β-strand complementation within tip initiation complexes licenses assembly of diverse type IV filaments

Area of Science:

• Microbiology
• Structural Biology
• Molecular Biology

Background:

• Type IV pili (T4P) are essential protein filaments involved in microbial motility, DNA uptake, and host cell adhesion.
• PilC/PilY1 proteins are tip adhesins crucial for T4P function, but their role in initiating fiber assembly remained unclear.

Purpose of the Study:

• To elucidate the mechanism by which PilC/PilY1 proteins control T4P assembly.
• To understand the interaction between PilC/PilY1, minor pilins, and the initiation complex.

Main Methods:

• Structural modeling
• Genetic experiments
• Biochemical experiments
• Comparative analysis of T4P systems (Neisseria gonorrhoeae, Acinetobacter baylyi, Caulobacter crescentus)

Main Results:

• A C-terminal peptide of PilC/PilY1 initiates T4P assembly via β-strand complementation with PilK-family proteins.
• This β-strand is necessary and partially sufficient for triggering fiber assembly.
• A quaternary 'licensing complex' involving PilK, PilC/PilY1, PilI, and PilJ templates and initiates fiber assembly.
• In T2SS and Tad T4P systems, homologs like PilK and CpaL provide the essential β-strand for assembly.

Conclusions:

• PilC/PilY1 proteins retain their position at the fiber tip under mechanical stress by utilizing a β-strand complementation mechanism.
• Diverse T4P systems employ β-strand complementation to license fiber assembly, highlighting an ancient and conserved mechanism.