The location of a disulfonic stilbene binding site in band 3, the anion transport protein of the red blood cell membrane.

The binding site for 4,4'-diisothiocyano-2,2'-stilbenedi sulfonic acid, a specific, potent, irreversible inhibitor of anion transport in red blood cells is located in a 15 000 dalton transmembrane segment of band 3, produced by chymotrypsin treatment of ghosts stripped of extrinsic proteins. The segment was cleaved into three fragments of 7000 daltons by CNBr. The C-terminus of the segment is located in the 7000 daltons by the N-terminus in one of the 4000 dalton fragment; the N-terminus in one of the 4000 dalton fragments; and the binding site for 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid in the middle 4000 dalton fragment. The latter was cleaved by N-bromosuccinimide into two fragments of 2000 daltons. The binding site for 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid was located on the fragment containing the newly formed N-terminus. It is concluded that the binding site is located about 9000 daltons from the C-terminus (at the outside face of the membrane) and 6000 daltons from the N-terminus (at the cytoplasmic face). In view of the existing evidence that the binding site may be located near the outside face of the membrane, it is suggested that the 15 000 dalton segment is folded, so that it crosses the bilayer three times.