PMID:15929981

Citation	Vaaje-Kolstad, G, Horn, SJ, van Aalten, DM, Synstad, B and Eijsink, VG (2005) The non-catalytic chitin-binding protein CBP21 from Serratia marcescens is essential for chitin degradation. J. Biol. Chem. 280:28492-7
Abstract	The Gram-negative soil bacterium Serratia marcescens uses three different family 18 chitinases to degrade chitin, an abundant insoluble carbohydrate polymer composed of beta(1,4)-linked units of N-acetylglucosamine. We show that efficient chitin degradation additionally depends on the action of a small non-catalytic protein, CBP21, which binds to the insoluble crystalline substrate, leading to structural changes in the substrate and increased substrate accessibility. CBP21 strongly promoted hydrolysis of crystalline beta-chitin by chitinases A and C, while it was essential for full degradation by chitinase B. CBP21 variants with single mutations on the largely polar binding surface lost their ability to promote chitin degradation, while retaining considerable affinity for the polymer. Thus, binding alone is not sufficient for CBP21 functionality, which seems to depend on specific, mostly polar interactions between the protein and crystalline chitin. This is the first time a secreted binding protein is shown to assist in the enzymatic degradation of an insoluble carbohydrate via non-hydrolytic disruption of the substrate. Interestingly, homologues of CBP21 occur in most chitin-degrading microorganisms, suggesting a general mechanism by which chitin-binding proteins enhance chitinolytic activity. Homologues also occur in chitinase-containing insect viruses, whose infectiousness is known to depend on chitinase efficiency.
Links	PubMed Online version:10.1074/jbc.M504468200
Keywords	Amino Acid Sequence; Bacterial Proteins/metabolism; Bacterial Proteins/ultrastructure; Carrier Proteins/metabolism; Carrier Proteins/ultrastructure; Chitin/metabolism; Chitin/ultrastructure; Conserved Sequence; Kinetics; Microscopy, Electron, Scanning; Models, Molecular; Protein Conformation; Recombinant Proteins/metabolism; Recombinant Proteins/ultrastructure; Serratia marcescens/metabolism
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Annotations

Gene product	Qualifier	GO Term	Evidence Code	with/from	Aspect	Extension	Notes	Status
SERMA:O83009		GO:0006032: chitin catabolic process	ECO:0000314:		P		Fig. 2 shows that addition of Serratia marcescens CBP21 increased reaction rates to levels comparable to those observed in reactions where the CBP21 was present from t = 0. In an additional control experiment with ChiB, CBP21 was added after 216 h, which led to full degradation of the substrate (Fig. 2B). Control reactions containing CBP21 without enzyme did not yield detectable amounts of soluble chitooligosaccharides. Taken together, these results demonstrate that CBP21 facilitates the degradation of Beta-chitin by family 18 chitinases in a non-enzymatic manner.	complete CACAO 12522

References

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