Mechanism underlying PilC/PilY1 role in type 4a pilus biogenesis at a late stage occurring after filament assembly

Type 4a pili (T4aP), the most widespread and functionally versatile subtype of the type 4 filaments (T4F) superfamily, are functionalised at their tip by the adhesin PilC/PilY1. Although AlphaFold models predict how this unusually large non-pilin protein is displayed at the pilus tip – by interacting through a short C-terminal stretch of residues with the PilHIJK complex of minor pilins – how PilC/PilY1 drives pilus biogenesis at a stage occurring after filament assembly remains unknown. Here, we answer this question in Neisseria meningitidis . After showing that the last 12 residues of PilC, which we name “plug”, are both necessary and sufficient for pilus biogenesis, we demonstrate that the plug interacts with PilK through β-strand augmentation by determining a crystal structure of a PilK-PLUG fusion protein. We establish that this interaction is strong and sequence-specific (by performing binding assays with purified PilK and synthetic peptides) and takes place at a late stage of pilus biogenesis occurring after filament assembly (by showing that extracellular addition of plug peptide to cultures of a ΔpilC mutant is able to restore pilus biogenesis by acting outside the cells). Finally, we demonstrate that the plug motif markedly stabilises PilK, providing an explanation for the requirement of PilC in pilus biogenesis and for the ability of a PilK-PLUG fusion protein to bypass this requirement. Together, these findings delineate the mechanism and chronology of events underlying PilC/PilY1 role in pilus biogenesis, reconcile all previous observations about this protein, and establish a broadly applicable model for T4aP biogenesis.