Altered structure and anion transport properties of band 3 (AE1, SLC4A1) in human red cells lacking glycophorin A

We have studied the properties of band 3 in different glycophorin A (GPA)-deficient red cells. These red cells lack either both GPA and glycophorin B (GPB) (M(k)M(k) cells) or GPA (En(a-) cells) or contain a hybrid of GPA and GPB (MiV cells). Sulfate transport was reduced in all three red cell types to approximately 60% of that in normal control red cells as a result of an increased apparent K(m) for sulfate. Transport of the monovalent anions iodide and chloride was also reduced. The reduced iodide transport resulted from a reduction in the V(max) for iodide transport. The anion transport site was investigated by measuring iodide fluorescence quenching of eosin-5-maleimide (EMA)-labeled band 3. The GPA-deficient cells had a normal K(d) for iodide binding, in agreement with the unchanged K(m) found in transport studies. However, the apparent diffusion quenching constant (K(q)) was increased, and the fluorescence polarization of band 3-bound EMA decreased in the variant cells, suggesting increased flexibility of the protein in the region of the EMA-binding site. This increased flexibility is probably associated with the decrease in V(max) observed for iodide transport. Our results suggest that band 3 in the red cell can take up two different structures: one with high anion transport activity when GPA is present and one with lower anion transport activity when GPA is absent.