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

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. © 2017 fabiani et al.