Building a flagellum outside the bacterial cell

Abstract

Flagella, the helical propellers that extend from the bacterial surface, are a paradigm for how complex molecular machines can be built outside the living cell. Their assembly requires ordered export of thousands of structural subunits across the cell membrane and this is achieved by a type III export machinery located at the flagellum base, after which subunits transit through a narrow channel at the core of the flagellum to reach the assembly site at the tip of the nascent structure, up to 20μm from the cell surface. Here we review recent findings that provide new insights into flagellar export and assembly, and a new and unanticipated mechanism for constant rate flagellum growth.

Keywords: bacterial flagellum; cell motility; chain mechanism; protein export; rotary nanomotor; type III secretion system.

Figure 1

The bacterial flagellum rotary nanomotor. The bacterial flagellum assembles from the inner membrane (IM) to span the PG cell wall and outer membrane (OM), finally extending into the extracellular space. Three contiguous hollow substructures – the rod, hook, and filament – are sequentially assembled. The drive-shaft rod (FliE, FlgB, FlgC, FlgF, and FlgG) is surrounded by a series of rings [lipopolysaccharide (L) ring, FlgH; PG (P) ring, FlgI; membrane–supramembrane (MS) ring, FliF; cytoplasmic (C) ring, FliGMN] and together these elements form the basal body. The C ring interacts with the stator units (MotAB) to drive flagellar rotation. The flexible hook (FlgE) extends from the cell surface with a defined length of ∼55 nm. Hook–filament junction proteins (FlgKL) connect the hook to the flagellar filament (flagellin, FliC). Subunits for the rod, hook, and filament are translocated across the cytoplasmic membrane by a dedicated type III export machinery that comprises an ATPase complex (FliI, FliJ, and FliH; the broken line is the predicted position of FliH), an unfolding cage (FlhAc), and a transmembrane export gate (FlhAB and FliOPQR). On crossing the membrane, subunits then transit through the central channel in the external flagellum to the distal tip, where they crystallise beneath specific cap foldases for the rod (FlgJ), hook (FlgD), and filament (FliD). The positions of individual protein structures are derived from cryoelectron tomography and represent the elements that remain in the mature structure.

Figure 2

Sequential interactions of flagellar subunits with the membrane export machinery. The rotary ATPase complex (FliI hexamer, 2DPY ; FliJ escort, 3AJW [28]) is associated with the C ring (FliG_MC, 1LKV ; FliM, 2HP7 ; FliN tetramer, 1YAB [70]) via FliH (proposed position indicated by broken lines). The chaperone–subunit complex (FliS–FliC, 1ORY [71]) docks initially at the ATPase complex before entering the export cage, comprising a nonameric ring of FlhA (FlhA_C, 3A5I [72]), and then passing to the export gate component FlhB_C (3B0Z [45]). Subunits are then translocated across the cell membrane into the export channel.

Figure 3

Model for hook-length control and the export-specificity switch mediated by the molecular ruler FliK. (A) During assembly of the flagellar hook (red), export of hook subunits is interrupted intermittently by the export of the molecular ruler FliK (purple). The N terminus of FliK interacts with the hook cap (FlgD, orange) at the tip of the nascent hook and, before the hook reaches its mature length of ∼55 nm, the C-terminal region of the transiting FliK does not interact with the FlhB export gate, which is instead occupied by incoming hook subunits. (B) When the hook reaches its mature length, a gate-binding region (red cylinder) in the C terminus of the transiting FliK becomes exposed and remains in the cytoplasm for a sufficient length of time to form a productive interaction with the FlhB export gate, causing a conformational change (FlhB*). The precise mechanisms underlying this export-specificity switch are unknown. (C) The specificity switch permits the export, and subsequent assembly, of filament subunits (blue).

Figure 4

A chain mechanism delivers subunits to the assembly tip of the external flagellum. Subunits are unfolded by the export machinery before recognition by the FlhB_C export gate component. The N terminus (blue) of the subunit docked at the export gate is captured by the free C terminus (red) of an exiting subunit in the flagellar channel, linking subunits head to tail in a chain. Linked unfolded subunits in the channel transit to the flagellum tip, where they crystallise sequentially beneath the cap foldase. As subunits fold into the flagellum tip, the chain becomes stretched, increasing the entropic pulling force at the cell-proximal end of the chain until a threshold force is reached and a new subunit is pulled from the export machinery into the channel. This process repeats, delivering subunits to the flagellum tip at a constant rate.