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J. Biol. Chem., Vol. 282, Issue 18, 13601-13609, May 4, 2007

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Amyloidogenesis of Type III-dependent Harpins from Plant Pathogenic Bacteria^*

Jonghee Oh, Jung-Gun Kim, Eunkyung Jeon, Chang-Hyuk Yoo, Jae Sun Moon, Sangkee Rhee, and Ingyu Hwang¹

From the Department of Agricultural Biotechnology and Center for Agricultural Biomaterials, Seoul National University, Seoul 151-921 and Plant Genome Research Center, Korea Research Institute of Bioscience and Biotechnology, Daejeon 305-633, Korea

Harpins are heat-stable, glycine-rich type III-secreted proteins produced by plant pathogenic bacteria, which cause a hypersensitive response (HR) when infiltrated into the intercellular space of tobacco leaves; however, the biochemical mechanisms by which harpins cause plant cell death remain unclear. In this study, we determined the biochemical characteristics of HpaG, the first harpin identified from a Xanthomonas species, under plant apoplast-like conditions using electron microscopy and circular dichroism spectroscopy. We found that His₆-HpaG formed biologically active spherical oligomers, protofibrils, and -sheet-rich fibrils, whereas the null HR mutant His₆-HpaG(L50P) did not. Biochemical analysis and HR assay of various forms of HpaG demonstrated that the transition from an -helix to -sheet-rich fibrils is important for the biological activity of protein. The fibrillar form of His₆-HpaG is an amyloid protein based on positive staining with Congo red to produce green birefringence under polarized light, increased protease resistance, and -sheet fibril structure. Other harpins, such as HrpN from Erwinia amylovora and HrpZ from Pseudomonas syringae pv. syringae, also formed curvilinear protofibrils or fibrils under plant apoplast-like conditions, suggesting that amyloidogenesis is a common feature of harpins. Missense and deletion mutagenesis of HpaG indicated that the rate of HpaG fibril formation is modulated by a motif present in the C terminus. The plant cytotoxicity of HpaG is unique among the amyloid-forming proteins that occur in several microorganisms. Structural and morphological analogies between HpaG and disease-related amyloidogenic proteins, such as A protein, suggest possible common biochemical characteristics in the induction of plant and animal cell death.

Received for publication, March 20, 2006 , and in revised form, February 20, 2007.

^* This work was supported by a grant from Crop Functional Genomics Center of the 21st Century Frontier R&D Program funded by Ministry of Science and Technology of the Republic of Korea and by Grant 20050401034743 from Biogreen21 Program funded by Rural Development Administration of the Republic of Korea. 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 supplemental Tables S1 and S2 and Fig. S1.

¹ To whom correspondence should be addressed: Dept. of Agricultural Biotechnology, Seoul National University, Seoul 151-921, Korea. Tel.: 82-2-880-4676; Fax: 82-2-873-2317; E-mail: ingyu{at}snu.ac.kr.

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