J. Biol. Chem., Vol. 259, Issue 7, 4652-4660, 04, 1984
Peptides of human erythrocyte band 3 protein produced by extracellular papain cleavage
ML Jennings, M Adams-Lackey and GH Denney
The structure of the human red blood cell membrane band 3 protein has been
investigated by proteolysis of intact cells with papain. Papain cleaves the
35,000-dalton chymotryptic peptide (CH35) of band 3 into two integral
fragments of Mr 7,500 (P7) and 28,000 (P28). The papain cleavage of CH35
causes inhibition of band 3-catalyzed Cl transport. The peptide P28 carries
the band 3 carbohydrate and also contains the 2 cysteine residues in CH35.
The anion transport inhibitor H2DIDS (4,4'-
diisothiocyanodihydrostilbene-2,2'-disulfonate) reacts covalently with a
lysine residue on P28, rather than P7, in native band 3. However, if the
cells are first digested with papain and then reacted with H2DIDS, there is
significant covalent reaction with P7, producing a covalent cross-link
between P7 and the 60,000-dalton chymotryptic peptide (CH60). Graded papain
digestion experiments and end group determinations indicate that the
alignment of the band 3 peptides is N- CH60-P7-P28-COOH. The NH2-terminal
sequence of P7 is identical with a segment of a peptic fragment of band 3
recently sequenced (Brock, C.J., Tanner, M.J.A., and Kempf, C. (1983)
Biochem. J. 213, 577-586). This published sequence, plus our sequence data
on CH35 and P7, show that papain cleaves the outer surface of CH35 at two
sites, which are 6 residues and 71 residues from the chymotrypsin cleavage
site. The 65- residue peptide (P7) between these sites is the best
characterized segment of band 3 thus far described: its sequence and
location in the band 3 primary structure are now known, and both ends of
the peptide are unambiguously exofacial.