Kosower E M
Biophysical Organic Chemistry Unit, School of Chemistry, Sackler Faculty of Exact Sciences, Tel-Aviv University, Israel.
Eur J Biochem. 1987 Oct 15;168(2):431-49. doi: 10.1111/j.1432-1033.1987.tb13437.x.
Folding of the five polypeptide subunits (alpha 2 beta gamma delta) of the nicotinic acetylcholine receptor (AChR) into a functional structural model is described. The principles used to arrange the sequences into a structure include: (1) hydrophobicity----membrane-crossing segments; (2) amphipathic character----ion-carrying segments (ion channel with single group rotations); (3) molecular shape (elongated, pentagonal cylinder)----folding dimensions of exobilayer portion; (4) choice of acetylcholine binding sites----specific folding of exobilayer segments; (5) location of reducible disulfides (near agonist binding site)----additional specification of exobilayer arrangement; (6) genetic homology----consistency of functional group choices; (7) noncompetitive antagonist labeling----arrangement of bilayer helices. The AChR model is divided into three parts: (a) exobilayer consisting of 11 antiparallel beta-strands from each subunit; (b) bilayer consisting of four hydrophobic and one amphiphilic alpha-helix from each subunit; (c) cytoplasmic consisting of one (folded) loop from each subunit. The exobilayer strands can form a closed 'flower' (the 'resting state') which is opened ('activated') by agonists bound perpendicular to the strands. Rearrangement of the agonists to a strand-parallel position and partial closing of the 'flower' leads to a desensitized receptor. The actions of acetylcholine and succinoyl and suberoyl bis-cholines are clarified by the model. The opening and closing of the exobilayer 'flower' controls access to the ion channel which is composed of the five amphiphilic bilayer helices. A molecular mechanism for ion flow in the channel is given. Openings interrupted by short duration closings (50 microseconds) depend upon channel group motions. The unusual photolabeling of intrabilayer serines in alpha, beta and delta subunits but not in gamma subunits near the binding site for non-competitive antagonists is explained along with a mechanism for the action of these antagonists such as phencyclidine. The unusual alpha 192Cys-193Cys disulfide may have a special peptide arrangement, such as a cis-peptide bond to a following proline (G.A. Petsko and E.M. Kosower, unpublished results). The position of phosphorylatable sites and proline-rich segments are noted for the cytoplasmic loops. The dynamic behavior of the AChR channel and many different experimental results can be interpreted in terms of the model. An example is the lowering of ionic conductivity on substitution of bovine for Torpedo delta M2 segment. The model represents a useful construct for the design of experiments on AChR.
描述了烟碱型乙酰胆碱受体（AChR）的五个多肽亚基（α2βγδ）折叠成功能结构模型的过程。用于将序列排列成结构的原则包括：（1）疏水性——跨膜片段；（2）两亲性特征——离子携带片段（具有单组旋转的离子通道）；（3）分子形状（细长的五角圆柱体）——胞外部分的折叠尺寸；（4）乙酰胆碱结合位点的选择——胞外片段的特定折叠；（5）可还原二硫键的位置（靠近激动剂结合位点）——胞外排列的进一步确定；（6）基因同源性——官能团选择的一致性；（7）非竞争性拮抗剂标记——双层螺旋的排列。AChR模型分为三个部分：（a）胞外部分，由每个亚基的11条反平行β链组成；（b）双层部分，由每个亚基的四条疏水α螺旋和一条两亲性α螺旋组成；（c）胞质部分，由每个亚基的一个（折叠）环组成。胞外链可形成一个封闭的“花”（“静息状态”），该“花”由垂直于链结合的激动剂打开（“激活”）。激动剂重新排列到与链平行的位置并使“花”部分关闭会导致受体脱敏。该模型阐明了乙酰胆碱以及琥珀酰胆碱和辛二酰胆碱的作用。胞外“花”的打开和关闭控制着由五个两亲性双层螺旋组成的离子通道的通路。给出了通道中离子流动的分子机制。被短时间关闭（50微秒）打断的开放取决于通道基团的运动。解释了在非竞争性拮抗剂结合位点附近α、β和δ亚基中双层内丝氨酸的异常光标记，而γ亚基中没有这种现象，同时还解释了这些拮抗剂（如苯环己哌啶）的作用机制。异常的α192Cys - 193Cys二硫键可能具有特殊的肽排列，例如与后续脯氨酸形成顺式肽键（G.A. Petsko和E.M. Kosower，未发表结果）。注意到了胞质环中可磷酸化位点和富含脯氨酸片段的位置。AChR通道的动态行为以及许多不同的实验结果都可以根据该模型进行解释。一个例子是用牛的δM2片段替代电鳐的相应片段后离子电导率的降低。该模型为设计关于AChR的实验提供了一个有用的构建框架。
