Sequences in the Amino Termini of GABA ρ and GABAA Subunits Specify Their Selective Interaction In Vitro

Abstract: Molecular cloning has revealed that there are six classes of subunits capable of forming GABA-gated chloride channel receptors. GABA_A receptors are composed of α, β, γ, δ, and ε/χ subunits, whereas GABA_C receptors appear to contain ρ subunits. However, retinal cells exhibiting GABA_C responses express α, β, and ρ subunits, raising the possibility that GABA_C receptors may be a mixture of subunit classes. Using in vitro translated protein, we determined that human GABA_A receptor subunits α1, α5, and β1 did not coimmunoprecipitate with full-length ρ1, ρ2, or the N-terminal domain of ρ1 that contains signals for ρ-subunit interaction. To explore the molecular mechanism underlying these apparently exclusive combinations, chimeric subunits were created and tested for interaction with the wild-type subunits. Transfer of the N terminus of β1 to ρ1 created a β1ρ1 chimera that coimmunoprecipitated with the α1 subunit but not with the ρ2 subunit. Furthermore, exchanging the N terminus of the ρ1 subunit with the corresponding region of β1 produced a ρ1β1 chimera that interfered with ρ1 receptor expression in Xenopus oocytes, whereas the full-length β1 subunit had no effect. Together, these results indicate that sequences in the N termini direct assembly of ρ subunits and GABA_A subunits into GABA_C and GABA_A receptors, respectively.