Role of the Outer beta -Sheet in Divalent Cation Modulation of alpha 7 Nicotinic Receptors

• Published in: Molecular pharmacology 2006-07, Vol.70 (1), p.16-22
• Main Authors: McLaughlin, James T, Fu, Jie, Sproul, Adrian D, Rosenberg, Robert L
• Format: Article
• Language: English
• ISSN: 0026-895X
• DOI: 10.1124/mol.106.023259

alpha -7 Nicotinic acetylcholine receptors (AChRs) exhibit a positive modulation by divalent cations similar to that observed in other AChRs. In the chick alpha 7 AChR, this modulation involves a conserved glutamate in loop 9 (Glu super(172)) that undergoes agonist-dependent movements during activation. From these observations, we hypothesized that movements of the nearby beta -sheet formed by the beta 7, beta 9, and beta 10 strands may be involved in agonist activation and/or divalent modulation. To test this hypothesis, we examined functional properties of cysteine mutations of the beta 7 and beta 10 strands, alone or in pairs. We postulated that reduced flexibility or mobility of the beta 7/ beta 9/ beta 10-sheet as a result of introduction of a disulfide bond between the beta strands would alter activation by agonists. Using a nondesensitizing alpha 7 mutant background (L super(247)T), we identified one mutant pair, K super(144)C + T super(198)C, that exhibited a unique characteristic: it was fully activated by divalent cations (Ca super(2+), Ba super(2+), or Sr super(2+)) in the absence of acetylcholine (ACh). Divalent-evoked currents were blocked by the alpha 7 antagonist methyllycaconitine and were abolished when Glu super(172) was mutated to glutamine. When the K super(144)C + T super(198)C pair was expressed in wild-type alpha 7 receptors, activation required both ACh and divalent cations. We conclude that the introduction of a disulfide bond into beta 7/ beta 9/ beta 10 lowers the energetic barrier between open and closed conformations, probably by reducing the torsional flexibility of the beta -sheet. In this setting, divalent cations, acting at the conserved glutamate in loop 9, act as full agonists or requisite coagonists.