Genetically distinct pathways guide effector export through the type VI secretion system

Abstract

Summary: Bacterial secretion systems often employ molecular chaperones to recognize and facilitate export of their substrates. Recent work demonstrated that a secreted component of the type VI secretion system (T 6 SS), haemolysin co‐regulated protein (Hcp), binds directly to effectors, enhancing their stability in the bacterial cytoplasm. Herein, we describe a quantitative cellular proteomics screen for T 6 S substrates that exploits this chaperone‐like quality of Hcp. Application of this approach to the Hcp secretion island I‐encoded T 6 SS (H 1‐T 6 SS) of Pseudomonas aeruginosa led to the identification of a novel effector protein, termed Tse 4 (t ype VI s ecretion e xported 4), subsequently shown to act as a potent intra‐specific H 1‐T 6 SS‐delivered antibacterial toxin. Interestingly, our screen failed to identify two predicted H 1‐T 6 SS effectors, Tse 5 and Tse 6, which differ from Hcp‐stabilized substrates by the presence of toxin‐associated PAAR‐repeat motifs and genetic linkage to members of the valine‐glycine repeat protein G (vgrG) genes. Genetic studies further distinguished these two groups of effectors: Hcp‐stabilized effectors were found to display redundancy in interbacterial competition with respect to the requirement for the two H 1‐T 6 SS‐exported VgrG proteins, whereas Tse 5 and Tse 6 delivery strictly required a cognate VgrG. Together, we propose that interaction with either VgrG or Hcp defines distinct pathways for T 6 S effector export.