Residues important for nitrate/proton coupling in plant and mammalian CLC transporters

Members of the CLC gene family either function as chloride channels or as anion/proton exchangers. The plant AtClC-a uses the pH gradient across the vacuolar membrane to accumulate the nutrient NO(3)(-) in this organelle. When AtClC-a was expressed in Xenopus oocytes, it mediated NO(3)(-)/H(+) exchange and less efficiently mediated Cl(-)/H(+) exchange. Mutating the "gating glutamate" Glu-203 to alanine resulted in an uncoupled anion conductance that was larger for Cl(-) than NO(3)(-). Replacing the "proton glutamate" Glu-270 by alanine abolished currents. These could be restored by the uncoupling E203A mutation. Whereas mammalian endosomal ClC-4 and ClC-5 mediate stoichiometrically coupled 2Cl(-)/H(+) exchange, their NO(3)(-) transport is largely uncoupled from protons. By contrast, the AtClC-a-mediated NO(3)(-) accumulation in plant vacuoles requires tight NO(3)(-)/H(+) coupling. Comparison of AtClC-a and ClC-5 sequences identified a proline in AtClC-a that is replaced by serine in all mammalian CLC isoforms. When this proline was mutated to serine (P160S), Cl(-)/H(+) exchange of AtClC-a proceeded as efficiently as NO(3)(-)/H(+) exchange, suggesting a role of this residue in NO(3)(-)/H(+) exchange. Indeed, when the corresponding serine of ClC-5 was replaced by proline, this Cl(-)/H(+) exchanger gained efficient NO(3)(-)/H(+) coupling. When inserted into the model Torpedo chloride channel ClC-0, the equivalent mutation increased nitrate relative to chloride conductance. Hence, proline in the CLC pore signature sequence is important for NO(3)(-)/H(+) exchange and NO(3)(-) conductance both in plants and mammals. Gating and proton glutamates play similar roles in bacterial, plant, and mammalian CLC anion/proton exchangers.