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GABA(A) receptor-associated protein (GABARAP) was isolated previously in a yeast two-hybrid screen using the intracellular loop of the gamma2 subunit of the GABA(A) receptor as bait. GABARAP has been shown to participate in the membrane-clustering and intracellular-trafficking of GABA(A) receptors, including a stimulation of the surface expression of GABA(A) receptors. To assess this quantitatively, we used Xenopus laevis oocytes expressing alpha1beta2gamma2S-containing GABA(A) receptors to demonstrate that coexpression of GABARAP increased net surface levels of GABA(A) receptors as shown by both increased GABA currents and surface-expressed protein. This GABARAP stimulation of GABA currents required the receptor gamma2 subunit and full-length GABARAP: deletion of the microtubule-binding domain (amino acids 1-22) or disrupting the polymerization of microtubules abolished the enhancement, indicating that the effect of GABARAP was derived from the interaction with microtubules. GABARAP coexpression did not alter the general properties of GABA(A) receptors such as sensitivity to GABA or benzodiazepines, but it increased surface levels of receptor protein in oocytes. Rather, it seems to supplement inadequate amounts of endogenous GABARAP to support optimum trafficking and/or stabilization of surface GABA(A) receptors.