Impact of subunit positioning on GABA_A receptor function

The major isoforms of the GABA_A (γ-aminobutyric acid type A) receptor are composed of two α, two β and one γ subunit. Thus α and β subunits occur twice in the receptor pentamer. As it is well documented that different isoforms of α and β subunits can co-exist in the same pentamer, the question is raised whether the relative position of a subunit isoform affects the functional properties of the receptor. We have used subunit concatenation to engineer receptors of well-defined subunit arrangement to study this question. Although all five subunits may be concatenated, we have focused on the combination of triple and dual subunit constructs. We review here what is known so far on receptors containing simultaneously α₁ and α₆ subunits and receptors containing β₁ and β₂ subunits. Subunit concatenation may not only be used to study receptors containing two different subunit isoforms, but also to introduce a point mutation into a defined position in receptors containing either two α or β subunits, or to study the receptor architecture of receptors containing unconventional GABA_A receptor subunits. Similar approaches may be used to characterize other members of the pentameric ligand-gated ion channel family, including nicotinic acetylcholine receptors, glycine receptors and 5-HT₃ (5-hydroxytryptamine) receptors.