The bicarbonate transport metabolon

To allow cells to control their pH and bicarbonate levels, cells express bicarbonate transport proteins that rapidly and selectively move bicarbonate across the plasma membrane. Physical interactions have been identified between the carbonic anhydrase isoform, CAII, and the erythrocyte membrane Cl- /HCO3- anion exchanger, AE1, mediated by an acidic motif in the AE1 C-terminus. We have found that the presence of CAII attached to AE1 accelerates AE1 HCO3- transport activity, as AE1 moves bicarbonate either into or out of the cell. In efflux mode the presence of CAII attached to AE1 will increase the local concentration of bicarbonate at the AE1 transport site. As bicarbonate is transported into the cell by AE1, the presence of CAII on the cytosolic surface accelerates transport by consumption of bicarbonate, thereby maximizing the transmembrane bicarbonate concentration gradient experienced by the AE1 molecule. Functional and physical interactions also occur between CAII and Na+/HCO3- co-transporter isoforms NBC1 and NBC3. All examined bicarbonate transport proteins, except the DRA (SLC26A3) Cl-/HCO3- exchange protein, have a consensus CAII binding site in their cytoplasmic C-terminus. Interestingly, CAII does not bind DRA. CAIV is anchored to the extracellular surface of cells via a glycosylphosphatidyl inositol linkage. We have identified extracellular regions of AE1 and NBC1 that directly interact with CAIV, to form a physical complex between the proteins. In summary, bicarbonate transporters directly interact with the CAII and CAIV carbonic anhydrases to increase the transmembrane bicarbonate flux. The complex of a bicarbonate transporter with carbonic anhydrase forms a "Bicarbonate Transport Metabolon."