Structural basis for effector transmembrane domain recognition by type VI secretion system chaperones

Shehryar Ahmad, Kara L Tsang, Kartik Sachar, Dennis Quentin, Tahmid M Tashin, Nathan P Bullen, Stefan Raunser, Andrew G McArthur, Gerd Prehna, & John C Whitney

Elife. 2020 Dec 15;9:e62816.

Type VI secretion systems (T6SSs) deliver antibacterial effector proteins between neighbouring bacteria. Many effectors harbor N-terminal transmembrane domains (TMDs) implicated in effector translocation across target cell membranes. However, the distribution of these TMD-containing effectors remains unknown. Here we discover prePAAR, a conserved motif found in over 6,000 putative TMD-containing effectors encoded predominantly by 15 genera of Proteobacteria. Based on differing numbers of TMDs, effectors group into two distinct classes that both require a member of the Eag family of T6SS chaperones for export. Co-crystal structures of class I and class II effector TMD-chaperone complexes from Salmonella Typhimurium and Pseudomonas aeruginosa, respectively, reveals that Eag chaperones mimic transmembrane helical packing to stabilize effector TMDs. In addition to participating in the chaperone-TMD interface, we find that prePAAR residues mediate effector-VgrG spike interactions. Taken together, our findings reveal mechanisms of chaperone-mediated stabilization and secretion of two distinct families of T6SS membrane protein effectors.

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