Interfacial binding sites for cholesterol on GABA_A receptors and competition with neurosteroids

γ-Aminobutyric acid type A (GABA_A) receptors in the brain are located in the outer membranes of brain cells where the concentration of cholesterol is high. Of the 25 available high-resolution structures available for GABA_A receptors, none were determined in the presence of cholesterol, but four include resolved molecules of cholesterol hemisuccinate (CHS). Here, a molecular docking procedure is used to sweep the transmembrane (TM) surfaces of the receptors for cholesterol binding sites. Cholesterol docking poses determined in this way match 89% of the resolved CHS when CHS molecules deemed unlikely to represent typical bound cholesterols are excluded. The receptors are pentameric, and their TM surfaces consist of a set of five facets, each including pairs of TM helices from two adjacent subunits. Each facet contains hydrophobic hollows running from one side of the membrane to the other, within which are six potential binding sites for cholesterol, three on each side of the membrane. High-resolution structures of GABA_A receptors with bound neurosteroids show that neurosteroids bind in these cholesterol binding sites, so the binding of neurosteroids and cholesterol will be competitive.