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