Sequences in the Amino Termini of GABA ρ and GABAA 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 alpha, beta, gamma, delta, and epsilon/chi subunits, whereas GABA(C) receptors appear to contain rho subunits. However, retinal cells exhibiting GABA(C) responses express alpha, beta, and rho 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 alpha1, alpha5, and beta1 did not coimmunoprecipitate with full-length rho1, rho2, or the N-terminal domain of rho1 that contains signals for rho-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 beta1 to rho1 created a beta1rho1 chimera that coimmunoprecipitated with the alpha1 subunit but not with the rho2 subunit. Furthermore, exchanging the N terminus of the rho1 subunit with the corresponding region of beta1 produced a rho1beta1 chimera that interfered with rho1 receptor expression in Xenopus oocytes, whereas the full-length beta1 subunit had no effect. Together, these results indicate that sequences in the N termini direct assembly of rho subunits and GABA(A) subunits into GABA(C) and GABA(A) receptors, respectively.