[Structure of human erythrocyte band 3: two-dimensional crystallographic analysis of the membrane domain]

Band 3 (also known as anion exchanger 1, AE1) is one of the most abundant membrane proteins in human erythrocytes. Band 3 has 911 amino acids and consists of two structurally and functionally distinct domains. One is a 40-kDa N-terminal cytoplasmic domain and the other is a 55-kDa C-terminal membrane domain. The cytoplasmic domain maintains red cell shape through interactions with cytoskeletal proteins, such as protein 4.1, protein 4.2, ankyrin, and spectrin. On the other hand, the membrane domain mediates electroneutral exchange of anions, such as bicarbonate and chloride across the erythrocyte membrane. We reported the three-dimensional structure of the outward-open membrane domain of band 3, which was cross-linked between K539 and K851 with H2DIDS, at 7.5 Å resolution using cryo-electron crystallography. Although the results showed significantly improved resolution as compared with previous structural analyses, we could not assign all α-helices because of low resolution and uncertainty persists regarding the fold of band 3. However, we recognized that band 3 has internal repeats, because the structure exhibited distinctive anti-parallel V-shaped motifs, which protrude from the membrane bilayer on both sides. One of the helices in the motif is very long and highly tilted with respect to the normal structure of the bilayer.