Open channel structure and ion binding sites of the nicotinic acetylcholine receptor channel

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.