Identification of a novel 4 kDa immunoglobulin-A-binding peptide obtained by the limited proteolysis of jacalin.

Jacalin, an IgA-binding lectin from jackfruit (Artocarpus heterophyllus) seeds, was isolated by the passage of PBS extracts of seeds over an affinity matrix containing IgA-Sepharose-4B. It was further purified by HPLC. When analyzed by SDS-PAGE under both reducing and nonreducing conditions, the native jacalin was dissociated into two subunits of 12 and 15.4 kDa. Both the subunits could bind IgA. Peptide mapping performed with radioiodinated jacalin indicated that both the subunits were susceptible to proteolysis by Staphylococcus aureus V8 proteinase. One degradation product was a small peptide of 4 kDa. This small proteolytic fragment also bound IgA. The amino-termini of the two major IgA binding subunits, 12 and 15.4 kDa, were identical. The 4 kDa IgA-binding proteolytic fragment of jacalin had a different amino-terminal sequence, suggesting that the region of jacalin which binds IgA does not remain close to the amino-terminus of the peptide.