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J. Biol. Chem., Vol. 279, Issue 9, 8070-8075, February 27, 2004

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Lipopolysaccharide 3-Deoxy-D-manno-octulosonic Acid (Kdo) Core Determines Bacterial Association of Secreted Toxins^*

Amanda L. Horstman, Susanne J. Bauman, and Meta J. Kuehn

From the Duke University Medical Center, Department of Biochemistry, Durham, North Carolina 27710

In contrast to cholera toxin (CT), which is secreted solubly by Vibrio cholerae across the outer membrane, heat-labile enterotoxin (LT) is retained on the surface of enterotoxigenic Escherichia coli (ETEC) via an interaction with lipopolysaccharide (LPS). We examined the nature of the association between LT and LPS. Soluble LT binds to the surface of LPS deep-rough biosynthesis mutants but not to lipid A, indicating that only the Kdo (3-deoxy-D-manno-octulosonic acid) core is required for binding. Although capable of binding truncated LPS and Kdo, LT has a higher affinity for longer, more complete LPS species. A putative LPS binding pocket is proposed based on the crystal structure of the toxin. The ability to bind LPS and remain associated with the bacterial surface is not unique to LT, as CT also binds to E. coli LPS. However, neither LT nor CT is capable of binding to the surface of Vibrio. The core structures of Vibrio and E. coli LPS differ in that Vibrio contains a phosphorylated single Kdo-lipid A, and E. coli LPS contains unphosphorylated Kdo2-lipid A. We determined that the phosphate group on the Kdo core of Vibrio LPS prevents CT from binding, resulting in the secretion of soluble toxin. Because LT binds E. coli LPS, it remains associated with the extracellular bacterial surface and is released in association with outer membrane vesicles. We propose that difference in the extracellular fates of LT and CT contribute to the differences in disease caused by ETEC and Vibrio cholerae.

Received for publication, August 5, 2003 , and in revised form, November 7, 2003.

^* This work was supported by a Burroughs Wellcome Fund Investigator in Pathogenesis of Infectious Disease Award (to M. J. K.) and the National Institutes of Health. 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.

To whom correspondence should be addressed: Duke University Medical Center, Box 3711, Durham, NC 27710. Tel.: 919-684-2545; Fax: 919-684-8885; E-mail: meta.kuehn{at}duke.edu.

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