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

Abstract

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 60x110 Å, and a thickness of 80 Å. 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 Å thick and probably spans the lipid bilayer. The upper part of the dimer consists of two elongated protrusions measuring 25x80 Å in projection, with a thickness of 40 Å. 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.