Summary: Neuropharmacology 41 (2001) 539545
Evidence for distinct conformations of the two 1 subunits in
diazepam-bound GABAA receptors
Daniel B. Williams a, b
, Myles H. Akabas a,*
Departments of Physiology and Biophysics and of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, USA
Integrated Program in Cellular, Molecular and Biophysical Studies, Columbia University, New York 10032, USA
Received 22 January 2001; received in revised form 19 April 2001; accepted 12 June 2001
Benzodiazepines allosterically modulate GABAA receptors to increase currents induced by submaximal GABA concentrations.
Benzodiazepine-induced conformational changes in the transmembrane domain increase the reactivity of cysteines substituted for
a subset of residues in the 1 subunit M3 membrane-spanning segment. With the cysteine-substitution mutant 1F296C12 we
previously noted that p-chloromercuribenzenesulfonate (pCMBS ) modification in the presence of diazepam potentiated subsequent
GABA-induced currents. In contrast, pCMBS modification in the presence of GABA caused inhibition of subsequent responses.
We now show that in the presence of diazepam, pCMBS only reacts with the engineered cysteine in one of the two subunits;
whereas, in the presence of GABA, pCMBS reacts with the cysteine in the other subunit, or with both cysteines. This implies
that the two subunits have distinct conformations in the diazepam-bound state. Based on analysis of single channel kinetic data,