Coexistence of two β subunit isoforms in the same γ-aminobutyric acid type A receptor

Abstract

Three novel subunit-specific antisera to the β1, β2, and β3 subunits of rat γ-aminobutyric acid type A (GABA(A)) receptors have been used to study the native receptor in the rat brain. Affinity-purified anti- β1, anti-β2, and anti-β3 antibodies recognized in immunoblots protein hands of 57, 55, and 57 kDa, respectively. Quantitative immunoprecipitation of solubilized GABA(A) receptors from various rat brain regions showed that the β2 subunit was the most abundant isoform in cerebellum (in 96% of the GABA(A) receptors) and cerebral cortex (64%) but that it was the least abundant isoform in hippocampus (44%). The β3 subunit was found most abundant in hippocampus (64%) followed by cerebral cortex (48%) and cerebellum (33%). The β1 subunit was present in a very small proportion of the cerebellar GABA(A) receptors (3%), but it was present in a high proportion of the GABA(A) receptors from the hippocampus (49%) and cerebral cortex (32%). Quantitative receptor immunoprecipitation or immunopurification followed by immunoblotting experiments have revealed the existence of colocalization of two different β subunit isoforms in a significant proportion of the brain GABA(A) receptors. Thus, in the rat cerebral cortex 33% of the GABA(A) receptors have both β2 and β3 subunits, and 19% of the receptors have both β1 and β3 subunits. The extent of colocalization of β subunit isoforms varied among brain regions, being highest in hippocampus and lowest in cerebellum. These and other results taken together suggest that the number of α, β, and γ subunits (stoichiometry) in the brain GABA(A) receptor pentamers might not be unique. It might vary depending on receptor type.