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Journal of Bacteriology, October 2000, p. 5365-5372, Vol. 182, No. 19
Department of Microbiology, University of
Illinois at Urbana-Champaign, Urbana, Illinois 61801
Received 9 March 2000/Accepted 11 July 2000
Bacteroides thetaiotaomicron, a gram-negative obligate
anaerobe, utilizes polysaccharides by binding them to its cell surface and allowing cell-associated enzymes to hydrolyze them into digestible fragments. We use the starch utilization system as a model to analyze
the initial steps involved in polysaccharide binding and breakdown. In
a recent paper, we reported that one of the outer membrane proteins
involved, SusG, had starch-degrading activity but was not sufficient
for growth on starch. Moreover, SusG alone did not have detectable
starch binding activity. Previous studies have shown that starch
binding is essential for starch utilization. In this paper, we report
that four other outer membrane proteins, SusC through SusF, are
responsible for starch binding. Results of 14C-starch
binding assays show that SusC and SusD both contribute a significant
amount of starch binding. SusE also appears to contribute substantially
to starch binding. Using affinity chromatography, we show in vitro that
these Sus proteins interact to bind starch. Moreover, protease
accessibility of either SusC or SusD greatly increased when one was
expressed without the other. This finding supports the hypothesis that
SusC and SusD interact in the outer membrane. Evidence from additional
protease accessibility studies suggests that SusC, SusE, and SusF are
exposed on the cell surface. Our results demonstrate that SusC and SusD
act as the major starch binding proteins on the cell surface, with SusE
enhancing binding. SusF's role in starch utilization has yet to be
determined, although the fact that starch protected it from proteolytic
attack suggests that it does bind starch.
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Characterization of Four Outer Membrane Proteins
Involved in Binding Starch to the Cell Surface of
Bacteroides thetaiotaomicron

and
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Corresponding author. Mailing address: Department of
Microbiology, 601 South Goodwin Ave., Urbana, IL 61801. Phone: (217) 333-7378. Fax: (217) 244-8485. E-mail: abigails{at}uiuc.edu.
Present address: Department of Microbiology and Molecular Genetics,
Harvard Medical School, Boston, MA 02115.
Present address: Department of Biology, Jordan Hall, Indiana
University, Bloomington, IN 47405.
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