Dani J A
Baylor College of Medicine, Department of Physiology, Houston, Texas 77030.
J Neurosci. 1989 Mar;9(3):884-92. doi: 10.1523/JNEUROSCI.09-03-00884.1989.
Clonal BC3H-1 muscle cells were studied using patch-clamp techniques. The structure and ion binding sites of the nicotinic ACh receptor channel were examined by measuring permeability ratios and streaming potentials. The permeability ratio of lithium to ammonium remained constant from 10 to 150 mM. That result indicates there is one primary binding site in the narrowest region of the channel over the concentration range tested. There are, however, other binding sites and many ions in the large entrance vestibules. The sites in the wider regions of the channel influence ionic permeation, but the main determinant of transport is the site directly in the permeation pathway. An estimate of the length of the narrow region was obtained from streaming potential measurements. The streaming potential is directly related to the number of water molecules coupled to the transport of a permeant ion through the pore. Under proper experimental conditions, streaming potential measurements indicated that the narrowest cross section of the pore holds only about 6 water molecules. Therefore, the narrowest cross section is very short, and it contains one main binding site. The overall results are consistent with the pore being lined by transmembrane helices with a low charge density. Since the open pore has a short narrow region, the helices that rim the pore may spread out from the narrowest cross section. The widening of the pore would expose a second set of transmembrane helices at the interstices between the first set of helices.
使用膜片钳技术对克隆的BC3H-1肌肉细胞进行了研究。通过测量渗透率比和流动电位来检查烟碱型乙酰胆碱受体通道的结构和离子结合位点。锂与铵的渗透率比在10至150 mM范围内保持恒定。该结果表明，在所测试的浓度范围内，通道最窄区域存在一个主要结合位点。然而，在大的入口前庭中有其他结合位点和许多离子。通道较宽区域的位点影响离子渗透，但运输的主要决定因素是直接位于渗透途径中的位点。通过流动电位测量获得了狭窄区域长度的估计值。流动电位与通过孔渗透离子的运输所耦合的水分子数量直接相关。在适当的实验条件下，流动电位测量表明孔的最窄横截面仅容纳约6个水分子。因此，最窄横截面非常短，并且它包含一个主要结合位点。总体结果与由低电荷密度的跨膜螺旋排列的孔一致。由于开放孔有一个短的狭窄区域，围绕孔的螺旋可能从最窄横截面展开。孔的变宽将在第一组螺旋之间的间隙处暴露第二组跨膜螺旋。
