Zinc ions are concentrated in the central nervous system and regulate GABAA receptors, which are pivotal mediators of inhibitory synaptic neurotransmission. Zinc ions inhibit GABAA receptor function by an allosteric mechanism that is critically dependent on the receptor subunit composition: αβ subunit combinations show the highest sensitivity, and αβγ isoforms are the least sensitive. Here we propose a mechanistic and structural basis for this inhibition and its dependence on the receptor subunit composition. We used molecular modeling to identify three discrete sites that mediate Zn2+ inhibition. One is located within the ion channel, and the other two are on the external amino (N)-terminal face of the receptor at the interfaces between α and β subunits. We found that the characteristically low Zn2+ sensitivity of GABAA receptors containing the γ2 subunit results from disruption to two of the three sites after receptor subunit co-assembly.
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This work was supported by the Medical Research Council (UK). E.L.D. was a University of London Maplethorpe Research Fellow. We thank P. Thomas and P. Miller for helpful comments.
Schematic model of Zn2+ inhibition on the GABAA receptor. The α1β3 and α1β3γ2 GABAA receptors are viewed as plans and also as transected sections that span the membrane. The plan view depicts the interfacial nature of the GABA, benzodiazepine (BZ) and Zn2+ binding sites where they are intact (black spheres) or disrupted (shaded spheres). In the transected views, the location of receptor subunit interfaces, the GABA binding site and the determinants of Zn2+ potency (shaded ovals) were derived by comparison with the AChBP30. The GABA and BZ binding sites, and the domains delineated by α1E122,D123, α1E137,H141, and β3E182, are located externally on the receptor (solid lines, shown also on the oblique side of the receptor as broken lines) in contrast to the channel mouth location of H267 and E270 in the β3 subunit. The three postulated types of Zn2+ binding site, formed by H267 and E270, and by E137, H141 and E182, act in a concerted, allosteric fashion to inhibit receptor function. The channel site will be severely disrupted by introduction of γ2 subunits. (PDF 503 kb)
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