MEMBRANE-SPANNING DOMAINS IN ACETYCHOLINE RECEPTOR

乙酰胆碱受体是一种跨越质膜的蛋白质， 在细胞质和细胞外之间形成阳离子的门控通道， 肌肉细胞。 这条通道的一部分， 膜本身由五个并列的疏水环形成， 域，五个亚基中的每一个。 这是长期目标， 来确定每一种的精确分子结构 域，从而，环本身的域和用于 阳离子。 四个亚基中的四个跨膜序列 乙酰胆碱受体已经被确定，Kendrew-Watson线 已经构建了四个跨膜螺旋的模型， γ亚单位。 这四个瓣门被排列成一个 这种跨膜结构域的假设分子模型 蛋白 该模型包含两个亚基内二硫键， 双层。 这两个二硫化物将在天然结构中寻找 乙酰胆碱受体通过化学实验进行的 纯化蛋白 含有这些二硫化物的肽将被分离出来 并通过已成功用于纯化 （Na+ + K+）-ATP酶的跨膜部分。 如果这些二硫化物 假设模型存在，那么假设模型是正确的概率很高。 如果它们不这样做，其他亚单位内二硫化物可能会出现在 天然结构中其他亚基的跨膜结构域，和 这些也将被寻求。 如果11个独特的半胱氨酸中的每一个 然而，膜内的残基是自由的，交联剂将 可以连接到它们中的任何一个，可以从烷烃核心的 双层。 这些半胱氨酸和邻近的半胱氨酸之间的共价交联 氨基酸侧链将形成，并且这些产物 交联反应将被纯化和鉴定。 如果一个或两 上述化学实验中的任何一个都提供了积极的结果，它们 应该为跨膜结构域定义一个独特的分子结构 乙酰胆碱受体四个亚单位中至少一个的基因。 这个， 然而，将定义来自每个领域的四个领域中的每个领域的结构， 四个亚基，因为每个亚基都必须折叠成相同的， 可重叠模式

Acetylcholine receptor is a protein that spans the plasma membrane and forms a gated channel for cations between the cytoplasm and the exterior of the muscle cell. The portion of this channel that passes through the membrane itself is formed by a ring of five, juxtaposed, hydrophobic domains, one from each of the five subunits. It is the long-term objective of this project to define the precise molecular structure of each of these domains and, thereby, that of the ring itself and the channel for the cations. The four membrane-spanning sequences in each of the four subunits of acetylcholine receptor have been identified, and Kendrew-Watson wire models have been constructed of the four membrane-spanning helices of the Gamma subunit. These four hilices have been aligned to create a hypothetical molecular model for the membrane-spanning domain of this protein. This model contains two intrasubunit disulfide bonds within the bilayer. These two disulfides will be sought within the native structure of acetylcholine receptor through chemical experiments performed on the purified protein. Peptides containing these disulfides will be isolated and identified by methods that have been used successfully to purify the membrane-spanning portions of (Na+ + K+)-ATPase. If these disulfides both exist, there is a high probability that the hypothetical model is correct. If they do not, other intrasubunit disulfides may occur within the membrane-spanning domains of other subunits in the native structure, and these will also be sought. If every one of the eleven unique cysteine residues within the membrane, however, are free, cross-linking agents will be attached to any of them that are accessible from the alkane core of the bilayer. Covalent cross-links between these cysteines and neighboring amino acid side chains will be formed, and the products of these cross-linking reactions will be purified and identified. If one or two of any of the foregoing chemical experiments provide positive results, they should define a unique molecular structure for the membrane-spanning domain of at least one of the four subunits of acetylcholine receptor. This, however, will define the structure of each of the four domains from each of the four subunits because every subunit is necessarily folded in the same, superimposable pattern.