Location of a threonine residue in the α -subunit M2 transmembrane segment that determines the ion flow through the acetylcholine receptor channel

By the combination of cDNA manipulation and functional analysis of normal and mutant acetylcholine receptor (AChR) channels of Torpedo expressed in Xenopus laevis oocytes determinants of ion flow were localized in the bends bordering the putative M2 transmembrane segment (Imoto et al. 1988). We now report that in the rat muscle AChR, substitution of a threonine residue in the $\alpha $-subunit localized in the M2 transmembrane segment increases or decreases the channel conductance, depending on the size of the amino acid side chain located at this position. This threonine residue ($\alpha $T264) is located adjacent to the cluster of charged amino acids that form the intermediate anionic ring (Imoto et al. 1988). This effect is pronounced for the large alkali cations Cs$^{+}$, Rb$^{+}$, K$^{+}$ whereas for Na$^{+}$ the effect is much smaller. Taken together the results suggest that the threonine residues at position 264 in the two $\alpha $-subunits together with the amino acids of the intermediate anionic ring form part of a narrow region close to the cytoplasmic mouth of the AChR channel.