Residues in the first transmembrane domain of the Caenorhabditis elegans GABA A receptor confer sensitivity to the neurosteroid pregnenolone sulfate

Abstract

1 The GABA A receptor is a target of endogenous and synthetic neurosteroids. Little is known about the residues required for neurosteroid action on GABA A receptors. We have investigated pregnenolone sulfate (PS) inhibition of the Caenorhabditis elegans UNC-49 GABA receptor, a close homolog of the mammalian GABA A receptor. 2 The UNC-49 locus encodes two GABA receptor subunits, UNC-49B and UNC-49C. UNC-49C is sensitive to PS but UNC-49B is not sensitive. By analyzing chimeric receptors and receptors containing site-directed mutations, we identified two regions required for PS inhibition. 3 Four residues in the first transmembrane domain are required for the majority of the sensitivity to PS, but a charged extracellular residue at the end of the M2 helix also plays a role. Strikingly, mutation of one additional M1 residue reverses the effect of PS from an inhibitor to an enhancer of receptor function. 4 Mutating the M1 domain had little effect on sensitivity to the inhibitor picrotoxin, suggesting that these residues may mediate neurosteroid action specifically, and not allosteric regulation in general. © 2006 Nature Publishing Group All rights reserved.