[Source: Proceedings of the National Academy of Sciences of the United States of America, full page: (LINK). Abstract, edited.]
Structural bases for F plasmid conjugation and F pilus biogenesis in Escherichia coli
Bo Hu, Pratick Khara, and Peter J. Christie
PNAS first published June 25, 2019 / DOI: https://doi.org/10.1073/pnas.1904428116
Edited by Scott J. Hultgren, Washington University School of Medicine, St. Louis, MO, and approved June 5, 2019 (received for review March 14, 2019)
Bacterial “sex,” or conjugation, is a central mechanism underlying the proliferation of antibiotic resistance. Despite the discovery of conjugation by F plasmids in Escherichia coliover seven decades ago, we have only now visualized the F-encoded transfer channel and F pilus-associated platforms in the E. coli cell envelope by cryoelectron tomography. The channel supports plasmid transfer or assembly of F pili, which remarkably upon synthesis are deposited onto alternative basal structures around the cell surface. The F plasmid transfer system is a paradigm for the bacterial type IV secretion system (T4SS) superfamily. Consequently, the F-encoded structures are broadly informative of mechanisms underlying the biogenesis and function of type IV secretion machines and associated conjugative pili.
Bacterial conjugation systems are members of the large type IV secretion system (T4SS) superfamily. Conjugative transfer of F plasmids residing in the Enterobacteriaceae was first reported in the 1940s, yet the architecture of F plasmid-encoded transfer channel and its physical relationship with the F pilus remain unknown. We visualized F-encoded structures in the native bacterial cell envelope by in situ cryoelectron tomography (CryoET). Remarkably, F plasmids encode four distinct structures, not just the translocation channel or channel-pilus complex predicted by prevailing models. The F1 structure is composed of distinct outer and inner membrane complexes and a connecting cylinder that together house the envelope-spanning translocation channel. The F2 structure is essentially the F1 complex with the F pilus attached at the outer membrane (OM). Remarkably, the F3 structure consists of the F pilus attached to a thin, cell envelope-spanning stalk, whereas the F4 structure consists of the pilus docked to the OM without an associated periplasmic density. The traffic ATPase TraC is configured as a hexamer of dimers at the cytoplasmic faces of the F1 and F2 structures, where it respectively regulates substrate transfer and F pilus biogenesis. Together, our findings present architectural renderings of the DNA conjugation or “mating” channel, the channel–pilus connection, and unprecedented pilus basal structures. These structural snapshots support a model for biogenesis of the F transfer system and allow for detailed comparisons with other structurally characterized T4SSs.
cryoelectron tomography – DNA conjugation – type IV secretion – pilus – protein transport
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Author contributions: B.H. and P.J.C. designed research; B.H. and P.K. performed research; B.H., P.K., and P.J.C. analyzed data; and P.J.C. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: Density maps and coordinate data that support the F-encoded channel structures determined by cryoelectron tomography have been deposited in The Electron Microscopy Data Bank (EMDB), https://www.ebi.ac.uk/pdbe/emdb (entry nos. EMD-9344 and EMD-9347).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1904428116/-/DCSupplemental.
Published under the PNAS license.
Keywords: Enterobacteriaceae; Plasmids.