Anion Exchanger 1 (AE1), also known as band 3 or capnaphorin (amongst other things), is the most prevalent integral membrane protein of the erythrocyte.
It is responsible for the exchange of bicarbonate ions (HCO3-) for chloride ions (Cl-) across the plasma membrane. The direction of transport depends on the environment:
1) The cell is in the peripheral circulation where respiring tissues are producing carbon dioxide (CO2).
The CO2 diffuses into the cell (as a small uncharged molecule it can freely cross the membrane) and is converted into bicarbonate by the action of carbonic anhydrase . Carbonic anhydrase is physically sited next to AE1 so that transport out of the cell by AE1 is optimised (this is an example of a metabolon). This is of crucial importance as the blood will only carry a very small amount of CO2 as a dissolved molecule, haemoglobin will only carry a small percentage more, but as (freely soluble) bicarbonate the blood can carry loads.
2) The cell is in the pulmonary circulation and by being forced through capillaries wrapped around alveoli is exposed to air.
Here oxygen (O2) import and binding to haemoglobin - using the UK spelling but linking to the US one to maintain both practicality and my standards of English :) - causes a drop in pH which favours conversion of bicarbonate back into CO2 (by carbonic anhydrase) and subsequent diffusion out of the cell (and then out of the circulation into the alveoli). This, by LeChatelier's principle, means that bicarbonate is drawn back into the cell - by exchanging for extracellular Cl- - through AE1. As a result carbonic anhydrase and AE1 have more CO2 to work on and can get as much of the dissolved bicarbonate out of the blood and into the air you're about to exhale. It is the relatively short transit time through the lungs that necessitates such humongous expression of AE1 in the erythrocyte - approximately 1.2 million copies per cell.
AE1 does more than just this though. It has been shown that it's transport function is performed by the 'membrane domain' whereas its large 'cytoplasmic domain' is involved in association with other erythrocyte proteins such as GPA, protein 4.2, ankyrin, haemoglobin and others - forming a large, multiprotein complex. These associations are essential for maintenance of the red blood cell's peculiar biconcave (kind of like a doughnut without a proper hole) morphology. Furthermore these associations are dynamic and are likely to be responsible for the cell's ability to squeeze through the narrow capillaries without being torn to shreds by the high shear stresses.
The Gods of Biochemistry
will probably never forgive me for such a summarised description - there's more to say for sure - but, to be honest, it's Friday night and I want to go home