Structural Study on Human Red Cell Band 3 Protein, AE1.

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The membrane domain of human erythrocyte anion exchanger 1(AE1) works as a Cl^-/HCO_3^-antiporter. This exchange is a key step for CO_2/O_2 circulation in the blood. In spite of their importance, structural information about AE1 and the AE(Anion Exchanger) family is still very limited. We used the electron microscopy and the X-ray diffraction methods to solve the three-dimensional structure of the AE1 membrane domain, fixed in an outward-open conformation by cross-linking. A dimer of AE1 membrane domains packed in two-dimensional array showed a projection map similar to that of the prokaryotic homolog of ClC chloride channel, a Cl^-/H^+antiporter. In a three-dimensional map, there are V-shaped densities near the center of the dimer, and slightly narrower V-shaped clusters at a greater distance from the center of the dimer. These appear to be inserted into membrane from opposite sides. The structural motifs, two homologous pairs of helices in internal repeats of ClC transporter(helices B+C and J+K), are well fitted to those AE1 densities after simple domain movement. We could get good enough results with a high resolution for examining the structure and function relationship of the anion exchange in red blood cells.