Sequences in the amino termini of GABA ρ and GABA(A) subunits specify their selective interaction in vitro

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.