Discovery of a Novel Inner Membrane-Associated Bacterial Structure Related to the Flagellar Type III Secretion System

Mohammed Kaplan, Catherine M. Oikonomou, Cecily R. Wood, Georges Chreifi, Debnath Ghosal, Megan J. Dobro, Qing Yao, Ritesh Ranjan Pal, Amit K. Baidya, Yuxi Liu, Stefano Maggi, Alasdair W. McDowall, Sigal Ben-Yehuda, Ilan Rosenshine, Ariane Briegel, Morgan Beeby, Yi Wei Chang, Carrie L. Shaffer, Grant J. Jensen

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


The bacterial flagellar type III secretion system (fT3SS) is a suite of membrane-embedded and cytoplasmic proteins responsible for building the flagellar motility machinery. Homologous nonflagellar (NF-T3SS) proteins form the injectisome machinery that bacteria use to deliver effector proteins into eukaryotic cells, and other family members were recently reported to be involved in the formation of membrane nanotubes. Here, we describe a novel, evolutionarily widespread, hat-shaped structure embedded in the inner membranes of bacteria, of yet-unidentified function, that is present in species containing fT3SS. Mutant analysis suggests a relationship between this novel structure and the fT3SS, but not the NF-T3SS. While the function of this novel structure remains unknown, we hypothesize that either some of the fT3SS proteins assemble within the hat-like structure, perhaps including the fT3SS core complex, or that fT3SS components regulate other proteins that form part of this novel structure. IMPORTANCE The type III secretion system (T3SS) is a fascinating suite of proteins involved in building diverse macromolecular systems, including the bacterial flagellar motility machine, the injectisome machinery that bacteria use to inject effector proteins into host cells, and probably membrane nanotubes which connect bacterial cells. Here, we accidentally discovered a novel inner membrane-associated complex related to the flagellar T3SS. Examining our lab database, which is comprised of more than 40,000 cryo-tomograms of dozens of species, we discovered that this novel structure is both ubiquitous and ancient, being present in highly divergent classes of bacteria. Discovering a novel, widespread structure related to what are among the best-studied molecular machines in bacteria will open new venues for research aiming at understanding the function and evolution of T3SS proteins.

Original languageEnglish
JournalJournal of Bacteriology
Issue number8
StatePublished - Aug 2022

Bibliographical note

Funding Information:
This project was funded by the NIH (grant R01 AI127401 to G.J.J., and grant P20 GM130456 to C.L.S.) and the European Research Council (ERC) Synergy grant (no. 810186 awarded to S.B.Y. and I.R.). M.K. acknowledges a Baxter postdoctoral fellowship from Caltech. Cryo-ET work was done in the Beckman Institute Resource Center for Transmission Electron Microscopy at the California Institute of Technology. We are grateful to Marc Erhardt (Humboldt-Universität zu Berlin) for critically reading an initial version of this work. We thank Elitza I. Tocheva for collecting the A. tumefaciens data, Jian Shi for collecting the H. neptunium data, and Martin Pilhofer for collecting the P. luteoviolacea data. We thank Dianne Newman’s lab for providing the P. aeruginosa transposon mutant. We are grateful to the lab of Robert Heinzen at the National Institute of Allergy and Infectious Diseases in Montana for growing the Coxiella burnetii cells.

Publisher Copyright:
Copyright © 2022 Kaplan et al.


  • bacteria
  • cryo-ET
  • flagella
  • secretion systems

ASJC Scopus subject areas

  • Microbiology
  • Molecular Biology


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