cDNA cloning and localization of a band 3-related protein from ileum

A Cl^--HCO₃/^- exchanger in the brush-border membrane mediates active Cl^- absorption and regulates intracellular pH in rabbit ileum. The molecular identity of the ileal Cl^--HCO₃/^- exchanger has not been established. The best-characterized plasma membrane Cl^--HCO₃/^- exchanger is erythroid band 3. Structurally related proteins in nonerythroid tissues comprise an anion exchanger (AE) family. We used the polymerase chain reaction to amplify and clone a cDNA encoding an ileal band 3-related protein (B3RP) from rabbit ileal enterocytes. The composite sequence is 3,909 bp and is predicted to encode a protein of 136 kDa. The deduced amino acid sequence is 95% identical to murine renal AE2, indicating that ileal B3RP is rabbit AE2. Antisera generated against a cytoplasmic fragment of ileal B3RP recognized a 160- to 170-kDa polypeptide in the brush-border membrane, but not the basolateral membrane, of ileal crypt and villus enterocytes. This correlates with previous studies indicating that a Cl^--HCO₃/^- exchange is present in brush-border but not basolateral membrane vesicles from rabbit ileal enterocytes. We conclude that ileal B3RP is a product of the AE gene family, and is present in the brush-border of ileal enterocytes, where it may mediate Cl^--HCO₃/^- exchange.