Expression and purification of a functional heteromeric GABA_A receptor for structural studies

The GABA-gated chloride channels of the Cys-loop receptor family, known as GABA_A receptors, function as the primary gatekeepers of fast inhibitory neurotransmission in the central nervous system. Formed by the pentameric arrangement of five identical or homologous subunits, GABA_A receptor subtypes are defined by the subunit composition that shape ion channel properties. An understanding of the structural basis of distinct receptor properties has been hindered by the absence of high resolution structural information for heteromeric assemblies. Robust heterologous expression and purification protocols of high expressing receptor constructs are vital for structural studies. Here, we describe a unique approach to screen for well-behaving and functional GABA_A receptor subunit assemblies by using the Xenopus oocyte as an expression host in combination with fluorescence detection size exclusion chromatography (FSEC). To detect receptor expression, GFP fusions were introduced into the α1 subunit isoform. In contrast to expression of α1 alone, co-expression with the β subunit promoted formation of monodisperse assemblies. Mutagenesis experiments suggest that the α and β subunits can tolerate large truncations in the non-conserved M3/M4 cytoplasmic loop without compromising oligomeric assembly or GABA-gated channel activity, although removal of N-linked glycosylation sites is negatively correlated with expression level. Additionally, we report methods to improve GABA_A receptor expression in mammalian cell culture that employ recombinant baculovirus transduction. From these methods we have identified a well-behaving minimal functional construct for the α1/β1 GABA_A receptor subtype that can be purified in milligram quantities while retaining high affinity agonist binding activity.