Structure and Dynamics of the Bacterial Flagellar Motor Complex

Abstract

Many bacteria swim in liquids and move over solid surfaces by rotating flagella. The bacterial flagellum is a supramolecular protein complex that is composed of about 30 different flagellar proteins ranging from a few to tens of thousands. Despite structural and functional diversities of the flagella among motile bacteria, the flagellum commonly consists of a membrane-embedded rotary motor fueled by an ion motive force across the cytoplasmic membrane, a universal joint, and a helical propeller that extends several micrometers beyond the cell surface. The flagellar motor consists of a rotor and several stator units, each of which acts as a transmembrane ion channel complex that converts the ion flux through the channel into the mechanical work required for force generation. The rotor ring complex is equipped with a reversible gear that is regulated by chemotactic signal transduction pathways. As a result, bacteria can move to more desirable locations in response to environmental changes. Recent high-resolution structural analyses of flagella using cryo-electron microscopy have provided deep insights into the assembly, rotation, and directional switching mechanisms of the flagellar motor complex. In this review article, we describe the current understanding of the structure and dynamics of the bacterial flagellum.

Keywords: bacterial flagellum; chemotaxis; cryo-electron microscopy (cryo-EM); motility; proton motive force; rotor; stator; torque generation; transmembrane proton channel complex.

Figure 4

Stator activation and assembly. (a) A four-state model for stator assembly. The proton channel is unplugged in the transition from the diffusion state to the loose-binding state, as shown in (b). Wadhwa et al. assumed that the transition between the loose-binding and tight-binding states depends on torque [139]: $k_{\mathrm{T}}/k_{\mathrm{L}}$ increases with torque in agreement with (b). In the transient-unbound state, the stator unit is hypothesized not to produce torque but to stay in the vicinity of the rotor and return to the bound, torque-producing state for a very short time: $k_{\mathrm{H}\to \mathrm{T}}\gg k_{\mathrm{T}\to \mathrm{H}}$. PG, peptidoglycan layer; CM, cytoplasmic membrane. (b) Catch-bond mechanism. (c) Stator binding probability depends on motor rotation. The time for stator assembly to a rotating motor (${\tau }_{\omega >0}$) is shorter than that of a non-rotating motor where no stator is docked (${\tau }_{\omega =0}$).

Figure 1

Schematic diagram of the Salmonella flagellum. The flagellum consists of a type III secretion system (fT3SS), a membrane-embedded basal body, a short flexible hook, a hook–filament junction, a long filament, and a filament cap. Several stator units, formed by 5 MotA subunits and 2 MotB subunits, surround the basal body, although they are not shown in this diagram. The inset shows the cryo-EM structure of the hook-basal body isolated from Salmonella (PDB ID: 7CGO). OM, outer membrane; PG, peptidoglycan layer; CM, cytoplasmic membrane.

Figure 2

Cryo-EM structures of ring complexes in the Salmonella basal body. (a) Cα ribbon diagram of the atomic model of the MS-ring (PDB ID: 8T8P). Only two Cα backbones (Mol-A and Mol-B) are color-coded from blue to red, going through the rainbow colors from the N-terminus to the C-terminus. FliF has three ring-building motifs: RBM1, RBM2, and RBM3. RBM3 forms the S-ring and β-collar with C34 symmetry. FliF has two different conformations in the MS-ring, so RBM2 in Mol-A faces inward, and in Mol-B faces outward. RBM1 is missing in Mol-A. As a result, 23 RBM2 domains face inward to form the inner core ring of the M-ring, while the remaining 11 RBM1-RBM2 domains form cog-like structures just outside the inner core ring. (b) Atomic models of the C-ring (PDB ID: 8UOX) and the FliG₁-FliM₁-FliN₃ complex (PDB ID: 8UMD) in Cα ribbon representation. The C-ring is composed of 34 FliG subunits, 34 FliM subunits, and 102 FliN subunits. FliG (blue) consists of three domains, FliG_N, FliG_M, and FliG_C and two helix linkers named Helix_NM and Helix_MC. FliM (cyan) consists of an intrinsically disordered N-terminal region and two compactly folded domains, FliM_M and FliM_C. FliN (magenta) consists of an intrinsically disordered N-terminal region and a compactly folded domain, FliN_C. FliM_M binds to FliG_M, whereas FliM_C forms a spiral structure along with the three FliN_C domains. (c) Cα ribbon diagram of the atomic model of the LP-ring complex (PDB ID: 8WHT). The LP-ring complex is composed of 26 FlgH subunits and 26 FlgI subunits. The three FlgH subunits are colored in coral, whereas the three FlgI subunits are colored in dodger blue. The N-terminal disordered chain of FlgI binds to FlgH.

Figure 3

Atomic models of flagellar axial proteins. (a) Cα ribbon diagrams of the distal rod protein (FlgG) (PDB ID: 7CBM), the hook protein (FlgE) (PDB ID: 7CBM), and the filament protein (FliC), namely flagellin (PDB ID:1UCU) derived from Salmonella. The N-terminal and C-terminal α-helices form an α-helical coiled-coil structure (domain D0). (b) Structural comparison of FlgE (plum) and FlgG (sky blue). Domains D0, Dc, and D1 of FlgE are structurally similar to those domains in FlgG (left panel), so newly transported FlgE molecule assembles into the hook at the tip of the distal rod (right panel). (c) Structural comparison of the R-type FliC subunit (PDB ID: 1UCU, magenta) and the R-type FljB subunit (PDB ID: 6JY0, green). Domains D0, D1, and D2 of FljB are nearly identical to those of FliC (top panel); however, the electron density corresponding to domain D3 of FliC is very poor in the FljB filament structure (lower panel); the cryo-EM structure of this domain is missing.

Figure 5

Structure of the stator complex and bi-directional rotation model. (a) MotA₅-MotB₂ complex of Campylobacter jejuni (PDB ID: 6YKM). (b) The structure of Clostridium sporogenes MotA–FliG_C fusion (PDB ID: 8UCS) overlaid on the FliG_C domain of the CCW (PDB ID: 8UMD) (upper left) and CW C-ring structures (PDB ID: 8UMX) (upper right) rotor subunits. A model for CCW/CW bidirectional rotor rotation based on unidirectional CW stator rotation (viewed from the cytoplasmic membrane looking down onto the FliG ring; lower panels).