DO BAND-3 PROTEIN CONFORMATIONAL-CHANGES MEDIATE SHAPE CHANGES OF HUMAN ERYTHROCYTES

The bilayer-couple model predicts a reversible membrane crenation for an increasing ratio of external to internal monolayer area. This was comprehensively proven. However, individual erythrocytes may undergo dramatic shape changes within seconds when the suspension medium is changed. In contrast, under physiological conditions with no addition of membrane active compounds, active phospholipid translocation and passive flip-flops are comparatively slow. We propose that conformational changes of the anion-exchange protein, band 3, may rapidly alter the monolayer area ratio. Band 3 occupies about 10% of the total membrane area of human erythrocytes. Under physiological conditions, its conformers are asymmetrically distributed with about 90% of the transport sites facing the cytoplasm. This distribution is altered when external conformations are recruited by changing the transmembranous CI- gradient, the external pH, or by the application of inhibitors. In experiments, recruitment by low ionic strength caused a rapid, temporary formation of echinocytes. This suspension effect could also be found at high ionic concentrations, when CI- was replaced by SO42-. Inhibitors known to recruit 4 the external band 3 conformation, like DIDS, SITS and flufenamic acid, are echinocytogenic. For inhibitors not recruiting a certain conformation, e.g. phenylglyoxal and niflumic acid, no shape effect was found. Since band 3 ensures a fast equilibrium of internal and external anions these ions are usually distributed according to the transmembrane potential (TMP). In the literature, a correlation of TMP and band 3 conformation, as well as a correlation of TMP and red cell shape, is described. in the proposed model, low external CI- concentrations, inhibitors, or a negative TMP may recruit the transport sit outwards. This increases the bulk of the protein in the external monolayer and, subsequently, leads to the formation of echinocytes.