The carboxyl-terminal domain of human erythrocyte band 3. Description, isolation, and location in the bilayer.

Band 3, the anion channel protein of the human erythrocyte, is the major transmembrane glycoprotein of the erythrocyte membrane. The protein is distributed in a broad 88,000-98,000-dalton band on gel electrophoresis. Previous investigations have defined an NH2-terminal cytoplasmic domain and an adjacent membrane-spanning domain of the Band 3 molecule. The fragments containing these domains appear as discrete bands by sodium dodecyl sulfate polyacrylamide gel electrophoresis. A carboxyl-terminal fragment of the Band 3 molecule was generated by digestion with chymotrypsin at the external face of intact erythrocytes. This fragment appears as a broad band of Mr = 34,000-45,000. It has a site accessible to lactoperoxidase at the internal face of the membrane and must, therefore, span the membrane. Slices from different regions of the broad electrophoretic band of the carboxyl-terminal fragment of Band 3 all have identical peptide maps. The same is true of Band 3. Therefore, despite their heterogeneous appearance on gels, Band 3 and its proteolytic fragments are homogeneous polypeptides. This conclusion is supported by the finding of an unique NH2-terminal tetrapeptide sequence for one Band 3 fragment. The apparent heterogeneity of Band 3 and its carboxyl-terminal region may reflect variability of glycosylation or sodium dodecyl sulfate binding.