Changes of cell shape
Human erythrocytes and the Band 3 protein
The bilayer-couple model predicts a reversible stomatocyte-echinocyte shape-transformation for an increasing ratio of external to internal monolayer area. Experimentally this ratio can be altered by introduction of lipophilic molecules, e.g. lipids, into one of the monolayers. However, under physiological conditions with no addition of membrane active compounds, active lipid translocation and passive flip-flop are comparatively slow. Nevertheless, individual erythrocytes undergo dramatic shape changes within seconds when the suspension medium is changed.
Band 3 is the most abundant protein of the erythrocyte's membrane. There are more than 106copies of band 3 anion-exchange protein per human erythrocyte which occupy about 10% of the total membrane area. Thus, may synchronized conformational changes of band 3 also alter the area ratio of the two monolayers?
Under physiological conditions the anion transport performed by band 3 is much faster than that of cations. This ensures the equilibration of internal and external anions like Cl- and HCO3- according to the transmembrane potential (TMP). A correlation of the TMP and the band 3 conformation as well as a correlation of the TMP and the erythrocyte shape are described in the literature. Our experiments on the influence of band 3-inhibitors on red cell morphology revealed a change of the predicted transmembrane potential-shape relationship.
Band 3 conformers are asymmetrically distributed with about 90% of the transport sites facing the inside. Specific band 3 inhibitors can induce echinocytes and these inhibitors are known to recruit the band 3 conformation with the transport site exposed to the external medium. DIDS, SITS, flufenamic acid, furosemide, and salicylic acid were tested. Other inhibitors e.g. phenylglyoxal and niflumic acid not recruiting a certain conformation are not echinocytogenic [33].
In the proposed model conformational changes may significantly alter the monolayer area ratio (see figure). Low external Cl--concentrations, negative TMPs or inhibitors may recruit the band 3 transport site towards the outside solution. It is proposed that this increases the bulk of the protein in the external monolayer and subsequently induces an echinocytogenic shape transformation according to the bilayer couple model.
More information: Jan Gimsa