A single histidine residue is essential for zinc inhibition of GABA ρ1 receptors

The GABA ρ1 subunit, cloned from a human retina library, can form homooligomeric receptors with properties similar to GABA(c) receptors characterized in retinal cells. The divalent cation Zn²⁺, abundant in the CNS and retina, was found to inhibit GABA ρ1 receptors in a voltage- independent manner. Varying the extracellular pH from 7.4 to 5.6 significantly reduced this inhibitory effect. This pH profile suggested that one or more histidine residues might play a role in the interaction between Zn²⁺ and the GABA ρ1 receptor. Site-directed mutagenesis revealed that a single histidine residue (His 156) in the putative extracellular domain of ρ1 was critical for Zn²⁺ sensitivity. Substitution of this amino acid with tyrosine (H156Y) created a functional GABA receptor with agonist and channel properties indistinguishable from wildtype. However, the H156Y mutant was insensitive to Zn²⁺, even at concentrations as high as 1 mM. Mutation to aspartic acid, an amino acid that can interact with Zn²⁺ in other proteins, preserved sensitivity to Zn²⁺ but abolished the pH-dependent effect. This histidine residue is also involved in Ni²⁺ and Cd²⁺ interaction since the H156Y mutation completely suppressed the inhibition effects of these two cations. These data demonstrate that an extracellular histidine residue is critical for transition metal cation sensitivity of GABA ρ1 receptors.