Band 3

Anion Exchanger 1 (AE1) or Band 3 is a phylogenetically preserved transport protein responsible for catalysing the electroneutral exchange of chloride (Cl-) for bicarbonate (HCO3-) across a plasma membrane.

It is ubiquitous throughout the vertebrates. In humans it is present in two specific sites:

• the erythrocyte (red blood cell) cell membrane and
• the basolateral surface of the alpha-intercalated cell (the acid secreting cell type) in the collecting duct of the kidney.

The erythrocyte and kidney forms are different isoforms of the same protein.

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Discovery

AE1 was discovered following SDS-PAGE gel electrophoresis of erythrocyte cell membrane. The large 'third' band on the electrophoresis gel represented AE1, which was thus initially termed 'Band 3'. The chloride-bicarbonate exchanger in the red cell membrane is not a pump, which would use metabolic energy. Nor is it strictly an enzyme. It is protein counter-transporter, known as band III. ^[1]

AE1 in Red Cells

AE1 is an important structural component of the erythrocyte cell membrane, making up to 25% of the cell membrane surface, indeed each red cell contains approximately one million copies of AE1.

Function

Here it performs two functions:

• Electroneutral chloride and bicarbonate exchange across the plasma membrane on a one-for-one basis.This is crucial for CO₂ uptake by the red cell and conversion (by hydration catalysed by carbonic anhydrase) into a proton and a bicarbonate ion. The bicarbonate is then extruded from the cell by the band 3 molecule.

• Physical linkage of the plasma membrane to the underlying membrane skeleton (via binding with ankyrin and protein 4.2). This appears to be to prevent membrane surface loss, rather than being to do with membrane skeleton assembly.

Pathology

Mutations of erythroid AE1 affecting the extracellular domains of the molecule may cause alterations in the individual's blood group, as band 3 determines the Diego blood group.

More importantly erythroid AE1 mutations cause between 15-25% of cases of Hereditary spherocytosis (a disorder associated with progressive red cell membrane loss), and also cause the hereditary conditions of Hereditary stomatocytosis ^[2] and Southeast Asian Ovalocytosis ^[3]

AE1 in Alpha-Intercalated cells

A different isoform of AE1, known as kAE1 (which is 65 amino acids shorter than erythroid AE1) is found in the basolateral surface of the alpha-intercalated cell in the cortical collecting duct of the kidney.

Function

This is the principal acid secreting cell of the kidney, which generates hydrogen ions and bicarbonate ions from carbon dioxide and water-a reaction catalysed by Carbonic anhydrase.

The hydrogen ions are pumped into the collecting duct tubule by vacuolar H+ATPase, the apical proton pump,which thus excretes acid into the urine.

kAE1 exchanges bicarbonate for chloride on the basolateral surface, essentially returning bicarbonate to the blood.

Pathology

Mutations of kidney AE1 cause distal (type1) renal tubular acidosis, which is an inability to acidify the urine, even if the blood is too acid. These mutations are disease causing as they cause mistargetting of the mutant band 3 proteins so that they are retained within the cell or occasionally addressed to the wrong (ie apical) surface.

References

1. ^ Hunter M (1977). "Human erythrocyte anion permeabilities measured under conditions of net charge transfer.". J Physiol 268 (1): 35-49. PMID 874904.
2. ^ LJ Bruce (Nov 2005). "Monovalent cation leaks in human red cells caused by single amino-acid substitutions in the transport domain of the band 3 chloride-bicarbonate exchanger, AE1.". Nature Genetics 37 (11): 1258-63.
3. ^ Jarolim p (1991). "Deletion in Erythrocyte Band 3 Gene in Malaria-Resistant Southeast Asian Ovalocytosis". PNAS 88: 11022-11026..

Further reading