A novel family of chitin-binding proteins from insect type 2 peritrophic matrix - cDNA sequences, chitin binding activity, and cellular localization

The peritrophic matrix is a prominent feature of the digestive tract of mos t insects, but its function, formation, and even its composition remain con tentious. This matrix is a molecular sieve whose toughness and elasticity a re generated by glycoproteins, proteoglycans, and chitin fibrils. We now de scribe a small, highly conserved protein, peritrophin-15, which is an abund ant component of the larval peritrophic matrices of the Old World screwworm fly, Chrysomya bezziana, and sheep blowfly, Lucilia cuprina. Their deduced amino acid sequences code for a 8-kDa secreted protein characterized by a highly conserved and novel register of six cysteines. Two Drosophila homolo gues have also been identified from unannotated genomic sequences. Recombin ant peritrophin-15 binds strongly and specifically to chitin; however, the stoichiometry of binding is low (1:10,000 N-acetyl glucosamine), We propose that peritrophin-15 caps the ends of the chitin polymer. Immunogold studie s localized peritrophin-15 to the peritrophic matrix and specific vesicles in cells of the cardia, the small organ of the foregut responsible for peri trophic matrix synthesis. The vesicular contents are disgorged at the base of microvilli underlying the newly formed peritrophic matrix. This is the f irst time that the process of synthesis and integration of a peritrophic ma trix protein into the nascent peritrophic matrix has been observed.