The γ-aminobutyric acid type A (GABA A ) receptor-associated protein (GABARAP) promotes GABA A receptor clustering and modulates the channel kinetics

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Abstract
A microtubule-associated protein, γ-aminobutyric acid type A (GABAA) receptor-associated protein (GABARAP), was previously identified as binding to the intracellular domain of GABAA receptors by using the yeast two-hybrid screen. In the present work, immunofluorescent staining and green fluorescent protein-tagged receptor subunits showed that GABARAP is associated with and promotes the clustering of GABAA receptors in QT-6 quail fibroblasts. The tubulin-binding motif of GABARAP and the γ2 subunit of the receptor are required. Disruption of microtubules prevents the clustering in a time-dependent manner. When green fluorescent protein-tagged α1 or γ2 subunit coexpressed with β2, γ2L, and GABARAP was used, recordings from visually identified cells revealed that clustered GABAA receptor had an EC50 of about 20 μM, vs. 5.7 μM for the diffuse receptor. Clustered receptors deactivated faster and desensitized slower than the diffuse receptors, because of decrease in the apparent affinity of GABA binding. Different properties for clustered receptors relative to unclustered receptors in heterologous cells suggest that homologous differences between extrasynaptic and synaptic clustered receptors in neurons may be due to the organization of the postsynaptic machinery.