Subcellular localization and endocytosis of homomeric gamma 2 subunit splice variants of gamma-aminobutyric acid type A receptors

The expression of alpha and beta gamma-aminobutyric acid type A receptor su bunits produces GABA-gated channels which require the incorporation of eith er the gamma 2 or gamma 3 subunit for benzodiazepine modulation. Here we ex amine the role of the gamma 2 subunit splice variants, gamma 2S and gamma 2 L which differ by eight amino acids in the major intracellular domain, in m ediating cell surface expression. Using immunocytochemistry we have demonst rated that when expressed alone, the gamma 2S subunit can access the cell s urface and internalize constitutively. In contrast, alpha 1, beta 2 and gam ma 2L are retained predominantly in the endoplasmic reticulum (ER) when exp ressed alone. Replacing the insert which differentiates gamma 2L from gamma 2S (LLRMFSFK) with eight alanines produces a phenotype identical to gamma 2S. Both gamma 2 subunits fail to produce high molecular weight oligomers o bserved for alpha 1 beta 2 and alpha 1 beta 2 gamma 2 heterooligomers and d o not form functional ion channels. Surface expression of gamma 2S is repre ssed upon the coexpression of alpha 1 or beta 2 subunits, resulting in ER-r etained heterooligomers, suggesting that homomeric gamma 2S is unlikely to occur in vivo. However, its independent maturation to surface competence an d preferential assembly with alpha and beta subunits may ensure the product ion of functional benzodiazepine-sensitive receptors. Furthermore, the pres ence of the gamma 2 subunit appears to confer an endocytotic capacity to th ese heterooligomeric receptors.