Synthetic peptides and four-helix bundle proteins as model systems for the pore-forming structure of channel proteins. II. Transmembrane segment M2 of the brain glycine receptor is a plausible candidate for the pore-lining structure

Abstract

A synthetic 23-mer peptide (M2GlyR) with the amino acid sequence of the putative transmembrane segment M2 of the strychnine-binding alpha subunit of the inhibitory glycine receptor forms anion-selective channels in phospholipid bilayers. The most frequent events show single-channel conductances, gamma, of 25 pS and 49 pS in symmetric 0.5 M KCl with channel open lifetimes, tau o, in the millisecond time range. These properties match those of authentic glycine receptors studied in inside-out patches of cultured rat spinal cord neurons, namely gamma = 27 pS and gamma = 45 pS, and tau o in the millisecond time range. The channel activity of M2GlyR is sequence-specific: 1) a synthetic peptide with the sequence of putative transmembrane segment M1 (M1GlyR), not considered to contribute to the channel lining, does not form channels; 2) an analog of M2GlyR with site-specific substitutions displays distinct channel properties: 2 arginine residues at the N and C termini of M2, postulated to contribute to the anion selectivity of the channel, are substituted by glutamic acids, and the analog peptide ([Glu3,22]M2GlyR) forms cation-selective channels. Further, a four-helix bundle protein (T4M2GlyR) formed by tethering four identical M2GlyR modules to a carrier template forms homogeneous anion-selective channels with gamma = 25 pS in 0.5 M KCl. These channels are blocked by picrotoxin and by the anion channel blockers 9-anthracene carboxylic acid and niflumic acid, but not by an analog of the local anesthetic lidocaine (QX-222), a cation channel blocker. Observed single-channel properties suggest that a pentameric assembly of alpha and beta subunits with a central pore lined by M2 segments would account for conductance properties of the authentic glycine receptor and the 2 arginines at either end of M2 could confer anion specificity to the receptor channel.