Type-III secretion pore formed by flagellar protein FliP

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• Bacterial flagellum assembly involves exporting protein subunits via a specialized secretion apparatus.
• This process, along with injectisomes in pathogens, is known as type-III secretion.
• The flagellar export apparatus includes five essential membrane proteins: FlhA, FlhB, FliP, FliQ, and FliR.

Purpose of the Study:

• To investigate the role of FliP in forming the protein-conducting channel of the type-III secretion apparatus.
• To determine if FliP alone can form a functional channel for protein export.

Main Methods:

• Chemical modification experiments to assess reagent accessibility within FliP transmembrane segments.
• Cell-based assays to evaluate the effects of FliP expression and mutations on chemical agent sensitivity.
• Multiple assays to test the channel-forming and conductance properties of FliP.

Main Results:

• Chemical modification revealed accessibility to polar reagents near the center of FliP transmembrane segments.
• FliP expression sensitized cells to chemical agents, with mutations altering this sensitivity.
• FliP demonstrated the ability to form a channel conducting medium-sized, polar molecules.
• Mutations in a methionine-rich loop of FliP significantly modulated channel conductance properties.

Conclusions:

• FliP is proposed as the primary, if not sole, component forming the protein-conducting conduit in the type-III secretion apparatus.
• A methionine-rich loop in FliP may act as a gasket, regulating cargo passage during export.
• These findings provide insights into the architecture and function of the cargo-conducting channel in type-III secretion.