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EMBO J. 2017 Dec 1;36(23):3517-3531. doi: 10.15252/embj.201797515. Epub 2017 Nov 6.

Hierarchical protein targeting and secretion is controlled by an affinity switch in the type III secretion system of enteropathogenic Escherichia coli.

Author information

1
Laboratory of Molecular Bacteriology, Department of Microbiology and Immunology, Rega Institute for Medical Research, KU Leuven, Leuven, Belgium.
2
Institute of Molecular Biology and Biotechnology, FORTH (Foundation of Research and Technology), University of Crete, Heraklion, Greece.
3
Department of Life Sciences, MRC Centre for Molecular Bacteriology and Infection, Imperial College London, London, UK.
4
University of Minnesota Biochemistry, Molecular Biology & Biophysics, Minneapolis, MN, USA.
5
Laboratory of Molecular Bacteriology, Department of Microbiology and Immunology, Rega Institute for Medical Research, KU Leuven, Leuven, Belgium lily.karamanou@kuleuven.be tassos.economou@kuleuven.be.

Abstract

Type III secretion (T3S), a protein export pathway common to Gram-negative pathogens, comprises a trans-envelope syringe, the injectisome, with a cytoplasm-facing translocase channel. Exported substrates are chaperone-delivered to the translocase, EscV in enteropathogenic Escherichia coli, and cross it in strict hierarchical manner, for example, first "translocators", then "effectors". We dissected T3S substrate targeting and hierarchical switching by reconstituting them in vitro using inverted inner membrane vesicles. EscV recruits and conformationally activates the tightly membrane-associated pseudo-effector SepL and its chaperone SepD. This renders SepL a high-affinity receptor for translocator/chaperone pairs, recognizing specific chaperone signals. In a second, SepD-coupled step, translocators docked on SepL become secreted. During translocator secretion, SepL/SepD suppress effector/chaperone binding to EscV and prevent premature effector secretion. Disengagement of the SepL/SepD switch directs EscV to dedicated effector export. These findings advance molecular understanding of T3S and reveal a novel mechanism for hierarchical trafficking regulation in protein secretion channels.

KEYWORDS:

EPEC ; chaperone; in vitro reconstitution; substrate switching; type III secretion

PMID:
29109154
PMCID:
PMC5709732
DOI:
10.15252/embj.201797515
[Indexed for MEDLINE]
Free PMC Article

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