Identification of residues within GABA_A receptor subunits that mediate specific assembly with receptor β subunits

GABA(A) receptors can be constructed from a range of differing subunit isoforms: alpha, beta, gamma, delta, and epsilon. Expression studies have revealed that production of GABA-gated channels is achieved after coexpression of alpha and beta subunits. The expression of a gamma subunit isoform is essential to confer benzodiazepine sensitivity on the expressed receptor. However, how the specificity of subunit interactions is controlled during receptor assembly remains unknown. Here we demonstrate that residues 58-67 within alpha subunit isoforms are important in the assembly of receptors comprised of alpha beta and alpha beta gamma subunits. Deletion of these residues from the alpha 1 or alpha 6 subunits results in retention of either alpha subunit isoform in the endoplasmic reticulum on coexpression with the beta 3, or beta 3 and gamma 2 subunits. Immunoprecipitation revealed that residues 58-67 mediated oligomerization of the alpha 1 and alpha 3 subunits, but were without affect on the production of alpha/gamma complexes. Within this domain, glutamine 67 was of central importance in mediating the production of functional alpha 1 beta 3 receptors. Mutation of this residue resulted in a drastic decrease in the cell surface expression of alpha 1 beta 3 receptors and the resulting expression of beta 3 homomers. Sucrose density gradient centrifugation revealed that this residue was important for the production of a 9S alpha 1 beta 3 complex representing functional GABA(A) receptors.

Therefore, our studies detail residues that specify GABA(A) receptor alpha beta subunit interactions. This domain, which is conserved in all alpha subunit isoforms, will therefore play a critical role in the assembly of GABA(A) receptors composed of alpha beta and alpha beta gamma subunits.