title: Amyloidogenesis of type III-dependent harpins from plant pathogenic bacteria.
abstract: 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 His(6)-HpaG formed biologically active spherical oligomers, protofibrils, and beta-sheet-rich fibrils, whereas the null HR mutant His(6)-HpaG(L50P) did not. Biochemical analysis and HR assay of various forms of HpaG demonstrated that the transition from an alpha-helix to beta-sheet-rich fibrils is important for the biological activity of protein. The fibrillar form of His(6)-HpaG is an amyloid protein based on positive staining with Congo red to produce green birefringence under polarized light, increased protease resistance, and beta-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 Abeta protein, suggest possible common biochemical characteristics in the induction of plant and animal cell death.
authors.affiliation: Department of Agricultural Biotechnology and Center for Agricultural Biomaterials, Seoul National University, Seoul 151-921, Korea.
authors.forename: Jonghee
authors.initials: J
authors.lastname: Oh
authors.forename: Jung-Gun
authors.initials: JG
authors.lastname: Kim
authors.forename: Eunkyung
authors.initials: E
authors.lastname: Jeon
authors.forename: Chang-Hyuk
authors.initials: CH
authors.lastname: Yoo
authors.forename: Jae Sun
authors.initials: JS
authors.lastname: Moon
authors.forename: Sangkee
authors.initials: S
authors.lastname: Rhee
authors.forename: Ingyu
authors.initials: I
authors.lastname: Hwang
mesh_terms: D020816:Amino Acid Motifs / Q000235:genetics; D000682:Amyloid / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D000818:Animals; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016923:Cell Death / Q000235:genetics; D006090:Gram-Negative Bacteria / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000472:pathogenicity; D016297:Mutagenesis, Site-Directed; D020125:Mutation, Missense; D010935:Plant Diseases / Q000235:genetics / Q000382:microbiology; D018515:Plant Leaves / Q000378:metabolism / Q000382:microbiology / Q000648:ultrastructure; D040681:Structural Homology, Protein; D014026:Nicotiana / Q000378:metabolism* / Q000382:microbiology / Q000648:ultrastructure
chemical_list: D000682:Amyloid; D001425:Bacterial Outer Membrane Proteins