Primary structure, isoforms, and molecular modeling of a chitin-binding mistletoe lectin

From mistletoe Viscum album L. extracts three chitin-binding lectin isoform s, cbML1, cbAM2, and cbML3, were isolated and their primary structure deter mined. All three cbML isoforms are composed of two protein chains of 48 or 49 amino acid residues, linked by an intermolecular disulfide bond. The seq uence of each single cbML chain is characterized by a relatively high numbe r of cysteine and glycine residues, 9 and 6, respectively, and contains fou r intramolecular disulfide bridges. On the basis of the combined interpreta tion of sequencing and MALDI MS data, the following results for the three c bML isoforms were obtained: the first one consists of two identical truncat ed polypeptide chains (1-48), the second is a heterodimer, containing one t runcated (1-48) and one full-length chain (1-49), and the third is composed of two fall length chains (1-49). The cbML sequence shows 55% identity to hevein, a single-chain chitin-binding protein of 43 amino acids, one of the most predominant proteins in natural rubber latex. On the basis of the NMR data on hevein from Hevea brasiliensis the three-dimensional structure of cbML3 was modelled. The 26 sequence changes between cbML3 and hevein were a ccommodated with only little perturbation in the main chain folding. A comp arison of the primary structures of cbML3 and hevein is shown and the effec ts of the sequence changes are discussed. Differences have been identified in the loop region of the molecule and the potential interface region of cb ML3 supporting the dimer formation. The high-affinity chitin-binding site s eems to be highly conserved. (C) 2001 Academic Press.