Homomeric rho 1 GABA channels: activation properties and domains

Expression of human rho 1 subunits in Xenopus laevis oocytes forms homomeric GABA-activated chloride channels with activation and pharmacological properties distinct from those of typical heteromeric GABA channels (e.g., alpha 1 beta 2 gamma 2). Here, we describe these rho 1 activation features and use site-directed mutagenesis to identify amino acids involved in GABA-mediated activation. In comparison to heteromeric GABA channels (alpha 1 beta 2 gamma 2), GABA channels comprised of rho 1 subunits are approximately 40-fold more sensitive to GABA, activate 8.3-fold more slowly (at a GABA concentration equal to their respective EC50S), do not desensitize with maintained agonist application, and close approximately 8-fold more slowly after agonist removal. Site-directed mutagenesis of rho 1 GABA channels has identified five amino acids (Y198, Y200, Y241, T244, and Y247) located between the N-terminal extracellular cysteine loop and first membrane spanning domain, that when conservatively mutated, significantly impaired GABA-mediated activation. These five residues are grouped in two domains that correspond in position to the putative agonist-binding domains previously identified for the beta 2 subunit of alpha 1 beta 2 gamma 2 GABA channels. Y198, T244, and Y247 correspond directly to crucial amino acids identified in the beta 2 subunit; Y200 and Y241 do not. These differences may account, in part, for the unique activation and pharmacological features of homomeric rho 1 GABA channels.