Corneal dystrophy mutations R125H and R804H disable SLC4A11 by altering the extracellular pH dependence of the intracellular pK that governs H⁺(OH^-) transport.

Mutations in the H⁺(OH^-) conductor SLC4A11 result in corneal endothelial dystrophy. In previous studies using mouse Slc4a11, we showed that the pK value that governs the intracellular pH dependence of SLC4A11 (pK_i) is influenced by extracellular pH (pH_e). We also showed that some mutations result in acidic or alkaline shifts in pK_i, indicating that the pH dependence of SLC4A11 is important for physiological function. An R125H mutant, located in the cytosolic amino terminus of SLC4A11, apparently causes a complete loss of function, yet the anion transport inhibitor 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) can partially rescue SLC4A11/R125H activity. In the present study we set out to determine whether the effect of R125H is explained by an extreme shift in pK_i. In <i>Xenopus</i> oocytes, we measured SLC4A11-mediated H⁺(OH^-) conductance while monitoring pH_i. We find that <i>1</i>) the human corneal variant SLC4A11-B has a more acidic pK_i than mouse Slc4a11, likely due to the presence of an NH₂-terminal appendage; <i>2</i>) pK_i for human SLC4A11 is acid-shifted by raising pH_e to 10.00; and <i>3</i>) R125H and R804H mutants mediate substantial H⁺(OH^-) conductances at pH_e = 10.00, with pK_i shifted into the wild-type range. These data suggest that the defect in each is a shift in pK_i at physiological pH_e, brought about by a disconnection in the mechanisms by which pH_e influences pK_i. Using de novo modeling, we show that R125 is located at the cytosolic dimer interface and suggest that this interface is critical for relaying the influence of pH_e on the external face of the transmembrane domain to the intracellular, pK_i-determining regions.