A Flagellum-Specific Chaperone Facilitates Assembly of the Core Type III Export Apparatus of the Bacterial Flagellum

PLoS Biol. 2017 Aug 3;15(8):e2002267. doi: 10.1371/journal.pbio.2002267. eCollection 2017 Aug.

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.

MeSH terms

  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Flagella / physiology*
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism*
  • Phylogeny
  • Type III Secretion Systems*

Substances

  • Bacterial Proteins
  • FliO protein, Bacteria
  • Membrane Proteins
  • Type III Secretion Systems

Grant support

Deutsche Forschungsgemeinschaft (grant number SFB 944 projects P4 and Z). Received by M.H.. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Deutsche Forschungsgemeinschaft (grant number SFB 766 project B14). Received by S.W.. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Helmholtz Association (grant number VH-GS-202). Received by F.D.F.. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Alexander von Humboldt Foundation. Received by T.T.R.. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Europeans Unions FP7 (grant number 334030). Received by M.E.. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Deutsche Forschungsgemeinschaft (grant number ER 778/2-1). Received by M.E.. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Helmholtz Association (grant number VH-NG-932). Received by M.E.. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.