3-DIMENSIONAL MAP OF THE DIMERIC MEMBRANE DOMAIN OF THE HUMAN ERYTHROCYTE ANION-EXCHANGER, BAND-3

The electroneutral exchange of chloride and bicarbonate across the hum an erythrocyte membrane is facilitated by Band 3, a 911 amino acid gly coprotein consisting of a 43 kDa N-terminal cytosolic domain that bind s the cytoskeleton, haemoglobin and glycolytic enzymes and a 52 kDa C- terminal membrane domain that mediates anion transport. Electron micro scopy 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 Angstrom, and a thickness o f 80 Angstrom. 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 Ang strom thick and probably spans the lipid bilayer. The upper part of th e dimer consists of two elongated protrusions measuring 25X80 Angstrom in projection, with a thickness of 40 Angstrom. The protrusions form the sides of a canyon, enclosing a wide space that narrows down and co nverges into a depression at the centre of the dimer on the top of the basal domain. This depression may represent the opening to a transpor t channel located at the dimer interface. Based on the available prote in-chemical data, the two protrusions face the cytosolic side of the m embrane and they appear to be dynamic.