RESUMO
The proteins encoded by the Brassica juncea chitinase gene BjCHI1 and its derived genes BjCHI2 and BjCHI3 were expressed by Multi-copy Pichia expression system. The chitinase activity of FPLC purified BjCHI1, BjCHI2 and BjCHI3 were tested and the results showed that all the three proteins degraded both CM-chitin-RBV and colloidal chitin. The Km values of BjCHI1, BjCHI2 and BjCHI3 for CM-chitin-RBV were estimated as 0.799 mg/mL, 0.544 mg/mL and 0.793 mg/mL, respectively. When the colloidal chitin was used as substrate, the Km values were 0.281 mg/mL, 0.388 mg/mL and 1.643 mg/mL, respectively, indicating chitin-binding domain can increase affinity of chitinase to insoluble substrate. In the agglutination activity assay, only BjCHI1 shows activity when the protein concentration was more than 33 micrograms/mL, while BjCHI2 and BjCHI3 without agglutination activity even when the concentration was increased as high as 800 micrograms/mL. This means that the two chitin-binding domains in BjCHI1 are essential for agglutination and BjCHI1 is the first protein which shows both chitinase and agglutination activity identified so far in plants.