Band 3 Protein Fragments

Definition
Band 3 protein fragments are phylogenetically preserved transport peptides located on the organelle membranes especially the erythrocyte membrane where they mediate anion exchange1.

Discovery
Band 3 peptides were originally identified following SDS-gel electrophoresis of erythrocyte cell membrane. The large third band on the gel turned out to be the peptide. Hence the name Band 31.

Classification
Band 3 proteins are members of anion-exchange (AE) family of proteins2. AE proteins are encoded by a multigene family that has atleast three genes, AE1, AE2, AE3 and several splicioforms2. They are all expressed by most cells, e.g., erythrocytes, cardiomyocytes.

Structural Characteristics
Band 3 fragments have two functional domains.The integral domain mediates a 1:1 exchange of inorganic anions across the membrane, whereas the cytoplasmic domain provides binding sites for cytoskeletal proteins, glycolytic enzymes, and hemoglobin3. Activation of Band 3 fragments occurs through tyrosine phosphorylation4.

Mode of action
Band 3 peptide is bound to the erythrocyte membrane5. Its N-terminal region protrudes from the main body towards the red cell cytoplasm allowing it to interact with various membrane bound and cytoplasmic red cell components such as glycolytic enzymes including aldolase, glyceraldehyde-3-phosphate dehydrogenase and phosphofructokinase as well as other membrane components such as ankyrin and protein 4.15,6. Recently it has been found that Band 3 also binds to Hemoglobin. Interaction of Band 3 with these proteins exerts several changes in the cells for example lowering oxygen affinity of Hemoglobin5.

Functions
Band 3 fragments are involved in the anion-exchange across the plasma membrane on a one-for-one basis. This is crucial for CO2 uptake by the red cell and conversion into a proton and a bicarbonate ion that is then extruded from the cell by the band 3 molecule2. Band 3 serves as a physical linkage between the plasma membrane and the underlying membrane skeleton via binding with ankyrin and protein 4.15. This appears to prevent membrane surface loss. A spliced variant of Band 3 nAE1 regulates pH in cardioventricular myocytes through anion-exchange2. Band 3 is also an allosteric regulator of Heamoglobin as it binds to Hb and decreases its affinity for oxygen5. Other functions of Band 3 fragments include maintenance of cell volume and osmotic homeostasis, red cell aging, IgG binding and cellular removal7. Band 3 alterations are implicated in neurological diseases such as familial paroxysmal dyskinesia, idiopathic generalized epilepsies, and neuro- or choreoacanthocytosis7.

References
1.Hunter M (1977). Human erythrocyte anion permeabilities measured under conditions of net charge transfer. J Physiol, 268 (1): 35–49.
2.Richards SM, Jaconi ME, Vassort G and Puceat M (1999). A spliced variant of AE1 gene encodes a truncated form of Band 3 in heart: the predominant anion exchanger in ventricular myocytes.
3.Jonathan DG, Lin W and Michael JAT (1998). Complementation studies with co-expressed fragments of human red cell band 3 (AE1): the assembly of the anion-transport domain in Xenopus oocytes and a cell-free translation system. Biochem. J., 332, 161-171.
4.Yannoukakos D, Vasseur C, Piau JP, Wajcman H, Bursaux E (1991). Phosphorylation sites in human erythrocyte band 3 protein. Biochim. Biophys. Acta, 1061, 253-266.
5.Yuxun Z, Lois RM, Jill F, Orah P and James MM (2003), Human Erythrocyte Membrane Band 3 Protein Influences Hemoglobin Cooperativity: Possible effect on oxygen transport. J. Biol. Chem., 278, 41, 39565-39571.
6.Chambers EJ, Askin D, Bloomberg GB, et al. (1998). Studies on the structure of a transmembrane region and a cytoplasmic loop of the human red cell anion exchanger (band 3, AE1), Biochem. Soc. Trans., 26 (3): 516–20.
7.Kay MM (2004). Band 3 and its alterations in health and disease. Cell Mol Bio, 52, 117-38.