Pseudomonas syringae type III secretion system: Secretion signals and putative docking stations.

摘要

Pseudomonas syringae pv. tomato DC3000, a causal agent of bacterial speck disease on tomato and model plant Arabidopsis, uses a Hrp (hypersensitive response and pathogenicity) type III secretion system (T3SS) to inject type III effector proteins (T3Es) into plant cells. The T3SS is conserved in many Gram-negative bacterial pathogens and is evolutionarily related to the flagellar biogenesis system, which is also considered a T3SS. A DC3000 T3E HopG1 was shown to be secreted in culture from both the Hrp T3SS and the flagellar T3SS. However, HopG1 could only be injected into the plant cells by the Hrp T3SS. We also show that FliC could be injected into the plant cells by the Hrp T3SS. The secretion signal recognized by the flagellar T3SS is localized within the first 100 amino acids of HopG1. This HopG1 secretion signal may represent an ancestral signal that is more similar to that of the flagellar type III-secreted substrates than the other DC3000 T3Es. To identify how type III-secreted substrates are recognized by the P. syringae T3SS, a comprehensive yeast two-hybrid screening was performed to identify protein-protein interactions between the type III apparatus proteins, regulatory proteins, type III chaperones (T3Cs) and T3Es. Many interactions between these proteins were identified. Interactions involving HrpE and the HrcN ATPase and HrpE with specific T3Es were further analyzed. The interaction of HrpE with HrcN ATPase and multiple effectors suggests HrpE plays a role in recruiting T3Es to the type III apparatus. HrpE and HrcN are essential for the function of the T3SS as the P. syringae pv. tomato DC3000 hrpE mutant UNL161 and hrcN mutant UNL235 lost the ability to injecting T3Es into plant cells. HopV1 is one of the T3Es that interacted with HrpE in the yeast two-hybrid assays. Further analysis indicated that HrpE primarily binds to the middle region of HopV1, but not the N-terminal secretion signal regions. Additionally, HopV1's T3C, ShcV, was shown to interact with the N-terminal half of HrcN ATPase. In summary, we present a model where HrpE/HrcN act together to form a docking station for T3E/T3C.