Structure-function relationships of AE2 regulation by Ca(i)(2+)-sensitive stimulators NH(4+) and hypertonicity

We showed previously that the nonerythroid anion exchanger AE2 and the erythroid anion exchanger AE1 differ greatly in their regulation by acute changes in intracellular pH (pH(i)) and extracellular pH (pH(o)). We have now examined how AE2, but not AE1, is activated by two stimuli with opposing effects on oocyte pH(i): an alkalinizing stimulus, hypertonicity, and an acidifying stimulus, NH(4)(+). We find that both NH(2)-terminal cytoplasmic and COOH-terminal transmembrane domains of AE2 are required for activation by either stimulus. Directed by initial deletion mutagenesis studies of the NH(2)-terminal cytoplasmic domain, an alanine scan of AE2 amino acids 336-347 identified residues whose individual mutation abolished or severely attenuated sensitivity to both or only one activating stimulus. Chelation of cytoplasmic Ca(2+) (Ca(i)(2+)) diminished or abolished AE2 stimulation by NH(4)(+) and by hypertonicity. Calmidazolium inhibited AE2 activity, but not that of AE1. AE2 was insensitive to many other modifiers of Ca(2+) signaling. Unlike AE2 stimulation by NH(4)(+) and by hypertonicity, AE2 inhibition by calmidazolium required only AE2's COOH-terminal transmembrane domain.