Concatenated γ-aminobutyric acid type A receptors revisited: Finding order in chaos

gamma-aminobutyric acid type A receptors (GABA(A)Rs), the major inhibitory neurotransmitter receptors in the mammalian central nervous system, are arguably the most challenging member of the pentameric Cys-loop receptors to study due to their heteromeric structure. When two or more subunits are expressed together in heterologous systems, receptors of variable subunit type, ratio, and orientation can form, precluding accurate interpretation of data from functional studies. Subunit concatenation is a technique that involves the linking of individual subunits and in theory allows the precise control of the uniformity of expressed receptors. In reality, the resulting concatemers from widely used constructs are flexible in their orientation and may therefore assemble with themselves or free GABA(A)R subunits in unexpected ways. In this study, we examine functional responses of receptors from existing concatenated constructs and describe refinements necessary to allow expression of uniform receptor populations. We find that dimers from two commonly used concatenated constructs, beta-23-alpha and alpha-10-beta, assemble readily in both the clockwise and the counterclockwise orientations when coexpressed with free subunits. Furthermore, we show that concatemers formed from new tetrameric alpha-10-beta-alpha-beta and alpha-10-beta-alpha-gamma constructs also assemble in both orientations with free subunits to give canonical alpha beta gamma receptors. To restrict linker flexibility, we systematically shorten linker lengths of dimeric and pentameric constructs and find optimized constructs that direct the assembly of GABA(A)Rs only in one orientation, thus eliminating the ambiguity associated with previously described concatemers. Based on our data, we revisit some noncanonical GABA(A)R configurations proposed in recent years and explain how the use of some concatenated constructs may have led to wrong conclusions. Our results help clarify current contradictions in the literature regarding GABA(A)R subunit stoichiometry and arrangement. The lessons learned from this study may guide future efforts in understanding other related heteromeric receptors.