-embedded surface, confers weak phototaxis signaling activity, and the additional two (surface substitutions Pro-200-Thr and Val-210-Tyr), expected to align bacteriorhodopsin and HtrII in similar juxtaposition as SRII and HtrII, greatly enhance the signaling. In SRII, the three residues form a chain of hydrogen bonds from the retinal's photoisomerized C(13)=C(14) double bond to residues in the membrane-embedded alpha-helices of HtrII. The results suggest a chemical mechanism for signaling that entails initial storage of energy of photoisomerization in SRII's hydrogen bond between Tyr-174, which is in contact with the retinal, and Thr-204, which borders residues on the SRII surface in contact with HtrII, followed by transfer of this chemical energy to drive structural transitions in the transducer helices. The results demonstrate that evolution accomplished an elegant but simple conversion: The essential differences between transport and signaling proteins in the rhodopsin family are far less than previously imagined.
在嗜盐古菌中，光驱动离子转运蛋白经过进化改造，产生了将光信号传递给控制运动行为的转导蛋白的感官受体。质子泵细菌视紫红质和趋光性受体感官视紫红质II（SRII）的氨基酸残基有74%不同，几乎所有保守残基都位于蛋白质膜嵌入中心的光反应性视黄醛结合口袋内。在这里，我们表明，细菌视紫红质中的三个残基被SRII中的相应残基取代后，能使细菌视紫红质有效地将视黄醛光异构化信号传递给SRII整合膜转导蛋白（HtrII），并诱导强烈的趋光反应。一个连接视黄醛和膜嵌入表面的单一取代（丙氨酸215 - 苏氨酸）赋予微弱的趋光信号活性，另外两个（表面取代脯氨酸200 - 苏氨酸和缬氨酸210 - 酪氨酸）预期会使细菌视紫红质和HtrII以与SRII和HtrII相似的并列方式排列，从而大大增强信号传递。在SRII中，这三个残基形成了一条氢键链，从视黄醛光异构化的C(13)=C(14)双键连接到HtrII膜嵌入α螺旋中的残基。结果表明了一种信号传递的化学机制，即视黄醛光异构化的能量最初存储在SRII中与视黄醛接触的酪氨酸174和与HtrII接触的SRII表面残基相邻的苏氨酸204之间的氢键中，随后这种化学能量转移以驱动转导螺旋中的结构转变。结果表明进化完成了一个巧妙而简单的转变：视紫红质家族中转运蛋白和信号蛋白之间的本质差异远比之前想象的要小得多。
