The anion channel protein (also called band 3 protein) is an integral membrane of the erythrocyte membrane (Figure 10.17, Figure 10.18). Anion channel protein is an 89-kilodalton protein that appears to exist in the membrane as a dimeric structure. The polypeptide chain passes back and forth through the membrane six times. Its major function is to serve as an anion channel, allowing the passage of HCO3^- and Cl^- through the membrane and thereby facilitate CO2 transport by the hemoglobin.

Band 3 protein does a number of additional interesting things. Its long N-terminal domain, which extends into the cytosol, may associate with a number of the enzymes of glycolysis, the major energy-producing pathway in the erythrocyte. These enzymes are also involved in providing bisphosphoglycerate (see here) to regulate hemoglobin O2 affinity. Finally, as mentioned above, the N-terminal domain of band 3 protein, which lies on the inner membrane surface, is associated with the peripheral protein ankyrin, which in turn anchors the band 3 complex to the membrane skeleton (Figure 10.18).