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Journal of Bacteriology, March 2000, p. 1549-1557, Vol. 182, No. 6
Graduate Institute of Agricultural
Biotechnology,1 Graduate Institute of
Molecular Biology,2 Graduate Institute
of Botany,3 Graduate Institute of
Biological Chemistry,4 Graduate
Institute of Veterinary Microbiology,6 and
Agricultural Biotechnology
Laboratories,7 National Chung Hsing University,
and Graduate Institute of Biochemistry, Chung Shan Medical
and Dental College,5 Taichung, Taiwan, Republic
of China
Received 8 November 1999/Accepted 22 December 1999
An xps gene cluster composed of 11 open reading frames
is required for the type II protein secretion in Xanthomonas
campestris pv. campestris. Immediately upstream of the
xpsD gene, which encodes an outer membrane protein that
serves as the secretion channel by forming multimers, there exists an
open reading frame (previously designated ORF2) that could encode a
protein of 261 amino acid residues. Its N-terminal hydrophobic region
is a likely membrane-anchoring sequence. Antibody raised against this
protein could detect in the wild-type strain of X. campestris pv. campestris a protein band with an apparent
molecular mass of 36 kDa by Western blotting. Its aberrant slow
migration in sodium dodecyl sulfate-polyacrylamide gels might be due to
its high proline content. We designated this protein XpsN. By
constructing a mutant strain with an in-frame deletion of the
chromosomal xpsN gene, we demonstrated that it is required
for the secretion of extracellular enzyme by X. campestris pv. campestris. Subcellular fractionation studies indicated that the
XpsN protein was tightly associated with the membrane. Sucrose gradient
sedimentation followed by immunoblot analysis revealed that it
primarily appeared in the cytoplasmic membrane fractions. Immune
precipitation experiments indicated that the XpsN protein was
coprecipitated with the XpsD protein. In addition, the XpsN protein was
co-eluted with the (His)6-tagged XpsD protein from the
metal affinity chromatography column. All observations suggested that
the XpsN protein forms a stable complex with the XpsD protein. In
addition, immune precipitation analysis of the XpsN protein with
various truncated XpsD proteins revealed that the C-terminal region of
the XpsD protein between residues 650 and 759 was likely to be involved
in complex formation between the two.
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Copyright © 2000, American Society for Microbiology. All rights reserved.
Association of the Cytoplasmic Membrane Protein
XpsN with the Outer Membrane Protein XpsD in the Type II Protein
Secretion Apparatus of Xanthomonas campestris pv.
Campestris


*
Corresponding author. Mailing address: Graduate
Institute of Biological Chemistry, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 40227, Taiwan, R.O.C. Phone: 886-4-2874754. Fax: 886-4-2861905. E-mail: nthu{at}nchu.edu.tw.
Deceased.
Present address: 269 San Fung Road, Fung Yuan, Taichung County,
Taiwan, R.O.C.
§
Present address: Kuo Kwang Biotech Corp., Tan Tsu Shiang,
Taichung County, Taiwan, R.O.C.
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