A flagellum-specific chaperone facilitates assembly of the core type III export apparatus of the bacterial flagellum.

Florian D Fabiani,Eric J C Gálvez,Marc Erhardt,Thibaud T Renault,Samuel Wagner,Britta Peters,Emmanuelle Charpentier,Karen Freier,Michael Hensel,Boris Macek,Tobias Dietsche,Mirita Franz-Wachtel,Alina Guse,Till Strowig,Ann Stock

doi:10.1371/journal.pbio.2002267

Abstract

Many bacteria move using a complex, self-assembling nanomachine, the bacterial flagellum. Biosynthesis of the flagellum depends on a flagellar-specific type III secretion system (T3SS), a protein export machine homologous to the export machinery of the virulence-associated injectisome. Six cytoplasmic (FliH/I/J/G/M/N) and seven integral-membrane proteins (FlhA/B FliF/O/P/Q/R) form the flagellar basal body and are involved in the transport of flagellar building blocks across the inner membrane in a proton motive force-dependent manner. However, how the large, multi-component transmembrane export gate complex assembles in a coordinated manner remains enigmatic. Specific for most flagellar T3SSs is the presence of FliO, a small bitopic membrane protein with a large cytoplasmic domain. The function of FliO is unknown, but homologs of FliO are found in >80% of all flagellated bacteria. Here, we demonstrate that FliO protects FliP from proteolytic degradation and promotes the formation of a stable FliP–FliR complex required for the assembly of a functional core export apparatus. We further reveal the subcellular localization of FliO by super-resolution microscopy and show that FliO is not part of the assembled flagellar basal body. In summary, our results suggest that FliO functions as a novel, flagellar T3SS-specific chaperone, which facilitates quality control and productive assembly of the core T3SS export machinery.

Highlights

Many bacteria use the bacterial flagellum for directed movement in various environments
The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
The ability of many bacteria to move on surfaces and swim through liquid environments depends on the function of a rotating nanomachine, the bacterial flagellum

Summary

Introduction

The ability of many bacteria to move on surfaces and swim through liquid environments depends on the function of a rotating nanomachine, the bacterial flagellum. The self-assembly of all other axial parts of the flagellum is dependent on protein export through the flagellar-specific type III secretion system (fT3SS) [3]. The core integral-membrane components of the flagellar export apparatus (FlhA/B and FliP/Q/R) are closely related to the virulence-associated T3SS (vT3SS) of the injectisome device used by many gram-negative bacteria to inject toxins into host cells [4]. Protein export via both fT3SS and vT3SS is primarily dependent on the proton motive force (pmf) [5,6,7,8,9]. How the bacterial cell coordinates the self-assembly of the multicomponent export apparatus complex in the inner membrane remains elusive

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