The crystal structure of the chitinase ChiA74 of Bacillus thuringiensis has a multidomain assembly

There is no structural information about any chitinase synthesized by Bacillus thuringiensis, the most successful microbial insect larvicide used worldwide. In this study, we solved the 3D structure of the chitinase ChiA74 at 2.26 angstrom. The crystal structure shows that ChiA74 is composed of a modular arrangement formed by (i) a catalytic region (CD), (ii) a chitinase insertion domain (CID), (iii) a fibronectin type III domain (FnIII), and (iv) a chitin binding domain (CBD). The location of the CBD with respect to the CD has no structural similarity to other chitinases with known structures. The activity of a ChiA74 lacking its secretion signal peptide (ChiA74 Delta sp) and a truncated version lacking its CBD/FnIII domains (ChiA74 Delta sp-50) did not have statistical differences in activity against colloidal chitin. However, ChiA74 Delta sp exhibits 4.5 and 2.0 higher activity than versions lacking the CBD (ChiA74 Delta sp-60) and CBD/FnIII domains (ChiA74 Delta sp-50), respectively, when crystalline chitin was used as substrate. Our data suggest that the CBD might plays a significant role in crystalline chitin hydrolysis. We also demonstrated the importance of the catalytic E211 in the CD, as mutants ChiA74 Delta sp(E211N) and ChiA74 Delta sp(D207N, E211N) were inactive against colloidal and crystalline chitins, chitosan and 4-MU-GIcNAc(3). ChiA74 has a processive activity producing oligosaccharides with degree of polymerization (DP) of 1 (GIcNAc) and 2 (GIcNAc(2)).