Flagellin polymerisation control by a cytosolic export chaperone1 1Edited by I. B. Holland

Abstract

Assembly of the long helical filament of the bacterial flagellum requires polymerisation of ca 20,000 flagellin (FliC) monomeric subunits into the growing structure extending from the cell surface. Here, we show that export of Salmonella flagellin is facilitated specifically by a cytosolic protein, FliS, and that FliS binds to the FliC C-terminal helical domain, which contributes to stabilisation of flagellin subunit interactions during polymerisation. Stable complexes of FliS with flagellin were assembled efficiently in vitro, apparently by FliS homodimers binding to FliC monomers. The data suggest that FliS acts as a substrate-specific chaperone, preventing premature interaction of newly synthesised flagellin subunits in the cytosol. Compatible with this view, FliS was able to prevent in vitro polymerisation of FliC into filaments.

Analysis of Pseudomonas fluorescens F113 genes implicated in flagellar filament synthesis and their role in competitive root colonization

“…In pseudomonads the role of the FliS protein remains unknown. Its distant homologue in enterobacteria has been described as a substrate-specific cytosolic chaperone that facilitates FliC secretion and contributes to the stabilization of the flagellin subunits during polymerization (Auvray et al, 2001;Ozin et al, 2003). The F113 fliS mutant has a very short and thin flagellum (Fig.…”

The SPI2-encoded SseA chaperone has discrete domains required for SseB stabilization and export, and binds within the C-terminus of SseB and SseD

“…In the case of SseB, it appears that the chaperone impinges upon domains (Fig. 6) likely to participate in subunit assembly, as demonstrated for the flagellar chapereone, FliS (Auvray et al, 2001;Ozin et al, 2003). In addition, a second instance where the stabilization and secretion functions of a chaperone can be uncoupled has been reported (Cheng & Schneewind, 1999;Fig.…”

“…A function provided by some virulence chaperones (Neyt & Cornelis, 1999; reviewed by Parsot et al, 2003) and by C-terminal binding flagellar chaperones (Auvray et al, 2001;Ozin et al, 2003) is to control association of subunits before export. It was anticipated that this is the role provided by SseA binding to SseB (Zurawski & Stein, 2003), and the location of the binding region supports this hypothesis.…”

“…This suggests that at least part of the domain occupied by SseA involves the putative coiled-coil. Occupancy of a self-association domain, as demonstrated for flagellar subunit chaperones (Auvray et al, 2001;Ozin et al, 2003), is predicted to interfere with SseB assembly into a translocon sheath. In addition, heterologous protein-protein interactions occurring with SseB are also likely, as suggested by the EPEC paradigm for Esc TTS phylogenetic class.…”

Mathematical model of flagella gene expression dynamics in Salmonella enterica serovar typhimurium

“…During this time, when the secretion is low, aggressive expression of class 3 genes could be deleterious to the cell. This is because class 3 products include FliC (flagelllin) and if not secreted effectively, FliC polymerizes inside the cell leading to death (Auvray et al 2001;Yokoseki et al 1995). Hence, from a cellular perspective it is very important that class 3 expression is commensurate with the secretion capability of the cell.…”