Multiple assembly signals in g-aminobutyric acid (type A) receptor subunits combine to drive receptor construction and composition

Mammalian gamma-aminobutyric acid type A (GABA(A)) receptors are constructed from a large repertoire of subunits (alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, theta and pi) into a pentameric ion channel. GABA(A) receptor assembly occurs within the endoplasmic reticulum (ER) and involves interactions with chaperone molecules. only specific subunit combinations can produce functional surface receptors (with a fixed stoichiometry); other subunit combinations are retained within the ER and degraded. Thus, receptor assembly occurs by defined pathways to limit the diversity of GABA(A) receptors. The key to understanding how receptor diversity is achieved and controlled is the identification of assembly signals capable of distinguishing between other subunit partners. Analysis of an assembly box in alpha1 (residues 57-68) has revealed an absolute requirement for this region in the assembly of alphabeta receptors. Furthermore, a selective requirement for a single amino acid (R66) is observed for the assembly of alpha1beta2, but not alpha1beta1 or alpha1beta3, receptors. in addition, we have characterized an assembly signal in the beta3 subunit that is capable of driving the assembly of beta3, gamma2beta3 and alpha1beta3 receptors. Interestingly, this signal does not appear to utilize the alpha1 assembly box, suggesting the presence of alternative assembly signals within the alpha1 subunit. Although this beta3 signal is sufficient to permit the formation of betagamma receptors it is not necessary, suggesting that alternative assembly signals also exist within the beta3 subunit. These findings support the belief that GABA(A) receptor assembly occurs via multiple defined pathways that may be determined by subunit availability.