PROBING THE DOMAIN-STRUCTURE OF ABRIN-A BY TRYPTIC DIGESTION

Abrin-a is a potent plant toxin that consists of A and B chains linked by a disulfide bond. The abrin-a A chain (AaTA) has N-glycosidase act ivity while the abrin-a B chain (AaTB) has galactose-binding activity. By partial tryptic digestion, the domain structure of abrin-a was inv estigated. Seven tryptic fragments with molecular masses greater than 3500 Da were isolated and characterized. One fragment, designated T-21 and consisting of 153 amino acid residues, contained the major part o f the second domain of AaTB and, after cross-linking of T-21 with glut araldehyde, the reaction product had the same level of hemagglutinatin g activity as native abrin. When the T-21 fragment was conjugated with AaTA, the conjugate inhibited protein biosynthesis in HeLa cells. Thi s suggests that the T-21 fragment is able to bind specifically to cell s; its conjugate facilitates membrane translocation of AaTA into cells and consequently inhibits protein biosynthesis. T-21, with a molecula r mass less than AaTB is therefore a potentially useful substance for the preparation of immunotoxins.