Subcellular localization and endocytosis of homomeric γ2 subunit splice variants of γ-aminobutyric acid type A receptors

The expression of a and ß ?-aminobutyric acid type A receptor subunits produces GABA-gated channels which require the incorporation of either the ?2 or ?3 subunit for benzodiazepine modulation. Here we examine the role of the ?2 subunit splice variants, ?2S and ?2L which differ by eight amino acids
in the major intracellular domain, in mediating cell surface expression. Using immunocytochemistry we have demonstrated that when expressed alone, the ?2S subunit can access the cell surface and internalize constitutively. In contrast, a1, ß2and ?2L are retained predominantly in the endoplasmic reticulum (ER)
when expressed alone. Replacing the insert which differentiates ?2L from
?2S (LLRMFSFK) with eight alanines produces a phenotype identical to ?2S. Both ?2 subunits fail to produce high molecular weight oligomers observed for a1ß2 and a1ß2?2 heterooligomers and do not form functional ion channels. Surface
expression of ?2S is repressed upon the coexpression of a1 or ß2 subunits, resulting in ER-retained heterooligomers, suggesting that homomeric
?2S is unlikely to occur in vivo. However, its independent maturation
to surface competence and preferential assembly with a and ß subunits may ensure the production of functional benzodiazepine-sensitive receptors. Furthermore, the presence of the ?2 subunit appears to confer an endocytotic capacity to these heterooligomeric receptors.