Identification, expression and bioactivity of a chitotriosidase-like homolog in amphioxus: Dependence of enzymatic and antifungal activities on the chitin-binding domain

The mammalian chitinase family 18 consists of two members, chitotriosidase (ChT) and acidic chitinase (AMCase). Despite the enormous progress on mammalian ChT study, little information regarding ChT is available to date in lower animals. In this study, we identified a chitotriosidase-like gene from the amphioxus Branchiostoma japonicum, named BjChTl, which consisted of a signal peptide, a catalytic domain, a Ser/Thr-rich linker region and a chitin-binding domain (CB domain). Sequence comparison and phylogenetic analysis showed that BjChTl was the common ancestor of ChTs and AMCases, implicating that ChT and AMCase evolved from an ancient gene like BjBhTl via gene duplication. qRT-PCR analysis revealed that BjChTl was expressed in the hepatic caecum and hind gut in a tissue-specific fashion. Both chitin-binding and enzymatic activities as well as antifungal activity assays demonstrated that like human ChT, recombinant BjChTl was able to bind to chitin particles, to hydrolyze artificial chitin substrate 4-methylumbelliferyl-β-d-N,N′,N″-triacetylchitotrioside, and to inhibit the growth of the fungus Candida albicans. Surprisingly, recombinant BjChTl-CD lacking CB domain retained partial capacity to bind to chitin, but its enzymatic activity was almost completely lost. These findings suggest that the CB domain is necessary for the execution of both enzymatic and antifungal activities of recombinant BjChTl. It is also the first study showing the presence of a ChT-like homolog with both chitinolytic activity and fungistatic activity in non-vertebrate species.