RESEARCH ARTICLE Structural basis for effector transmembrane domain recognition by type VI secretion system chaperones Shehryar Ahmad1,2, Kara K Tsang1,2†, Kartik Sachar3†, Dennis Quentin4, Tahmid M Tashin1,2, Nathan P Bullen1,2, Stefan Raunser4, Andrew G McArthur1,2,5, Gerd Prehna3*, John C Whitney1,2,5* 1Michael DeGroote Institute for Infectious Disease Research, McMaster University, Hamilton, Canada; 2Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Canada; 3Department of Microbiology, University of Manitoba, Winnipeg, Canada; 4Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, Dortmund, Germany; 5David Braley Centre for Antibiotic Discovery, McMaster University, Hamilton, Canada Abstract Type VI secretion systems (T6SSs) deliver antibacterial effector proteins between neighboring 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 6000 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 *For correspondence: class I and class II effector TMD-chaperone complexes from Salmonella Typhimurium and
[email protected] (GP); Pseudomonas aeruginosa, respectively, reveals that Eag chaperones mimic transmembrane helical
[email protected] (JCW) packing to stabilize effector TMDs. In addition to participating in the chaperone-TMD interface, we †These authors contributed find that prePAAR residues mediate effector-VgrG spike interactions. Taken together, our findings equally to this work reveal mechanisms of chaperone-mediated stabilization and secretion of two distinct families of T6SS membrane protein effectors.