An intrinsically disordered linker controlling the formation and the stability of the bacterial flagellar hook

Abstract

BackgroundIn a macro-molecular complex, any minor change may prove detrimental. For a supra-molecular nano-machine like the bacterial flagellum, which consists of several distinct parts with specific characteristics, stability is important. During the rotation of the bacterial flagellar motor, which is located in the membrane, the flagella rotate at speeds between 200 and 2000 rpm, depending on the bacterial species. The hook substructure of the bacterial flagellum acts as a universal joint connecting the motor to the flagellar filament. We investigated the formation of the bacterial flagellar hook and its overall stability between the FlgE subunits that make up the hook and attempted to understand how this stability differs between bacteria.ResultsAn intrinsically disordered segment plays an important role for overall hook stability and for its structural cohesion during motor rotation. The length of this linker segment depends on the species of bacteria; for Salmonella enterica and Campylobacter jejuni it is approximately 37 and 54 residues, respectively. Few residues of the linker are conserved and mutating the conserved residues of the linker yields non-flagellated cells. In the case of Campylobacter, which rotates its flagella at a speed much higher than that of Salmonella, shortening the linker leads to a rupture of the hook at its base, decreasing cell motility. Our experiments show that this segment is required for polymerization and stability of the hook, demonstrating a surprising role for a disordered region in one of the most finely tuned and closely studied macromolecular machines.ConclusionsThis study reveals a detailed functional characteristic of an intrinsically disordered segment in the hook protein. This segment evolved to fulfill a specific role in the formation of the hook, and it is at the core of the stability and flexibility of the hook. Its length is important in the case of bacteria with high-speed rotating flagella. Finding a way of disrupting this linker in Campylobacter might help in preventing infections.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-017-0438-7) contains supplementary material, which is available to authorized users.

Dynamic stiffening of the flagellar hook

“…The fact that stiffening in bending occurs with increasing twist suggests that a coupling may exist between these two classically decoupled directions, as described in actin filaments and DNA 41 , 44 , 45 . However, we note that this formalism does not explain a change in stiffness (see Supplementary Note 2) .…”

“…The D2 domain of subunit 0 also interacts with the D1 domain of subunit 11, and MD simulations have shown large axial sliding upon compression and extension 13 , 14 , 37 . An intrinsically disordered region that connects D0 and D1 governs inter-subunit interactions and has been shown to play a role in the stability of the hook structure 41 – 43 . It is therefore conceivable that, under the strain imposed by a global twist, changes in these interactions could give rise to decreased bending flexibility, a hypothesis that could be explored by future MD simulations.…”

Campylobacter jejuni genotypes are associated with post-infection irritable bowel syndrome in humans

“…kpsM mutant of strain 81–176 showed a 10-fold reduction in intestinal epithelial cells invasion in vitro 65 . flgE encodes for flagellar hook, which is a joint between the motor and flagellar filament, and is found across a number of motile bacterial species 66 , 67 . Thus, in addition to stress response genes, variants in flagellar- and motility-associated genes are significantly associated with PI-IBS.…”

Genetic Variation in Toll-Like Receptor 5 and Colonization with Flagellated Bacterial Vaginosis-Associated Bacteria

“…These domains can affect protein structure and function through many mechanisms, such as by posttranslational modification, folding, creation of “fuzzy complexes,” and interactions with other proteins ( 31 ). Disordered protein regions have been described in genes encoding flagellin and affect function of the flagellar motor ( 32 , 33 ). We hypothesize that M. mulieris uses this disordered domain for steric inhibition of TLR5 dimerization and downstream immune response.…”