The Small Heat-shock Protein HspL Is a VirB8 Chaperone Promoting Type IV Secretion-mediated DNA Transfer |
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Authors: | Yun-Long Tsai Yin-Ru Chiang Franz Narberhaus Christian Baron Erh-Min Lai |
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Institution: | From the ‡Institute of Plant and Microbial Biology, Academia Sinica, Taipei 11529, Taiwan.;the §Lehrstuhl für Biologie der Mikroorganismen, Ruhr-Universität Bochum, D-44780 Bochum, Germany, and ;the ¶Département de Biochimie, Université de Montréal, Montréal, Quebec H3C 3J7, Canada |
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Abstract: | Agrobacterium tumefaciens is a plant pathogen that utilizes a type IV secretion system (T4SS) to transfer DNA and effector proteins into host cells. In this study we discovered that an α-crystallin type small heat-shock protein (α-Hsp), HspL, is a molecular chaperone for VirB8, a T4SS assembly factor. HspL is a typical α-Hsp capable of protecting the heat-labile model substrate citrate synthase from thermal aggregation. It forms oligomers in a concentration-dependent manner in vitro. Biochemical fractionation revealed that HspL is mainly localized in the inner membrane and formed large complexes with certain VirB protein subassemblies. Protein-protein interaction studies indicated that HspL interacts with VirB8, a bitopic integral inner membrane protein that is essential for T4SS assembly. Most importantly, HspL is able to prevent the aggregation of VirB8 fused with glutathione S-transferase in vitro, suggesting that it plays a role as VirB8 chaperone. The chaperone activity of two HspL variants with amino acid substitutions (F98A and G118A) for both citrate synthase and glutathione S-transferase-VirB8 was reduced and correlated with HspL functions in T4SS-mediated DNA transfer and virulence. This study directly links in vitro and in vivo functions of an α-Hsp and reveals a novel α-Hsp function in T4SS stability and bacterial virulence. |
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Keywords: | Bacteria Chaperone Chaperonin Heat Shock Protein Membrane Proteins Protein Secretion Agrobacterium tumefaciens DNA Transfer Type IV Secretion System Small Heat-shock Protein Virulence |
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