Straight and rigid flagellar hook made by insertion of the FlgG specific sequence into FlgE

Koichi D Hiraoka,Tomoko Miyata,Takashi Fujii,Keiichi Namba,Fumiaki Makino,Yusuke V Morimoto,Yumi Inoue,Tohru Minamino

doi:10.1038/srep46723

Abstract

The bacterial flagellar hook connects the helical flagellar filament to the rotary motor at its base. Bending flexibility of the hook allows the helical filaments to form a bundle behind the cell body to produce thrust for bacterial motility. The hook protein FlgE shows considerable sequence and structural similarities to the distal rod protein FlgG; however, the hook is supercoiled and flexible as a universal joint whereas the rod is straight and rigid as a drive shaft. A short FlgG specific sequence (GSS) has been postulated to confer the rigidity on the FlgG rod, and insertion of GSS at the position between Phe-42 and Ala-43 of FlgE actually made the hook straight. However, it remains unclear whether inserted GSS confers the rigidity as well. Here, we provide evidence that insertion of GSS makes the hook much more rigid. The GSS insertion inhibited flagellar bundle formation behind the cell body, thereby reducing motility. This indicates that the GSS insertion markedly reduced the bending flexibility of the hook. Therefore, we propose that the inserted GSS makes axial packing interactions of FlgE subunits much tighter in the hook to suppress axial compression and extension of the protofilaments required for bending flexibility.

Highlights

The bacterial flagellum is a supramolecular motility machine consisting of the basal body, the hook and the filament
To examine whether the GSS consisting of 18 residues, YQTIRQPGAQSSEQTTLP (Fig. 1a), plays an important role in conferring rigidity on the FlgG rod, we first analyzed motility of the flgE+GSS mutant, which has the GSS insertion between Phe-42 and Ala-43 of FlgE in soft agar to see the function of this mutant hook (Fig. 1b)
The tertiary folds of domains D0, Dc and D1 of FlgG are nearly identical to domains D0, Dc and D1 of FlgE25,26, and the FlgG rod has the same helical symmetry and repeat distance as those of the hook[26]

Summary

Introduction

The bacterial flagellum is a supramolecular motility machine consisting of the basal body, the hook and the filament. The rod and hook have distinct mechanical properties, namely the rod being straight and rigid whereas the hook being supercoiled and flexible in bending, the hook is directly connected to the distal end of the FlgG rod[11]. The atomic model of the straight hook shows that the axial packing of the D1 and D2 domains in the outer part of the hook is relatively loose, which is primarily responsible for the bending flexibility of the hook structure[21,23]. The most plausible mechanism of hook supercoiling that has been suggested by the atomic model of a curved supercoiled hook is that the close axial packing of the D2 domains on the inner side of the maximally curved hook, which is made possible by its bending flexibility, is responsible for the supercoiling[21]. The curvature and twist of each supercoil presumably depends on the direction of intermolecular D2-D2 interactions in the protofilaments, suggesting that hook supercoiling could not occur if the bending flexibility is reduced or domain D2 is removed

Methods

Results

Conclusion