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Determinants of Zinc Potentiation on the 4 Subunit of Neuronal Nicotinic Receptors

Departments of Molecular & Cellular Pharmacology (B.H., K.B.M., S.E.R., D.E., A.H.M., C.W.L.) and Biochemistry & Molecular Biology (A.M.), Miller School of Medicine, University of Miami, Miami, Florida

We have shown previously that the function of neuronal nicotinic acetylcholine receptors can be modulated by zinc. This modulation varies from potentiation to inhibition, depending on receptor subunit composition and zinc concentration, with the 42 and 44 receptors displaying the most dramatic potentiation. In this study, we used site-directed mutagenesis to identify glutamate 59 and histidine 162 on the rat 4 subunit as potential mediators of zinc potentiation. By modeling the extracellular domain of the receptor pentamer, we locate these residues to two subunit-subunit interfaces that alternate with the two acetylcholine-binding interfaces. Substitution of a cysteine at either position allows additional reduction of zinc potentiation upon treatment with the methanethiosulfonate reagents N-biotinoylaminoethyl methanethiosulfonate (MTSEA-biotin) and [2-(trimethylammonium)ethyl] methanethiosulfonate. Mutagenesis and methanethiosulfonate treatment are most effective at position 162, and the presence of zinc hinders the reaction of MTSEA-biotin with the substituted cysteine at this position, suggesting that 4His162 participates in forming a coordination site for zinc. Mutagenesis and methanethiosulfonate treatment are less effective at position 59, suggesting that whereas 4Glu59 may be near the zinc coordination site, it may not be participating in coordination of the zinc ion. It is noteworthy that the position of 4Glu59 within the neuronal nAChR is identical to that of a residue that lines the benzodiazepine-binding site on GABA_A receptors. We suggest that the zinc potentiation sites on neuronal nAChRs are structurally and functionally similar to the benzodiazepine-binding sites on GABA_A receptors.

Address correspondence to: Dr. Charles W. Luetje, Molecular and Cellular Pharmacology, R-189, University of Miami, P.O. Box 016189, Miami, FL 33101. E-mail: cluetje{at}chroma.med.miami.edu

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