Three-dimensional map of the dimeric membrane domain of the human erythrocyte anion exchanger, Band 3.

The electroneutral exchange of chloride and bicarbonate across the human erythrocyte membrane is facilitated by Band 3, a 911 amino acid glycoprotein consisting of a 43 kDa N-terminal cytosolic domain that binds the cytoskeleton, haemoglobin and glycolytic enzymes and a 52 kDa C-terminal membrane domain that mediates anion transport. Electron microscopy and three-dimensional image reconstruction of negatively stained two-dimensional crystals of the dimeric membrane domain revealed a U-shaped structure with dimensions of 60 x 110 A, and a thickness of 80 A. The structure is open on the top and at the sides, with the monomers in close contact at the base. The basal domain is 40 A thick and probably spans the lipid bilayer. The upper part of the dimer consists of two elongated protrusions measuring 25 x 80 A in projection, with a thickness of 40 A. The protrusions form the sides of a canyon, enclosing a wide space that narrows down and converges into a depression at the centre of the dimer on the top of the basal domain. This depression may represent the opening to a transport channel located at the dimer interface. Based on the available protein-chemical data, the two protrusions face the cytosolic side of the membrane and they appear to be dynamic.