How pH regulates a pH regulator: a regulatory hot spot in the N-terminal cytoplasmic domain of the AE2 anion exchanger.

Regulation of cell pH and cell volume require homeostatic control of intracellular cations and anions. Bicarbonate transporters play an important role in these cellular functions. The SLC4 and SLC26 gene families both encode bicarbonate transporter polypeptides. The SLC4 gene family includes four Na+-independent chloride-bicarbonate exchanger genes and multiple Na+-bicarbonate cotransporter and Na+-dependent anion-exchanger genes. The acute regulatory properties of the recombinant polypeptides encoded by these genes remain little studied. The most extensively studied among them are the Na+-independent anion exchangers AE1, AE2, and AE3. The widely expressed AE2 anion exchanger participates in recovery from alkaline load and in regulatory cell volume increase following shrinkage. AE2 can also be regulated by the ammonium ion. These properties are not shared by the closely related AE1 anion exchanger of the erythrocyte and the renal collecting duct Type A intercalated cell. Structure-function studies of recombinant proteins involving chimeras, deletions, and point mutations have delineated regions of AE2, which are important in the exhibition of the regulatory properties absent from AE1. These include regions of the transmembrane domain and the N-terminal cytoplasmic domain. Noncontiguous regions in the middle of the N-terminal cytoplasmic domain are of particular importance for acute regulation by several types of stimulus.