Structural basis of chaperone-subunit complex recognition by the type 1 pilus assembly platform FimD

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Type 1 pili are crucial virulence factors in uropathogenic Escherichia coli (UPEC).
• Pilus assembly is mediated by outer membrane platforms like FimD, which interact with periplasmic chaperone-subunit complexes.
• Understanding the molecular basis of FimD-chaperone-subunit interactions is key to deciphering pilus biogenesis.

Purpose of the Study:

• To determine the structural basis of FimD-mediated recognition of type 1 pilus chaperone-subunit complexes.
• To elucidate the role of the N-terminal substrate recognition domain of FimD (FimD(N)) in this process.
• To investigate the mechanism of discrimination between different chaperone-subunit complexes.

Main Methods:

• Nuclear magnetic resonance (NMR) and X-ray crystallography were employed to obtain structures of FimD(N).
• Structures were determined for FimD(N) alone and in complex with a chaperone-subunit complex.
• In vivo complementation studies were performed to assess functional implications.

Main Results:

• FimD(N) possesses a novel fold comprising a flexible N-terminal segment, a structured core, and a C-terminal hinge.
• The N-terminal residues (1-24) of FimD(N) directly interact with both the chaperone (FimC) and the pilus subunit in the ternary complex.
• These interactions suggest a sensor mechanism by which FimD(N) recognizes loaded FimC molecules.

Conclusions:

• The N-terminal segment of FimD(N) acts as a specific sensor for correctly loaded chaperone-subunit complexes.
• This mechanism allows bacterial pilus assembly platforms to accurately recognize and discriminate between various chaperone-subunit complexes.
• The findings provide critical insights into the molecular machinery governing type 1 pilus biogenesis in UPEC.