High-level expression and purification of Cys-loop ligand-gated ion channels in a tetracycline-inducible stable mammalian cell line: GABAA and serotonin receptors.

The human neuronal Cys-loop ligand-gated ion channel superfamily of ion channels are important determinants of human behavior and the target of many drugs. It is essential for their structural characterization to achieve high-level expression in a functional state. The aim of this work was to establish stable mammalian cell lines that enable high-level heterologous production of pure receptors in a state that supports agonist-induced allosteric conformational changes. In a tetracycline-inducible stable human embryonic kidney cells (HEK293S) cell line, GABA(A) receptors containing α1 and β3 subunits could be expressed with specific activities of 29-34 pmol/mg corresponding to 140-170 pmol/plate, the highest expression level reported so far. Comparable figures for serotonin (5-HT(3A)) receptors were 49-63 pmol/mg and 245-315 pmol/plate. The expression of 10 nmol of either receptor in suspension in a bioreactor required 0.3-3.0 L. Both receptor constructs had a FLAG epitope inserted at the N-terminus and could be purified in one step after solubilization using ANTI-FLAG affinity chromatography with yields of 30-40%. Purified receptors were functional. Binding of the agonist [(3)H]muscimol to the purified GABA(A)R was enhanced allosterically by the general anesthetic etomidate, and purified 5-hydroxytryptamine-3A receptor supported serotonin-stimulated cation flux when reconstituted into lipid vesicles.