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J. Biol. Chem., Vol. 280, Issue 31, 28492-28497, August 5, 2005

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The Non-catalytic Chitin-binding Protein CBP21 from Serratia marcescens Is Essential for Chitin Degradation^*

Gustav Vaaje-Kolstad, Svein J. Horn, Daan M. F. van Aalten¶, Bjørnar Synstad, and Vincent G. H. Eijsink||

From the Department of Chemistry, Biotechnology, and Food Science, Norwegian University of Life Sciences, P. O. Box 5003, 1432 Ås, Norway and the Division of Biological Chemistry & Molecular Microbiology, School of Life Sciences, University of Dundee, Dundee DD1 5EH, Scotland, United Kingdom

The Gram-negative soil bacterium Serratia marcescens uses three different family 18 chitinases to degrade chitin, an abundant insoluble carbohydrate polymer composed of (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 -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.

Received for publication, April 25, 2005 , and in revised form, May 26, 2005.

^* This work was supported by Grant 140497/420 from the Norwegian Research Council. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

This article was selected as a Paper of the Week.

^¶ Supported by a Wellcome Trust Research Career Development Fellowship and an European Molecular Biology Organization Young Investigator Award.

^|| To whom correspondence should be addressed. Tel.: 47-64965892; Fax: 47-64965901; E-mail: vincent.eijsink{at}umb.no.

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