Two protonation switches control rhodopsin activation in membranes
Two protonation switches control rhodopsin activation in membranes
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DOI：
10.1073/pnas.0804541105
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发表时间：
2008-11-18
影响因子：
11.1
通讯作者：
Vogel, Reiner
中科院分区：
文献类型：
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作者：
Mahalingam, Mohana;Martinez-Mayorga, Karina;Vogel, Reiner
G蛋白偶联受体（GPCR）视紫红质的激活是由光诱导的视网膜配体的异构化引发的，这触发了2个质子化开关的构象转变为活性受体状态Meta It。第一个开关涉及破坏螺旋间的盐桥由内部质子转移从视网膜质子化席夫碱（PSB）到其counterparts，Glu-113，在跨膜结构域。第二个开关由Glu-134在螺旋3的胞质末端的保守E（D）RY基序从溶剂中摄取质子组成，导致pH依赖性受体活化。通过结合紫外可见光谱和红外光谱，我们研究了视紫红质在不同的膜环境中的激活机制，并表明这2个质子化开关在生理温度下部分解偶联。这种部分解偶联导致约50%的群体处于熵稳定的Meta 11状态，其中螺旋间PSB盐桥断裂，并发生了激活螺旋运动，但其中Glu-134保持未质子化。这种部分活化仅通过与来自溶剂的Glu-134的pH依赖性质子化偶联而转化为完全活化，这通过降低其焓来稳定活性受体构象。因此，在膜环境中，Glu-134的质子化是激活螺旋运动的热力学而不是结构先决条件。鉴于E（D）RY基序在视紫红质样GPCR中的保守性，该羧酸的质子化也可以在该受体家族的其他成员的信号转导中发挥类似的功能。
Activation of the G protein-coupled receptor (GPCR) rhodopsin is initiated by light-induced isomerization of the retinal ligand, which triggers 2 protonation switches in the conformational transition to the active receptor state Meta It. The first switch involves disruption of an interhelical salt bridge by internal proton transfer from the retinal protonated Schiff base (PSB) to its counterion, Glu-113, in the transmembrane domain. The second switch consists of uptake of a proton from the solvent by Glu-134 of the conserved E(D)RY motif at the cytoplasmic terminus of helix 3, leading to pH-dependent receptor activation. By using a combination of UV-visible and FTIR spectroscopy, we study the activation mechanism of rhodopsin in different membrane environments and show that these 2 protonation switches become partially uncoupled at physiological temperature. This partial uncoupling leads to approximate to 50% population of an entropy-stabilized Meta 11 state in which the interhelical PSB salt bridge is broken and activating helix movements have taken place but in which Glu-134 remains unprotonated. This partial activation is converted to full activation only by coupling to the pH-dependent protonation of Glu-134 from the solvent, which stabilizes the active receptor conformation by lowering its enthalpy. In a membrane environment, protonation of Glu-134 is therefore a thermodynamic rather than a structural prerequisite for activating helix movements. In light of the conservation of the E(D)RY motif in rhodopsin-like GPCRs, protonation of this carboxylate also may serve a similar function in signal transduction of other members of this receptor family.