Channel-lining residues of the AMPA receptor M2 segment: Structural environment of the Q/R site and identification of the selectivity filter

In AMPA receptor channels, a single amino acid residue (Q/R site) of the M2 segment controls permeation of calcium ions, single-channel conductance, b lockade by intracellular polyamines, and permeation of anions. The structur al environment of the Q/R site and its positioning with regard to a narrow constriction were probed with the accessibility of substituted cysteines to positively and negatively charged methanethiosulfonate reagents, applied f rom the extracellular and cytoplasmic sides of the channel. The accessibili ty patterns confirm that the M2 segment forms a pore loop with the Q/R site positioned at the tip of the loop (position 0) facing the extracellular ve stibule. Cytoplasmically accessible residues on the N- and C-terminal sides of position 0 form the ascending alpha -helical (-8 to -1) and descending random coil (+1 to +6) components of the loop, respectively. Substitution o f a glycine residue at position +2 with alanine strongly decreased the perm eability of organic cations, indicating that position +2 contributes to the narrow constriction. The anionic 2-sulfonatoethyl-methanethiosufonate reac ted with a cysteine at position 0 only from the external side and with cyst eines at positions +1 to +4 only from the cytoplasmic side. These results s uggest that charge selectivity occurs external to the constriction (+2) and possibly involves interactions of ions with the negative electrostatic pot ential created by the dipole of the alpha -helix formed by the ascending li mb of the loop.