A lysine residue in the β3 subunit contributes to the regulation of GABAA receptor activity by voltage

GABA(A) receptors (GABARs) are responsible for most fast inhibitory neurotransmission in the mammalian brain. The whole-cell currents of many native and recombinant GABARs exhibit outward rectification. This property has been shown to depend in part upon the identity of the alpha and beta subtype, but the structural determinants controlling this characteristic are not well known. An extracellular lysine residue conserved in the beta subunit family and located near the third transmembrane domain was examined. Mutations were made in the alpha1, beta3, and gamma2L sub-units, exchanging the wild-type residue for either the lysine found in the beta subunit family or the threonine found in the alpha and gamma families. GABARs containing the mutated alpha1 or gamma2L subunits showed a large increase in outward rectification. Conversely, replacing the lysine in the beta3 subunit with threonine resulted in a nearly linear current-voltage relationship and an increased sensitivity to GABA. Replacing this lysine with uncharged or negatively charged residues consistently eliminated outward rectification, with varying effects on GABA sensitivity. Similar mutations of the four other charged residues within the beta3 subunit M2-M3 domain did not alter rectification. These results suggest that lysine279 of the beta3 subunit plays an important role in the regulation of GABAR activity by membrane voltage.