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

摘要

class="enumerated" style="list-style-type:decimal">The sensitivity of GABAA receptors (GABARs) to Zn²⁺ inhibition depends on subunit composition. The predominant neuronal forms of mammalian GABARs, αβγ and αβδ, are differentially sensitive to Zn²⁺ inhibition; αβγ receptors are substantially less sensitive than αβδ receptors. Recently, functional domains involved in Zn²⁺ sensitivity have been identified in α and β subunits. Our aim in the present study was to localize functional domains of low Zn²⁺ sensitivity within γ2L subunits.Chimeric subunits were constructed by progressively replacing the rat γ2L subunit sequence with that of the rat δ subunit sequence. Whole-cell currents were recorded from mouse L929 fibroblasts coexpressing wild-type rat α1 and β3 subunits with a chimeric δ-γ2L subunit.Unlike α and β subunits, the γ2L 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 γ2L subunit that are homologous to those found in β and α 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.