Bacterial Flagellar Protein Export Mechanisms

Bacterial motility relies on the assembly of the flagellum, a complex extracellular filament powered by an internal rotary motor. Construction of this nanomachine requires the coordinated export of component proteins from the cytosol through a specialised type III secretion system (T3SS). The T3SS comprises a transmembrane export gate complex—formed by integral membrane proteins such as FlhA, FlhB, FliP, FliQ and FliR—and a cytoplasmic ATPase complex including FliH, FliI and FliJ. Proteins destined for the growing flagellum carry specific N- and C-terminal export signals and often require dedicated chaperones to prevent premature folding and to guide substrates to the export apparatus. Energy for translocation is derived from the proton motive force across the inner membrane, augmented by ATP hydrolysis. Substrate recognition and ordered secretion are governed by conformational switches within gate components, ensuring that rod and hook proteins are exported before filament subunits. Once docked, substrates unfold and traverse an inner channel to reach the distal tip, where assembly proceeds by diffusion-driven insertion. Recent work has refined our understanding of substrate targeting, gate assembly and energy coupling, revealing multiple layers of regulation that ensure rapid and accurate construction of the flagellar filament under diverse environmental conditions.

Research from Nature Portfolio

Recent studies have illuminated how periplasmic factors and cytoplasmic ATPase components coordinate export fidelity. One investigation demonstrated that a periplasmic chaperone prevents aberrant assembly by ensuring correct rod formation, thereby safeguarding cell integrity. Super-resolution microscopy and mutational analysis revealed that absence of this chaperone leads to mislocalised rod segments and compromised cell walls. In parallel, biophysical and genetic approaches have shown that the conserved motif within the FlhA C-terminal domain undergoes ATPase-dependent remodelling to switch substrate specificity upon hook completion. Alteration of this motif impairs the dynamic ring architecture of FlhA and disrupts the sequential export of filament subunits, underscoring the critical interplay between the ATPase complex and the gate in maintaining export order and efficiency.

Bacterial Flagellar Protein Export Mechanisms publication trend

The graph below shows the total number of articles in bacterial flagellar protein export mechanisms across all publications each year (not limited to Nature Index journals).

Technical terms

Type III secretion system (T3SS): A specialised protein export apparatus that translocates unfolded substrates across the inner membrane for assembly or virulence.

Export gate complex: A transmembrane assembly (FlhA, FlhB, FliP, FliQ, FliR) forming the channel through which flagellar proteins are secreted.

ATPase complex: A cytosolic machinery (FliH, FliI, FliJ) that hydrolyses ATP to energise and regulate protein export.

Proton motive force (PMF): An electrochemical gradient of protons across the membrane that provides energy for protein translocation.

Export signal: Short sequence motifs on substrate proteins that mediate recognition and docking at the export machinery.

Chaperone: A protein that binds export substrates to maintain them in a secretion-competent state and direct them to the gate.

Substrate specificity switch: A conformational change in gate components that alters their affinity for rod or filament proteins in temporal order.

References

1. FlhE functions as a chaperone to prevent formation of periplasmic flagella in Gram-negative bacteria. Nature Communications (2024).
2. FliH and FliI help FlhA bring strict order to flagellar protein export in Salmonella. Communications Biology (2024).
3. Identification of a new export signal that targets early subunits to the flagellar type III secretion export machinery. mBio (2024).
4. Assembly and stoichiometry of the core structure of the bacterial flagellar type III export gate complex. PLOS Biology (2017).
5. The Bacterial Flagellar Type III Export Gate Complex Is a Dual Fuel Engine That Can Use Both H+ and Na+ for Flagellar Protein Export. PLOS Pathogens (2016).
6. Bacterial flagella grow through an injection-diffusion mechanism. eLife (2017).