Homology of the D-galactose-specific lectins from Artocarpus integrifolia and Maclura pomifera and the role of an unusual small polypeptide subunit.

The Maclura pomifera agglutinin (MPA) was purified by affinity chromatography from a seed extract and its properties were compared with those of the Artocarpus integrifolia lectin, jacalin. Reverse-phase high-performance liquid chromatography showed both proteins had multiple forms of a small approximately 20-residue polypeptide chain in addition to the major 12,000 Mr subunit. The amino acid sequences of the small chains and the N-terminal sequences of the large subunits showed considerable similarity between the two proteins, approximately 60% identical residues. The homology of the proteins was confirmed by the similarity of their circular dichroism and fluorescence emission spectra. MPA showed much greater spectral changes upon binding methyl alpha-D-galactoside, suggesting it has complete activity rather than the partial activity found for jacalin. The binding of methyl alpha-D-galactoside by MPA was measured by fluorescence titration; the KA was 1.9 X 10(4) M-1 compared to 3.4 X 10(4) M-1 for jacalin. MPA also precipitated human IgA1 in the same manner as jacalin. The spectra indicate the involvement of tryptophan and tyrosine residues in the binding site of these lectins. Since a tryptophan residue is conserved in all the small subunits, they may form part of the binding site.