Deficient HCO3- transport in an AE1 mutant with normal Cl- transport can be rescued by carbonic anhydrase II presented on an adjacent AE1 protomer

Abstract

Cl-/HCO3- exchange activity mediated by the AE1 anion exchanger is reduced by carbonic anhydrase II (CA2) inhibition or by prevention of CA2 binding to the AE1 C-terminal cytoplasmic tail. This type of AE1 inhibition is thought to represent reduced metabolic channeling of HCO3- to the intracellular HCO3- binding site of AE1. To test the hypothesis that CA2 binding might itself allosterically activate AE1 in Xenopus oocytes, we compared Cl-/Cl- and Cl-/HCO3- exchange activities of AE1 polypeptides with truncation and missense mutations in the C-terminal tail. The distal renal tubular acidosis-associated AE1 901X mutant exhibited both Cl-/Cl- and Cl-/HCO3- exchange activities. In contrast, AE1 896X, 891X, and AE1 missense mutants in the CA2 binding site were inactive as Cl-/HCO3- exchangers despite exhibiting normal Cl-/Cl- exchange activities. Co-expression of CA2 enhanced wild-type AE1-mediated Cl-/HCO3- exchange, but not Cl-/Cl- exchange. CA2 co-expression could not rescue Cl-/HCO3- exchange activity in AE1 mutants selectively impaired in Cl-/HCO3- exchange. However, co-expression of transport-incompetent AE1 mutants with intact CA2 binding sites completely rescued Cl-/HCO3- exchange by an AE1 missense mutant devoid of CA2 binding, with activity further enhanced by CA2 co-expression. The same transport-incompetent AE1 mutants failed to rescue Cl-/HCO3- exchange by the AE1 truncation mutant 896X, despite preservation of the latter's core CA2 binding site. These data increase the minimal extent of a functionally defined CA2 binding site in AE1. The inter-protomeric rescue of HCO3- transport within the AE1 dimer shows functional proximity of the C-terminal cytoplasmic tail of one protomer to the anion translocation pathway in the adjacent protomer within the AE1 heterodimer. The data strongly support the hypothesis that an intact transbilayer anion translocation pathway is completely contained within an AE1 monomer.