The location of a chymotrypsin cleavage site and of other sites in the primary structure of the 17,000-dalton transmembrane segment of band 3, the anion transport protein of red cell.

A 17,000-dalton transmembrane segment of band 3 protein is further cleaved by chymotrypsin treatment of red blood cell ghosts to 15,000 daltons. The location of this particular chymotrypsin cleavage site was determined by comparing the fragmentation pattern of the 17,000- and 15,000-dalton peptides using cyanogen bromide (CNBr). Each peptide is cleaved at its two methionine residues into three fragments. For each peptide two of the fragments are the same size, 7000 and 4000 daltons, the latter containing, in each case, the binding site of the anion transport inhibitor 4,4'-diisothiocyano-2,2' disulfonic acid (DIDS). The third fragment is 2000 daltons larger in the case of the 17,000-dalton peptide (6000 compared to 4000 daltons). These findings indicate that the chymotrypsin cleavage site is located at the cytoplasmic side of the membrane, 2000 daltons from the N-terminus of the 17,000-dalton peptide. This information allows the mapping of a number of defined sites of the 15,000-dalton segment within the primary structure of band 3. These sites support the suggestion that this peptide segment is folded within the bilayer.