Identification of amino acid residues within GABA(A) receptor beta subunits that mediate both homomeric and heteromeric receptor expression

GABA(A) receptors are believed to be heteropentamers that can be constructe d from six subunit classes: alpha(1-6), beta(1-4), gamma(1-3), delta, epsil on, and pi. Given that individual neurons often express multiple receptor s ubunits, it is important to understand how these receptors assemble. To det ermine which domains of receptor subunits control assembly we have exploite d the differing capabilities of the beta 2 and beta 3 subunits to form func tional cell surface homomeric receptors. Using a chimeric approach, we have identified four amino acids in the N-terminal domain of the beta 3 subtlni t that mediate functional cell surface expression of this subunit compared with beta 2, which is retained within the endoplasmic reticulum. Substituti on of these four amino acids-glycine 171, lysine 173, glutamate 179, and ar ginine 180-into the beta 2 subunit was sufficient to enable the beta 2 subu nit to homo-oligomerize. The effect of this putative "assembly signal" on t he production of heteromeric receptors composed of crp and pr subunits was also analyzed. This signal was not critical for the formation of receptors composed of either alpha 1 beta 2 or alpha 1 beta 3 subunits, suggesting th at mutation of these residues did not disrupt subunit folding. However, thi s signal was important in the formation of beta gamma 2 receptors. These re sidues did not seem to affect the initial association of beta 2 and gamma 2 subunits but appeared to be important for the subsequent production of fun ctional receptors. Our studies identify, for the first time, key residues w ithin the N-terminal domains of receptor beta subunits that mediate the sel ective assembly of GABA(A) receptors.