Unwin N, Miyazawa A, Li J, Fujiyoshi Y
MRC Laboratory of Molecular Biology, Neurobiology Division, Hills Road, Cambridge CB2 2QH, UK.
J Mol Biol. 2002 Jun 21;319(5):1165-76. doi: 10.1016/S0022-2836(02)00381-9.
The nicotinic acetylcholine (ACh) receptor belongs to a superfamily of synaptic ion channels that open in response to the binding of chemical transmitters. Their mechanism of activation is not known in detail, but a time-resolved electron microscopic study of the muscle-type ACh receptor had suggested that a local disturbance in the ligand-binding region and consequent rotations in the ligand-binding alpha subunits, connecting to the transmembrane portion, are involved. A more precise interpretation of this structural change is given here, based on comparison of the extracellular domain of the ACh receptor with an ACh-binding protein (AChBP) to which a putative agonist is bound. We find that, to a good approximation, there are two alternative extended conformations of the ACh receptor subunits, one characteristic of either alpha subunit before activation, and the other characteristic of all three non-alpha subunits and the protomer of AChBP. Substitution in the three-dimensional maps of alpha by non-alpha subunits mimics the changes seen on activation, suggesting that the structures of the alpha subunits are modified initially by their interactions with neighbouring subunits and switch to the non-alpha form when ACh binds. This structural change, which entails 15-16 degrees rotations of the inner pore-facing parts of the alpha subunits, most likely acts as the trigger that opens the gate in the membrane-spanning pore.
烟碱型乙酰胆碱（ACh）受体属于突触离子通道超家族，该家族通道会因化学递质的结合而开放。其激活机制尚不完全清楚，但对肌肉型ACh受体的时间分辨电子显微镜研究表明，配体结合区域的局部干扰以及随之而来的连接到跨膜部分的配体结合α亚基的旋转参与其中。基于将ACh受体的胞外结构域与结合了假定激动剂的ACh结合蛋白（AChBP）进行比较，本文给出了对这种结构变化更精确的解释。我们发现，大致来说，ACh受体亚基有两种交替的伸展构象，一种是激活前任一α亚基的特征构象，另一种是所有三个非α亚基以及AChBP原体的特征构象。在三维图谱中用非α亚基替换α亚基会模拟激活时看到的变化，这表明α亚基的结构最初是通过与相邻亚基的相互作用而改变的，并且在ACh结合时会转变为非α形式。这种结构变化需要α亚基面向孔内部部分进行15 - 16度的旋转，很可能是打开跨膜孔道闸门的触发因素。
