Type‐III secretion pore formed by flagellar protein FliP

Abstract

SummaryDuring assembly of the bacterial flagellum, protein subunits that form the exterior structures are exported through a specialized secretion apparatus energized by the proton gradient. This category of protein transport, together with the similar process that occurs in the injectisomes of gram-negative pathogens, is termed type-III secretion. The membrane-embedded part of the flagellar export apparatus contains five essential proteins: FlhA, FlhB, FliP, FliQ and FliR. Here, we have undertaken a variety of experiments that together support the proposal that the proteinconducting conduit is formed primarily, and possibly entirely, by FliP. Chemical modification experiments demonstrate that positions near the center of certain FliP trans-membrane (TM) segments are accessible to polar reagents. FliP expression sensitizes cells to a number of chemical agents, and mutations at predicted channel-facing positions modulate this effect. Multiple assays are used to show that FliP suffices to form a channel that can conduct a variety of mediumsized, polar molecules. Conductance properties are strongly modulated by mutations in a methionine-rich loop that is predicted to lie at the inner mouth of the channel, which might form a gasket around cargo molecules undergoing export. The results are discussed in the framework of an hypothesis for the architecture and action of the cargo-conducting part of the type-III secretion apparatus.

The FlgN chaperone activates the Na+-driven engine of the flagellar protein export apparatus

“…The export gate complex is located inside the basal body MS ring and acts as a proton/protein antiporter to drive H + -coupled protein translocation across the cytoplasmic membrane 3,4 . FliP, FliQ and FliR form a polypeptide channel complex for the translocation of export substrates across the cytoplasmic membrane 6,7 . FlhB associates with the FliP/FliQ/FliR complex and is postulated to coordinate gate opening of the polypeptide channel 8 .…”

In situ structures of periplasmic flagella reveal a distinct cytoplasmic ATPase complex inBorrelia burgdorferi

“…3H). It has been recently suggested FliP/Q/R are likely to form the central channel complex (Kuhlen et al, 2018; Ward et al, 2018). Our study provided evidence that the assembly of the FlhA complex depends on the formation of the FliP/Q/R channel.…”

A LuxR‐type regulator, AcrR, regulates flagellar assembly and contributes to virulence, motility, biofilm formation, and growth ability of Acidovorax citrulli

“…Why the three genes, fliP , fliQ , and fliR , which code for FliP, FliQ, and FliR, respectively, in flagellar biosynthesis were highly expressed in the acrR mutant in comparison to the wild‐type strain is unclear. FliP, FliQ, and FliR are components of the flagellar secretion apparatus that anchors at the cell membrane (Ward et al , ) and belongs to the flagellar type III secretion system (fT3SS). They reside within the MS‐ring, a subdomain of the hook‐basal‐body, which is located within the cytoplasmic membrane (Figure ) (Zhuang and Shapiro, ; Fan et al , ).…”