The type VI lipase effectors Tle1-Tle5 secreted by the bacterial type VI secretion system (T6SS) have recently been identified as antibacterial effectors that hydrolyze membrane phospholipids. The Tle1-4 display phospholipase A1 and A2 (PLA1) (PLA2) activities. Tle1 belongs to Duf_2235 (T6SS-TLE1) family, TLE2 to Lipase_3, TLE3 to alpha/ beta hydrolase(now the family (T6SS-TLE3) and Tle4 belongs to PGAP1. The adjacent genes code for VgrG proteins and putative periplasmic immunity proteins Tle5 are unrelated phospholipase D type. The bacterial type VI secretion system (T6SS) is a macromolecular machine that injects effectors into prokaryotic and eukaryotic cells. In enteroaggregative Escherichia coli, the Tle1 phospholipase binds the C-terminal extension of the VgrG trimeric spike. The inhibition of Tle1 phospholipase activity once bound to VgrG suggests that Tle1 dissociation from VgrG is required upon delivery
The bacterial type VI secretion system (T6SS) is a macromolecular machine that injects effectors into prokaryotic and eukaryotic cells. The mode of action of the T6SS is similar to contractile phages: the contraction of a sheath structure pushes a tube topped by a spike into target cells. Effectors are loaded onto the spike or confined into the tube. In enteroaggregative Escherichia coli, the Tle1 phospholipase binds the C-terminal extension of the VgrG trimeric spike. Here, we purify the VgrG-Tle1 complex and show that a VgrG trimer binds three Tle1 monomers and inhibits their activity. Using covalent cross-linking coupled to high-resolution mass spectrometry, we provide information on the sites of contact and further identify the requirement for a Tle1 N-terminal secretion sequence in complex formation. Finally, we report the 2.6-A-resolution cryo-electron microscopy tri-dimensional structure of the (VgrG)(3) -(Tle1)(3) complex revealing how the effector binds its cargo, and how VgrG inhibits Tle1 phospholipase activity. The inhibition of Tle1 phospholipase activity once bound to VgrG suggests that Tle1 dissociation from VgrG is required upon delivery.
Title: Structure of the type VI secretion phospholipase effector Tle1 provides insight into its hydrolysis and membrane targeting Hu H, Zhang H, Gao Z, Wang D, Liu G, Xu J, Lan K, Dong Y Ref: Acta Crystallographica D Biol Crystallogr, 70:2175, 2014 : PubMed
A diverse superfamily of phospholipases consisting of the type VI lipase effectors Tle1-Tle5 secreted by the bacterial type VI secretion system (T6SS) have recently been identified as antibacterial effectors that hydrolyze membrane phospholipids. These effectors show no significant homology to known lipases, and their mechanism of membrane targeting and hydrolysis of phospholipids remains unknown. Here, the crystal structure of Tle1 ( approximately 96.5 kDa) from Pseudomonas aeruginosa refined to 2.0 A resolution is reported, representing the first structure of this superfamily. Its overall structure can be divided into two distinct parts, the phospholipase catalytic module and the putative membrane-anchoring module; this arrangement has not previously been observed in known lipase structures. The phospholipase catalytic module has a canonical alpha/beta-hydrolase fold and mutation of any residue in the Ser-Asp-His catalytic triad abolishes its toxicity. The putative membrane-anchoring module adopts an open conformation composed of three amphipathic domains, and its partial folds are similar to those of several periplasmic or membrane proteins. A cell-toxicity assay revealed that the putative membrane-anchoring module is critical to Tle1 antibacterial activity. A molecular-dynamics (MD) simulation system in which the putative membrane-anchoring module embedded into a bilayer was stable over 50 ns. These structure-function studies provide insight into the hydrolysis and membrane-targeting process of the unique phospholipase Tle1.
Membranes allow the compartmentalization of biochemical processes and are therefore fundamental to life. The conservation of the cellular membrane, combined with its accessibility to secreted proteins, has made it a common target of factors mediating antagonistic interactions between diverse organisms. Here we report the discovery of a diverse superfamily of bacterial phospholipase enzymes. Within this superfamily, we defined enzymes with phospholipase A1 and A2 activity, which are common in host-cell-targeting bacterial toxins and the venoms of certain insects and reptiles. However, we find that the fundamental role of the superfamily is to mediate antagonistic bacterial interactions as effectors of the type VI secretion system (T6SS) translocation apparatus; accordingly, we name these proteins type VI lipase effectors. Our analyses indicate that PldA of Pseudomonas aeruginosa, a eukaryotic-like phospholipase D, is a member of the type VI lipase effector superfamily and the founding substrate of the haemolysin co-regulated protein secretion island II T6SS (H2-T6SS). Although previous studies have specifically implicated PldA and the H2-T6SS in pathogenesis, we uncovered a specific role for the effector and its secretory machinery in intra- and interspecies bacterial interactions. Furthermore, we find that this effector achieves its antibacterial activity by degrading phosphatidylethanolamine, the major component of bacterial membranes. The surprising finding that virulence-associated phospholipases can serve as specific antibacterial effectors suggests that interbacterial interactions are a relevant factor driving the continuing evolution of pathogenesis.
