The cytoplasmic domain of band 3 serves as a center of erythrocyte membrane organization and constitutes the major substrate of erythrocyte tyrosine kinases. Tyrosine phosphorylation of band 3 is induced by several physiologic stimuli, including malaria parasite invasion, cell shrinkage, normal cell aging, and oxidant stress (thalassemias, sickle cell disease, glucose-6-phosphate dehydrogenase deficiency, etc). In an effort to characterize the biologic sequelae of band 3 tyrosine phosphorylation, we looked for changes in the polypeptide's function that accompany its phosphorylation. We report that tyrosine phosphorylation promotes dissociation of band 3 from the spectrin-actin skeleton as evidenced by: (1) a decrease in ankyrin affinity in direct binding studies, (2) an increase in detergent extractability of band 3 from ghosts, (3) a rise in band 3 cross-linkability by bis-sulfosuccinimidylsuberate, (4) significant changes in erythrocyte morphology, and (5) elevation of the rate of band 3 diffusion in intact cells. Because release of band 3 from its ankyrin and adducin linkages to the cytoskeleton can facilitate changes in multiple membrane properties, tyrosine phosphorylation of band 3 is argued to enable adaptive changes in erythrocyte biology that permit the cell to respond to the above stresses. (Blood. 2011; 117(22):5998-6006)
IntroductionEarly views of the human erythrocyte argued that the cell was inert to external stimuli and that its complement of protein kinases, phospholipases, G proteins, phosphatases, and hormone receptors simply constituted nonfunctional vestiges of signaling pathways that were once operational in erythroid precursor cells. More recent evidence, however, has revealed that the human erythrocyte is highly responsive to its environment and that the cell's rich ensemble of signaling proteins likely comprise critical components in the cell's communication with its extracellular milieu. Classic hormones/signaling molecules such as prostaglandin E 2 , insulin, epinephrine, endothelin, ADP, and NO are now known to modulate erythrocyte properties in an adaptive manner, and the functional activities of many intracellular signaling intermediates have been demonstrated to regulate erythrocyte behavior. [1][2][3][4][5] One of the major targets of erythrocyte signaling appears to be the predominant membrane-spanning protein, band 3. Band 3 (AE1) catalyzes the exchange of anions (primarily HCO 3 Ϫ for Cl Ϫ ) across the erythrocyte membrane, 6 anchors the spectrin/ actin cytoskeleton to the lipid bilayer, 7 organizes and regulates a complex of glycolytic enzymes, 8,9 participates in control of erythrocyte lifespan, 10,11 nucleates several important membrane-spanning proteins, 12 and serves as a docking site for multiple peripheral membrane proteins, including protein 4.1, protein 4.2, and several kinases and phosphatases. [13][14][15][16] Not surprisingly, mutations in band 3 are frequently associated with various hemolytic diseases. 17 Perhaps because of its many important functions, band 3 is a...