The substrate specificity switch FlhB assembles onto the export gate to regulate type three secretion

Area of Science:

• Microbiology
• Structural Biology
• Molecular Biology

Background:

• Type-three secretion systems (T3SS) are essential for bacterial motility and virulence.
• Protein transport via T3SS involves an export gate complex, including a crucial switch protein (FlhB/SctU).

Purpose of the Study:

• To determine the high-resolution structure of the T3SS export gate complex, including the switch protein.
• To elucidate the structural mechanism of substrate specificity switching in T3SS.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to obtain the structure of the export gate at 3.2 Å resolution.
• Structure-informed mutagenesis was employed to investigate the function of key protein domains.

Main Results:

• The cryo-EM structure reveals how the FlhB/SctU switch protein integrates into the export gate, interacting with FliPQR/SctRST.
• An unusual loop structure within FlhB/SctU was identified, which appears critical for regulating gate function.
• Mutagenesis studies confirmed the importance of this loop in controlling protein secretion.

Conclusions:

• The structure provides unprecedented insight into the T3SS export gate mechanism.
• Conformational changes involving FlhB/SctU and FliPQR/SctRST are proposed to regulate gate opening and substrate secretion.
• This work lays the foundation for understanding T3SS regulation and developing novel antimicrobial strategies.