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Journal of Bacteriology, March 2000, p. 1549-1557, Vol. 182, No. 6
0021-9193/00/$04.00+0
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

Hsien-Ming Lee,¹ Kuan-Cheng Wang,^2, Yi-Ling Liu,^3, Hsin-Yan Yew,^4,§ Ling-Yun Chen,⁵ Wei-Ming Leu,¹ David Chanhen Chen,⁶ and Nien-Tai Hu^4,7,*

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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^* 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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Journal of Bacteriology, March 2000, p. 1549-1557, Vol. 182, No. 6
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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