Interaction between GABAA receptor β subunits and the multifunctional protein gC1q-R

gamma -Aminobutyric acid type A (GABA,) receptors were immunopurified from bovine brain using a monoclonal antibody directed against the alpha1 subunit, Of the several proteins that copurified, a 34-kDa protein was analyzed further. After enrichment and tryptic proteolysis, the resulting fragments were sequenced, and the protein was identified as gC1q-R. Using anti-gC1q-R and anti-GABA(A) receptor antibodies, mutual coimmunoprecipitation could be demonstrated from solubilized rat brain membranes. The stability of this interaction was estimated to be very high, Using the yeast two-hybrid system, various GABAA receptor subunit intracellular loop constructs were tested for an interaction with gC1q-R. All beta subunits, but not alpha1 and gamma2 subunits, were found to bind to gC1q-R, NH2- and COOH-terminally truncated beta2 subunit loops were used to find the region responsible for the interaction with gC1q-R, A stretch of 15 amino acids containing 7 positively charged residues was identified (amino acids 399-413), This region contains residue Ser-410, which is a protein kinase substrate, and it is known that phosphorylation of this residue leads to an alteration in receptor activity. Localization studies suggested a predominantly intracellular localization. Our observations therefore suggest a tight interaction between gC1q-R and the GABA(A) receptor which might be involved in receptor biosynthesis or modulation of the mature function.