A structural study of the carboxyl terminal region of the human erythrocyte band 3 protein

Two peptides derived from the carboxyl terminal region of the human erythrocyte band 3 protein were identified as fragments releasable from cell membranes on trypsin digestion. These peptides, Asn-880-Lys-892 and Ala-893-Val-911-COOH, however, were resistant to trypsin, unless the cell membranes had been treated with high concentrations of NaOH. This suggests that the carboxyl terminal region is located in situ within the native band 3 molecule. Unlike in the cases of other portions of the band 3 protein, such as Gly-647-Arg-656, Ser-731-Lys-743, and Tyr-818-Lys-826, the release of the carboxyl terminal region was not inhibited by pretreatment of erythrocytes with 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS), indicating that there is no major structural difference in the carboxyl terminal portion between the outward and inward facing forms. The carboxyl terminal region of the band 3 protein has a negative charge cluster. In the middle of the negative charge cluster, consensus sequences, Val-Asp-X-X-X-Leu-Asp-Ala-Asp-Asp and Thr-Phe-Asp-Glu (TFDE), were found in the carboxyl terminal regions of aquaporin CHIP and glucose transporter 1, respectively. The sequence, TFDE, exists in the highly amphipathic 11-residue sequence of glucose transporter 1, and this amphipathic sequence has been suggested to promote normal membrane insertion of polytopic membrane proteins such as glucose transporter 1 and serine chemoreceptor. The role of the carboxyl terminal region of the band 3 protein is discussed.