Recognition of discrete export signals in early flagellar subunits during bacterial Type III secretion

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Abstract

Type III Secretion Systems (T3SS) deliver subunits from the bacterial cytosol to nascent cell surface flagella. Early flagellar subunits that form the rod and hook substructures are unchaperoned and contain their own export signals. A gate recognition motif (GRM) docks them at the FlhBc component of the FlhAB-FliPQR export gate, but the gate must then be opened and subunits must be unfolded to pass through the flagellar channel. This induced us to seek further signals on the subunits. Here, we identify a second signal at the extreme N-terminus of flagellar rod and hook subunits and determine that key to the signal is its hydrophobicity. We show that the two export signal elements are recognised separately and sequentially, as the N-terminal signal is recognised by the flagellar export machinery only after subunits have docked at FlhB_C via the GRM. The position of the N-terminal hydrophobic signal in the subunit sequence relative to the GRM appeared to be important, as a FlgD deletion variant (FlgD_short), in which the distance between the N-terminal signal and the GRM was shortened, 'stalled' at the export machinery and was not exported. The attenuation of motility caused by FlgD_short was suppressed by mutations that destabilised the closed conformation of the FlhAB-FliPQR export gate, suggesting that the hydrophobic N-terminal signal might trigger opening of the flagellar export gate.

Data availability

Data generated or analysed during this study are included in the manuscript and supporting files or have been submitted to Dryad (https://doi.org/10.5061/dryad.66t1g1k3x.)

The following data sets were generated

1. Fraser GM et al
(2022) Data from: Recognition of discrete export signals in early flagellar subunits during bacterial Type III secretion
Dryad Digital Repository, doi:10.5061/dryad.66t1g1k3x.

http://dx.doi.org/10.5061/dryad.66t1g1k3x

Article and author information

Author details

Owain J Bryant

Department of Biochemistry, University of Oxford, Oxford, United Kingdom

Competing interests
The authors declare that no competing interests exist.
Paraminder Dhillon

Department of Pathology, University of Cambridge, Cambridge, United Kingdom

Competing interests
The authors declare that no competing interests exist.
Colin Hughes

Department of Pathology, University of Cambridge, Cambridge, United Kingdom

Competing interests
The authors declare that no competing interests exist.
Gillian M Fraser

Department of Pathology, University of Cambridge, Cambridge, United Kingdom

For correspondence
gmf25@cam.ac.uk

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0002-4874-8734

Funding

Biotechnology and Biological Sciences Research Council (BB/M007197/1)

Gillian M Fraser

The Wellcome Trust

Colin Hughes

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Copyright

This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.