The type III flagellar export specificity switch is dependent on FliK ruler and a molecular clock

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• The flagellar hook is a crucial component of the bacterial type III secretion system, essential for motility.
• Hook length is precisely regulated, but the underlying molecular mechanisms remain incompletely understood.
• The FlhB protein and FliK sensor are implicated in controlling hook length by modulating export substrate specificity.

Purpose of the Study:

• To investigate the role of FliK and FlgE in Salmonella flagellar hook length determination.
• To elucidate the molecular interactions governing hook assembly and length control.
• To identify the factors influencing the timing of export substrate switching.

Main Methods:

• Assaying the binding affinity of FliK domains to hook proteins (FlgD, FlgE).
• Isolation and characterization of Salmonella flgE mutants with altered motility.
• Analysis of hook length and substrate secretion in wild-type and mutant strains, including overexpression studies.

Main Results:

• FliK N-terminal domain (FliK(N)) exhibits high affinity for FlgD and low affinity for FlgE.
• flgE mutants produced significantly shorter flagellar hooks, despite normal secretion of hook-type substrates.
• Overexpression of mutant FlgE restored normal hook length, while wild-type FlgE overexpression resulted in longer hooks.

Conclusions:

• Hook length is dependent on the hook polymerization rate and elongation speed.
• A "time countdown" mechanism, initiated at the start of polymerization, likely regulates the export switch.
• FliK(N) may function as a molecular ruler, but hook length is also influenced by elongation rate and switch timing.