A GABA‐A alpha1 third transmembrane cysteine mutant with unusual sensitivity to allopregnanolone

Abstract

GABA‐A receptors are known to be modulated by neurosteroids. At wild type receptors, low concentrations of allopregnanolone can potentiate GABA currents while higher concentrations can activate GABA‐A receptors. Regions of the receptor involved in the binding or transduction of steroid are unclear. Regions may be N terminal to M2 or in M4 or in the cavities between transmembrane domains. While investigating the third transmembrane domain (M3) of the rat alpha1 subunit, I discovered a cysteine mutant that has unusual sensitivity to allopregnanolone. When expressed with wild type beta1 and gamma2s subunits, a mutant in alpha1 M3, S299C, showed activation by allopregnanolone with an EC50 in the range of 60–90 nM. This EC50 is normally in the range for potentiation at wild type receptors. This data could allow a parallel to the data gathered for ethanol at S270 and A291: to pinpoint a potential binding site or transduction area for neurosteroids. Further, this residue would likely be located in the cavities between transmembrane domains where some steroid actions have been localized.