Crystal structure and binding properties of the Serratia marcescens chitin-binding protein CBP21

Gustav Vaaje-Kolstad, Douglas R. Houston, Anna H. K. Riemen, Vincent G. H. Eijsink, Daan M. F. van Aalten

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    Abstract

    Chitin proteins are commonly found in bacteria that utilize chitin as a source of energy. CBP21 is a chitin-binding protein from Serratia marcescens, a Gram-negative soil bacterium capable of efficient chitin degradation. When grown on chitin, S. marcescens secretes large amounts of CBP21, along with chitin-degrading enzymes. In an attempt to understand the molecular mechanism of CBP21 action, we have determined its crystal structure at 1.55 Å resolution. This is the first structure to be solved of a family 33 carbohydrate-binding module. The structure reveals a "budded" fibronectin type III fold consisting of two ß-sheets, arranged as a ß-sheet sandwich, with a 65-residue "bud" consisting of three short helices, located between ß-strands 1 and 2. Remarkably, conserved aromatic residues that have been suggested previously to play a role in chitin binding were mainly found in the interior of the protein, seemingly incapable of interacting with chitin, whereas the structure revealed a surface patch of highly conserved, mainly hydrophilic residues. The roles of six of these conserved surface-exposed residues (Tyr-54, Glu-55, Glu-60, His-114, Asp-182, and Asn-185) were probed by site-directed mutagenesis and subsequent binding studies. All single point mutations lowered the affinity of CBP21 for ß-chitin, as shown by 3-8-fold increases in the apparent binding constant. Thus, binding of CBP21 to chitin seems to be mediated primarily by conserved, solvent-exposed, polar side chains.
    Original languageEnglish
    Pages (from-to)11313-11319
    Number of pages7
    JournalJournal of Biological Chemistry
    Volume280
    Issue number12
    DOIs
    Publication statusPublished - 25 Mar 2005

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