Flagellar Hook Flexibility Is Essential for Bundle Formation in Swimming Escherichia coli Cells

Abstract

dSwimming Escherichia coli cells are propelled by the rotary motion of their flagellar filaments. In the normal swimming pattern, filaments positioned randomly over the cell form a bundle at the posterior pole. It has long been assumed that the hook functions as a universal joint, transmitting rotation on the motor axis through up to ϳ90 o to the filament in the bundle. Structural models of the hook have revealed how its flexibility is expected to arise from dynamic changes in the distance between monomers in the helical lattice. In particular, each of the 11 protofilaments that comprise the hook is predicted to cycle between short and long forms, corresponding to the inside and outside of the curved hook, once each revolution of the motor when the hook is acting as a universal joint. To test this, we genetically modified the hook so that it could be stiffened by binding streptavidin to biotinylated monomers, impeding their motion relative to each other. We found that impeding the action of the universal joint resulted in atypical swimming behavior as a consequence of disrupted bundle formation, in agreement with the universal joint model. An Escherichia coli cell swimming is arguably one of the simplest examples of behavior in a living organism. As a consequence, much is understood about the system: the process of flagellum assembly (8), the motor's molecular architecture (25) and physiology (3,32), and the physics of swimming (7, 29). The flagellum is a macromolecular complex made up of ϳ30 different proteins (3), which are assembled in a particular order (8). It can be divided into three parts: the motor, the hook, and the filament. The motor is embedded in the cell envelope, spanning both the inner and outer membranes. In E. coli, motor rotation is powered by the diffusion of protons from the periplasm, down their electrochemical potential gradient, and into the cytoplasm (32). The motor is coupled via the hook to the filament. The hook consists of a flexible curved helical structure 55 nm long (19), made up of ϳ120 copies of the FlgE protein which form 11 helical protofilaments (12,13,33). The filament also consists of a single protein (FliC) made into 11 protofilaments, but it is 10 to 15 m long and rigid for its function as a propeller (3). An E. coli cell typically possesses ϳ4 to 10 flagella which coalesce to form a bundle at the posterior pole when swimming. When one or more motors reverse direction in response to environmental cues, they break from the bundle, causing the cell to reorientate (10).The hook is thought to act as a universal joint, translating rotation from the axis of rotation at the motor, through 90 o , to the filament participating in the bundle. This idea was first postulated by Berg and Anderson in 1973 (4). Since then, models describing the universal joint have been developed based on ever-advancing structural information (17,30,31). FlgE is composed of four domains (D0, Dc, D1, and D2) positioned radially from the inside to the outside of the hook (17). The D0 domains form the inn...

Structural Characterization of the Fla2 Flagellum of Rhodobacter sphaeroides

“…Similar long S-shaped hooks were described in the lateral flagella of S. ruminantium (31). The fact that the curved hooks are present in Fla2 flagella could be related to the fact that these hooks are more flexible than the straight Fla1 hook, and this characteristic may allow several filaments to rotate together in a bundle (48).…”

Bacteria can exploit a flagellar buckling instability to change direction

“…Flexibility also plays a subtle role in the locomotion of bacteria with multiple flagella (peritrichous), such as Escherichia coli, which bundles its flagella together for propulsion (a run 1 ) by exploiting the compliance of the flagellum's base 21,22 . When one or more flagella leave the bundle following a change in the direction of rotation of their motor, the torque resulting from the unbundling, or from the associated polymorphic transformation of the flagellar filament, reorients the cell (a tumble 1,21 ).…”

Biological Magnetometry: Torque on Superparamagnetic Beads in Magnetic Fields

“…2(a)] to rotate magnetic beads inside a static external magnetic field. We fixed E.coli cells (MTB32 [35] ΔcheY strain, i.e., the motor rotates solely in the counterclockwise direction) to glass cover slips (Menzel-Gläser) coated with poly-Llysine (Sigma-Aldrich, P4707). Streptavidin-coated magnetic beads (Dynabeads® MyOne™; 1 μm diameter) were attached to the biotinylated hooks of the flagellar motors anchored in the membranes of these cells [36] In the absence of an external magnetic field, the magnetic beads were freely and continuously rotated by the flagellar motors.…”