Transport activity of AE3 chloride/bicarbonate anion-exchange proteins and their regulation by intracellular pH

Plasma membrane Q-/HCO3- anion-exchange (AE) proteins contribute to regulation of intracellular pH (pH(i)). We characterized the transport activity and regulation by pH(i) of full-length AE3 and the cardiac isoform, AE3c, both of which are expressed in the heart. AE3c is an N-terminal variant of AE3. We also characterized AE1, AE2 and a deletion construct (AE3tr) coding for the common region of AE3 and AE3c. AE proteins were expressed by transient transfection of HEK-293 cells, and transport activity was monitored by following changes of intracellular pH or intracellular chloride concentration associated with anion exchange. Transport activities, measured as proton flux (mM H+ . min(-1)), were as follows: AE1, 24; AE2, 32; full-length AE3, 9; AE3c, 4 and AE3tr, 4. The wide range of transport activities is not explained by variation of cell surface processing since approx. 30% of each isoform was expressed on the cell surface. pH(i) was clamped at a range of values from 6.0-9.0 to examine regulation of AE proteins by pH(i). Whereas AE2 was steeply inhibited by acid pH(i), AE1, AE3 and AE3c were essentially insensitive to changes of pH(i). We conclude that AE3 and AE3c can contribute to pH(i) recovery after cellular-acid loading.