A single histidine residue is essential for zinc inhibition of GABA rho 1 receptors.

The GABA rho 1 subunit, cloned from a human retina library, can form homooligomeric receptors with properties similar to GABAc receptors characterized in retinal cells. The divalent cation Zn2+, abundant in the CNS and retina, was found to inhibit GABA rho 1 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 rho 1 receptor. Site-directed mutagenesis revealed that a single histidine residue (His 156) in the putative extracellular domain of rho 1 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 rho 1 receptors.