Direct binding of GABAA receptor β2 and β3 subunits to gephyrin

GABAergic transmission is essential to brain function, and a large repertoire of GABA type A receptor (GABAAR) subunits is at a neuron's disposition to serve this function. The glycine receptor (GlyR)-associated protein gephyrin has been shown to be essential for the clustering of a subset of GABAAR. Despite recent progress in the field of gephyrin-dependent mechanisms of postsynaptic GABAAR stabilisation, the role of gephyrin in synaptic GABAAR localisation has remained a complex matter with many open questions. Here, we analysed comparatively the interaction of purified rat gephyrin and mouse brain gephyrin with the large cytoplasmic loops of GABAAR 1, 2, 2 and 3 subunits. Binding affinities were determined using surface plasmon resonance spectroscopy, and showed an 20-fold lower affinity of the 2 loop to gephyrin as compared to the GlyR loopgephyrin interaction. We also probed in vivo binding in primary cortical neurons by the well-established use of chimaeras of GlyR 1 that harbour respective gephyrin-binding motifs derived from the different GABAAR subunits. These studies identify a novel gephyrin-binding motif in GABAAR 2 and 3 large cytoplasmic loops.