Orientation of the band 3 polypeptide from human erythrocyte membranes. Identification of NH2-terminal sequence and site of carbohydrate attachment.

The NH2-terminal sequence and carbohydrate attachment site of the 95,000-dalton transmembrane polypeptide (Band 3) from human erythrocyte membranes have been studied. The blocked NH2-terminal sequence is Ac-Met-Glu-Glu; the presence of this sequence in specific fragments of the polypeptide confirms that the end of the polypeptide which is inside the cell is the NH2-terminal. The carbohydrate associated with the Band 3 polypeptide appears to be attached at a single site in the COOH-terminal third of the molecule, to a region with composition Asx1Ser2; this confirms that part of the polypeptide toward the COOH-terminal is outside the cell. The carbohydrate structure appears to be extremely heterogeneous both in size and composition, which probably causes the Band 3 polypeptide to migrate as a diffuse band on dodecyl sulfate gel electrophoresis.