Identification of two new peritrophic membrane proteins from larval Trichoplusia ni: structural characteristics and their functions in the protease rich insect gut

Peritrophic membrane (PM) proteins are important determinants for the structural formation and function of the PM. We identified two new chitin binding proteins, named CBP1 and CBP2, from the PM of Trichoplusia ni larvae by cDNA cloning. The proteins contain 12 and 10 tandem chitin binding domains in CBP1 and CBP2, respectively. Chitin binding studies demonstrated the chitin binding activity of CBP1 and CBP2, and confirmed the chitin binding domain sequence predicted by sequence analysis. Both CBP1 and CBP2 were not mucin-like glycoproteins, however, they were highly resistant to proteolytic degradation by trypsin. We found that in CBP1 and CBP2, potential trypsin and chymotrypsin cleavage sites reside primarily within the chitin binding domain sequences, limiting exposure of the potential cleavage sites to the digestive proteinases. This finding suggests a proteinase-resistance mechanism for non-mucin PM proteins to function in the proteinase rich gut environment. Immunohistochemical analysis showed that CBP1 and CBP2 are specifically localized in the PM. However, intact CBP1 and CBP2 proteins were not present in the PM, indicating that their partially degraded fragments were assembled into the PM. This observation suggests that the presence of a large number of chitin binding domains in PM proteins allows the proteins to tolerate limited proteolytic degradation in the midgut without loss of their chitin binding activity with multiple chitin binding domains. Alignment of the chitin binding sequences suggested that CBP1 and CBP2 evolved by gene duplication and the tandem chitin binding domains in the proteins arose from domain duplications.