2-DIMENSIONAL STRUCTURE OF THE MEMBRANE DOMAIN OF HUMAN BAND-3, THE ANION TRANSPORT PROTEIN OF THE ERYTHROCYTE-MEMBRANE

The membrane domain of human erythrocyte Band 3 protein (M(r) 52 000) was reconstituted with lipids into two-dimensional crystals in the form of sheets or tubes. Crystalline sheets were monolayers with six-fold symmetry (layer group p6, a = b = 170 angstrom, gamma = 60-degrees), whereas the symmetry of the tubular crystals was p2 (a = 104 angstrom, b = 63 angstrom, gamma = 104-degrees). Electron image analysis of negatively stained specimens yielded projection maps of the protein at 20 angstrom resolution. Maps derived from both crystal forms show that the membrane domain is a dimer of two monomers related by two-fold symmetry, with each monomer consisting of three subdomains. In the dimer, two subdomains of each monomer form a roughly rectangular core (40 x 50 angstrom in projection), surrounding a central depression. The third subdomain of the monomer measures approximately 15 x 25 angstrom in projection and appears to be connected to the other two by a flexible link. We propose that the central depression may represent the channel for anion transport while the third subdomain appears not to be directly involved in channel formation.