The small heat-shock protein HspL is a VirB8 chaperone promoting type IV secretion-mediated DNA transfer.
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 alpha-crystallin type small heat-shock protein (alpha-Hsp), HspL, is a molecular chaperone for VirB8, a T4SS assembly factor. HspL is a typical alpha-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 alpha-Hsp and reveals a novel alpha-Hsp function in T4SS stability and bacterial virulence.
Tsai
Yun-Long
YL
Institute of Plant and Microbial Biology, Academia Sinica, Taipei 11529, Taiwan.
Chiang
Yin-Ru
YR
Narberhaus
Franz
F
Baron
Christian
C
Lai
Erh-Min
EM
D016960:Agrobacterium tumefaciens / Q000235:genetics / Q000378:metabolism* / Q000472:pathogenicity; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D015153:Blotting, Western; D002462:Cell Membrane / Q000378:metabolism; D002950:Citrate (si)-Synthase / Q000378:metabolism; D050888:Heat-Shock Proteins, Small / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018832:Molecular Chaperones / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D055503:Protein Multimerization; D055550:Protein Stability; D013379:Substrate Specificity; D013696:Temperature; D014170:Transformation, Genetic; D014774:Virulence
D001426:Bacterial Proteins; D050888:Heat-Shock Proteins, Small; D018832:Molecular Chaperones; D002950:Citrate (si)-Synthase