Two γ2L subunit domains confer low Zn2+ sensitivity to ternary GABAA receptors

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Abstract
The sensitivity of GABAA receptors (GABARs) to Zn2+ inhibition depends on subunit composition. The predominant neuronal forms of mammalian GABARs, alpha(beta)gamma and, alpha(beta)delta are differentially sensitive to Zn2+ inhibition; alpha(beta)gamma receptors are substantially less sensitive than alpha(beta)delta receptors. Recently, functional domains involved in Zn2+ sensitivity have been identified in and subunits. Our aim in the present study was to localize functional domains of low Zn2+ sensitivity within gamma2L subunits. Chimeric subunits were constructed by progressively replacing the rat gamma2L subunit sequence with that of the rat delta subunit sequence. Whole-cell currents were recorded from mouse L929 fibroblasts coexpressing wild-type rat alpha1 and beta3 subunits with a chimeric delta-gamma2L subunit. Unlike alpha and beta subunits, the gamma2L subunit was found to contain a determinant of low Zn2+ sensitivity in the N-terminal extracellular region. In addition, we identified determinants in the M2 segment and the M2-M3 loop of the gamma2L subunit that are homologous to those found in beta and alpha subunits. We postulate that the interface between the latter two domains, which may form the outer vestibule of the channel, represents a single functional domain modulating Zn2+ sensitivity. Thus, the Zn2+ sensitivity of ternary GABARs appears to be determined by two functional domains, one in the N-terminal extracellular region and one near the outer mouth of the channel.