Structure and Enzymatic Properties of a Two-Domain Family GH19 Chitinase from Japanese Cedar (Cryptomeria japonica) Pollen

CJP-4 is an allergen found in pollen of the Japanese cedar Cryptomeria japonica. The protein is a two-domain family GH19 (class IV) Chitinase consisting of an N-terminal CBM18 domain and a GH19 catalytic domain. Here, we produced recombinant CJP-4 and CBM18-truncated CJP-4 (CJP-4-Cat) proteins. In addition to solving the crystal structure of CJP-4-Cat by X-ray crystallography, we analyzed the ability of both proteins to hydrolyze chitin oligosaccharides, (GlcNAc)_n, polysaccharide substrates, glycol chitin, and β-chitin nanofiber and examined their inhibitory activity toward fungal growth. Truncation of the CBM18 domain did not significantly affect the mode of (GlcNAc)_n hydrolysis. However, significant effects were observed when we used the polysaccharide substrates. The activity of CJP-4 toward the soluble substrate, glycol chitin, was lower than that of CJP-4-Cat. In contrast, CJP-4 exhibited higher activity toward β-chitin nanofiber, an insoluble substrate, than did CJP-4-Cat. Fungal growth was strongly inhibited by CJP-4 but not by CJP-4-Cat. These results indicate that the CBM18 domain assists the hydrolysis of insoluble substrate and the antifungal action of CJP-4-Cat by binding to chitin. CJP-4-Cat was found to have only two loops (loops I and III), as reported for ChiA, an allergenic class IV Chitinase from maize.