Structure, heterologous expression, and properties of rice (oryza sativa L.) family 19 chitinases

We identified four new family 19 chitinases in Oryza sativa L. cv. Nipponbare: one class I (OsChia1d), two class II (OsChia2a and OsChia2b), and one class IV (OsChia4a). OsChia2a resembled (about 60% identity) the catalytic domains of class I chitinases, but OsChia2b was almost identical (95% identity) to that of the class IV enzyme. OsChia1c, OsChia1cΔCBD (a deletion of OsChia1c lacking a chitin-binding domain, CBD), and OsChia2b were separately expressed and purified in Pichia pastoris. OsChia1c inhibited fungal growth significantly more than OsChia1cΔCBD or OsChia2b. The activities of these enzymes on chitin polymers were similar, but they acted differently on N-acetylchitooligosaccharides, (GlcNAC)_n. OsChia1c slowly hydrolyzed (GlcNAC)₆ and very poorly hydrolyzed (GlcNAC)₄ and (GlcNAC)₅. In contrast, OsChia2b efficiently hydrolyzed these oligosaccharides. The high antifungal activity and low hydrolytic activity of the class I enzyme towards (GlcNAC)_n imply that it participates in the generation of N-acetylchitooligosaccharide elicitors from the cell walls of infecting fungi.