Structure and dynamics of neurosteroid binding to the αβγ GABA receptor.

Neurosteroids are the principal endogenous modulators of the γ-Aminobutyric acid receptors (GABARs), pentameric membrane-bound proteins that can be assembled from at least 19 subunits. In the most abundant GABAR arrangement (αβγ), neurosteroids can potentiate the GABA action as well as produce a direct activation of the channel. The recent crystal structures of neurosteroids bound to α homopentameric GABAR reveal binding to five equivalent sites. However, these results have been obtained using receptors that are not physiologically relevant, suggesting a need to investigate neurosteroid binding to heteropentameric receptors that exist in the central nervous system. In a previous work, we predicted the neurosteroid binding site by applying molecular modeling methods on the β homopentamer. Here we construct a homology model of the transmembrane domain of the heteropentameric αβγ receptor and then, by combining docking and molecular dynamics simulations, we analyzed neurosteroid binding. Results show that the five neurosteroid cavities are conserved in the αβγ receptor and all of them are able to bind neurosteroids. Two different binding modes were detected depending on the identity of the residue at position 241 in the transmembrane helix 1. These theoretical findings provide microscopic insights into neurosteroid binding at the heteropentameric GABAR. The existence of two classes of sites may be associated with how neurosteroids modulate GABAR. Our finding would represent the essential first step to reach a comprehensive understanding of how these endogenous molecules regulate the central nervous system.