Evolution of chitin-binding proteins in invertebrates

Analysis of a group of invertebrate proteins, including chitinases and peri trophic matrix proteins, reveals the presence of chitin-binding domains tha t share significant amino acid sequence similarity. The data suggest that t hese domains evolved from a common ancestor which may be a protein containi ng a single chitin-binding domain. The duplication and transposition of thi s chitin-binding domain may have contributed to the functional diversificat ion of chitin-binding proteins. Sequence comparisons indicated that inverte brate and plant chitin binding domains do not share significant amino acid sequence similarity, suggesting that they are not coancestral. However, bot h the invertebrate and the plant chitin-binding domains are cysteine-rich a nd have several highly conserved aromatic residues. In plants, cysteines ha ve been elucidated in maintaining protein folding and aromatic amino acids in interacting with saccharides [Wright HT, Sanddrasegaram G, Wright CS (19 91) J Mol Evol 33:283-294], It is likely that these residues perform simila r functions in invertebrates. We propose that the invertebrate and the plan t chitin-binding domains share similar mechanisms for folding and saccharid e binding and that they evolved by convergent evolution. Furthermore, we pr opose that the disulfide bonds and aromatic residues are hallmarks for sacc haride-binding proteins.