Glutamate 172, essential for modulation of L super(247)T alpha sub(7) ACh receptors by Ca super(2+), lines the extracellular vestibule

Neuronal alpha sub(7) nicotinic ACh receptors (nAChRs) are permeable to and modulated by Ca super(2+), Ba super(2+), and Sr super(2+). These permeant divalent cations interact with slowly desensitizing L super(247)T alpha sub(7) nAChRs to increase the potency and maximal efficacy of ACh, increase the efficacy of dihydro- beta -erythroidine (DH beta E), and increase agonist-independent activity. Mutation of glutamate 172 (E super(172)) to glutamine or cysteine eliminated these effects of permeant divalent cations. 2-(Trimethylammonium)ethyl methanethiosulfonate (MTSET), a cysteine-modifying reagent directed at water-accessible thiols, inhibited ACh-evoked currents of E super(172)C/L super(247)T alpha sub(7) nAChRs by >90%, demonstrating that E super(172) was accessible to permeant ions. The data are consistent with a model of alpha sub(7) receptors, derived from the crystal structure of the ACh binding protein (AChBP) from Lymnaea stagnalis, in which E super(172) projects toward the lumen of the extracellular vestibule. The observations that E super(172) was essential for divalent cation modulation of L super(247)T alpha sub(7) nAChRs and was accessible to permeating ions suggest that this residue participates in coupling ion permeation with modulation of receptor activity.