Interaction between GABAA receptor subunit intracellular loops:: implications for higher order complex formation

The majority of fast inhibitory neurotransmission in the CNS is mediated by the GABA type-A (GABA(A)) receptor, a ligand-gated chloride channel. Of the approximately 20 different subunits composing the hetero-pentameric GABA(A) receptor, the gamma2 subunit in particular seems to be important in several aspects of GABA(A) receptor function, including clustering of the receptor at synapses. In this study, we report that the intracellular loop of the gamma2 subunit interacts with itself as well as with gamma1, gamma3 and beta1-3 subunits, but not with the alpha subunits. We further show that gamma2 subunits interact with photolabeled pentameric GABA(A) receptors composed of alpha1, alpha2/3 and gamma2 subunits, and calculate the dissociation constant to be in the micromolar range. By using deletion constructs of the gamma2 subunit in a yeast two-hybrid assay, we identified a 23-amino acid motif that mediates self-association, residues 389-411. We confirmed this interaction motif by inhibiting the interaction in a glutathione-S-transferase pull-down assay by adding a corresponding gamma2-derived peptide. Using similar approaches, we identified the interaction motif in the gamma2 subunit mediating interaction with the beta2 subunit as a 47-amino acid motif that includes the gamma2 self-interacting motif. The identified gamma2 self-association motif is identical to the interaction motif reported between GABA(A) receptor and GABA(A) receptor-associated protein (GABARAP). We propose a model for GABA(A) receptor clustering based on GABARAP and GABA(A) receptor subunit-subunit interaction.