Amphipathic analysis and possible formation of the ion channel in an acetylcholine receptor.

Abstract

Fourier analysis of the hydrophobicities of the acetylcholine receptor subunit sequences reveals regions of amphipathic secondary structure. Prediction of a consensus secondary structure based on this analysis and on an empirical prediction method leads to a testable hypothesis about how the ion channel is formed and might function. Knowledge of the three-dimensional structure of acetylcholine receptors is consistent with features of the model proposed and provides some constraints.The acetylcholine receptor (AcChoR) is an '-'250-kilodalton (kDa) complex of five homologous glycoprotein subunits in stoichiometry a2P3y8 (1)(2)(3). The four subunits each contain four hydrophobic stretches of at least 20-25 contiguous residues (4-9). The structural similarity of the five subunits and the transmembrane orientation of secondary structural features (10) imply that they surround the central ion channel like staves (11). This is the simplest arrangement that preserves quasi-equivalent interactions at the subunit interfaces. Huang et al. (12) conclude that the channel is a waterfilled pore containing hydrogen-accepting groups and at least one negatively charged site. Lewis and Stevens (13) calculated the simplest potential energy surface that can account for electrophysiological data (14). Their profile is characterized by two energy barriers separated by a shallow negative energy well, which Horn and Stevens (15) hypothesize results from one or more negatively charged sites within the channel. The dependence of conduction upon ion size for both monovalent and divalent cations has led Lewis and Stevens to conclude (i) that the ion channel environment is similar to that of free water, (ii) that the ion channel may be lined with hydrophilic residues, but (iii) that ion selectivity does not result from direct interaction of ions with amino acid side chains (16).Inspection of the y-subunit sequence for regions that might interface with a hydrophobic domain and form a hydrophilic wall of the ion channel identifies regions in which charged or very hydrophilic residues are interspersed with neutral or hydrophobic residues in a periodic fashion characteristic of regular secondary structures. This observation prompted us to quantitate the local periodicity in the hydrophobicities of the primary structures of all subunits by Fourier analysis. McLachlan has discussed Fourier analysis as a means of identifying periodic properties of amino acid sequences (17). We used Fourier analysis for amphipathic secondary structure correlation together with a secondary structure prediction method for the -75% sequence external to the bilayer and separate evidence for oriented helices in the transmembrane regions to develop an overall structural scheme. The overall predictions gain statistical significance from the congruence of all four subunit types and are consistent with the low-resolution, three-dimensional structure of AcChoR. METHODSThe premise for the analytic method is that regular secondary structural features located at an inter...

Helix Stability in Barstar Peptides

“…A very similar measure of amphipathy, although formally different, has been used by Finer-Moore and Stroud (1984) and Finer-Moore et al (1989), who used the power spectrum of the Fourier transform of the hydrophobic profile. We have found a quantitative relationship for our peptides between the amphipathic index, as defined in Eqn (3, and their helical contents in water and in trifluoroethanol (in terms of dG) (Figs4 and 6).…”

“…Periodicity in the hydrophobicity pattern was analyzed according to Finer-Moore and Stroud (1984), calculating the power spectrum of the hydrophobic profile with the expression:…”

Effects of chlorpromazine and phencyclidine on mouse C2 acetylcholine receptor kinetics.

“…On this basis, however, one would expect the concentration profiles (Fig. 4) (Oswald & Changeux, 1981;Heidmann et al 1983) suggests that it is located along the axis of fivefold symmetry which has been assumed (yet without proof) to trace the path of the permeant ions through the receptor molecule (see Changeux et al 1984;Guy, 1984;Finer-Moore & Stroud, 1984 (Giraudat, Dennis, Heidmann, Chang & Changeux, 1986) or, indirectly, by in vitro mutagenesis (Mishina, Kurosaki, Tabimatsu, Morimoto, Noda, Yamamoto, Terao, Lindstrom, Takahashi, Kuno & Numa, 1984) might lead to the demonstration of the correct model among the diverse ones suggested.…”

A synaptic basis for T-lymphocyte activation

“…Instead of separate independent channel sub-units, the AChR has five stably attached homologous chains [60]. The actual binding site for acetylcholine is thought to be in a region with β-pleated sheet characteristics [61] suggestive of a domain-like structure. Although the exact nature of this binding is unknown, it is possible that the binding site may be between two domains like the binding suggested above for T cells, and that this binding pulls together these domains resulting in a receptor conformational change, opening the channel.…”