Identification and Characterization of the Three Chitin-Binding Domains within the Multidomain Chitinase Chi92 from Aeromonas hydrophila JP101

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Abstract
The gene ( chi92 ) encoding the extracellular chitinase of Aeromonas hydrophila JP101 has been cloned and expressed in Escherichia coli . The mature form of Chi92 is an 842-amino-acid (89.830-kDa) modular enzyme comprised of a family 18 catalytic domain, an unknown-function region (the A region), and three chitin-binding domains (ChBDs; Chi92-N, ChBD CI , and ChBD CII ). The C-terminally repeated ChBDs, ChBD CI and ChBD CII , were grouped into family V of cellulose-binding domains on the basis of sequence homology. Chitin binding and enzyme activity studies with C-terminally truncated Chi92 derivatives lacking ChBDs demonstrated that the ChBDs are responsible for its adhesion to unprocessed and colloidal chitins. Further adsorption experiments with glutathione S -transferase (GST) fusion proteins (GST-CI and GST-CICII) demonstrated that a single ChBD (ChBD CI ) could promote efficient chitin and cellulose binding. In contrast to the two C-terminal ChBDs, the Chi92-N domain is similar to ChiN of Serratia marcescens ChiA, which has been proposed to participate in chitin binding. A truncated derivative of Chi92 that contained only a catalytic domain and Chi92-N still exhibited insoluble-chitin-binding and hydrolytic activities. Thus, it appears that Chi92 contains Chi92-N as the third ChBD in addition to two ChBDs (ChBD CI and ChBD CII ).

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