In vivo structure of the Legionella type II secretion system by electron cryotomography

The type II secretion system (T2SS) is a multiprotein envelope-spanning assembly that translocates a wide range of virulence factors, enzymes and effectors through the outer membrane of many Gram-negative bacteria. Here, using electron cryotomography and subtomogram averaging methods, we reveal the in vivo structure of an intact T2SS imaged within the human pathogen Legionella pneumophila. Although the T2SS has only limited sequence and component homology with the evolutionarily related type IV pilus (T4P) system, we show that their overall architectures are remarkably similar. Despite similarities, there are also differences, including, for example, that the T2SS–ATPase complex is usually present but disengaged from the inner membrane, the T2SS has a much longer periplasmic vestibule and it has a short-lived flexible pseudopilus. Placing atomic models of the components into our electron cryotomography map produced a complete architectural model of the intact T2SS that provides insights into the structure and function of its components, its position within the cell envelope and the interactions between its different subcomplexes. ; © 2019 Springer Nature Limited. Received 19 January 2019; Accepted 07 October 2019; Published 21 November 2019. Data availability: The subtomogram averages of the L. pneumophila T2SS have been deposited in the Electron Microscopy Data Bank under the following accession codes: EMD-20713 (wild type, aligned on the OM part) and EMD-20712 (wild type, aligned on the IM part). All additional data/information are available from the authors upon request. The authors declare that all data supporting the findings of this study, including source data for Fig. 1, are available within the paper and its Supplementary Information. This work was supported by National Institutes of Health grants AI127401 to G.J.J. and AI043987 to N.P.C. ECT data were recorded at the Beckman Institute Resource Center for Transmission Electron Microcopy at Caltech and the cryo-EM facility at Janelia Research Campus. We ...