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J. Biol. Chem., Vol. 280, Issue 34, 30310-30319, August 26, 2005

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The C-terminal Domain of the Nucleotide-binding Domain Protein Wzt Determines Substrate Specificity in the ATP-binding Cassette Transporter for the Lipopolysaccharide O-antigens in Escherichia coli Serotypes O8 and O9a^*

Leslie Cuthbertson, Jacqueline Powers, and Chris Whitfield, Recipient of a Tier 1 Canada Research Chair¶

From the Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada

The polymannan O-antigenic polysaccharides (O-PSs) of Escherichia coli O8 and O9a are synthesized via an ATP-binding cassette (ABC) transporter-dependent pathway. The group 2 capsular polysaccharides of E. coli serve as prototypes for polysaccharide synthesis and export via this pathway. Here, we show that there are some fundamental differences between the ABC transporter-dependent pathway for O-PS biosynthesis and the capsular polysaccharide paradigm. In the capsule system, mutants lacking the ABC transporter are viable, and membranes isolated from these strains are no longer able to synthesize polymer using an endogenous acceptor. In contrast, E. coli strains carrying mutations in the membrane component (Wzm) and/or the nucleotide-binding component (Wzt) of the O8 and O9a polymannan transporters are nonviable under conditions permissive to O-PS biosynthesis and take on an aberrant elongated cell morphology. Whereas the ABC transporters for capsular polysaccharides with different structures are functionally interchangeable, the O8 and O9a exporters are specific for their cognate polymannan substrates. The E. coli O8 and O9a Wzt proteins contain a C-terminal domain not present in the corresponding nucleotide-binding protein (KpsT) from the capsule exporter. Whereas the Wzm components are functionally interchangeable, albeit with reduced efficiency, the Wzt components are not, indicating a specific role for Wzt in substrate specificity. Chimeric Wzt proteins were constructed in order to localize the region involved in substrate specificity to the C-terminal domain.

Received for publication, April 21, 2005 , and in revised form, June 2, 2005.

^* This work was supported by Canadian Institutes of Health Research Grant MOP-62877 (to C. W.). 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.

The on-line version of this article (available at http://www.jbc.org) contains Tables 2 and 3.

Recipient of a Postgraduate Scholarship (PGS-A) from the Natural Sciences and Engineering Research Council.

Recipient of a Canadian Institutes for Health Research Postdoctoral Fellowship.

^¶ To whom correspondence should be addressed: Dept. of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada. Tel.: 519-824-4120 (ext. 53361); Fax: 519-837-1802; E-mail: cwhitfie{at}uoguelph.ca.

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