The band 3 anion exchange protein (AE1, SLC4A1) exchanges chloride (Cl-) for bicarbonate (HCO3-) across the plasma membrane according to the concentration gradients of the anions (Knauf et al. 1996, Dahl et al. 2003). SLC4A1 may be part of a complex ("metabolon") with carbonic anhydrase II (CA2) which would facilitate the transport of HCO3- (Sterling et al. 2001).