pilins

Pilins are small proteins that serve as the building blocks of pili, filamentous surface appendages found on many bacteria. In Gram-negative bacteria, the best-characterized pilins form Type IV pili, which participate in adhesion, surface sensing, motility, DNA uptake, and conjugation. Major pilin subunits typically form the pilus shaft, while minor pilins and tip-associated proteins modulate binding and function.

Most Type IV pilins are synthesized with an N-terminal signal sequence that directs insertion into the inner membrane. A prepilin peptidase (PilD) cleaves the leader peptide and often methylates the new N-terminus. Mature pilins contain an amphipathic N-terminal alpha-helix that drives insertion into the membrane and drives incorporation into a growing pilus filament during assembly.

Pilus assembly relies on a multi-protein machine. Inner-m membrane components and cytoplasmic ATPases, such as PilF, power pilus extension, while PilT and related ATPases mediate retraction. The outer membrane secretin complex, including PilQ in many species, forms a channel through which the pilus is extruded. PilC and other pilus-associated proteins coordinate base assembly with this channel.

Functions of pili are diverse. They mediate attachment to host cells and abiotic surfaces, enabling colonization and biofilm formation. They drive twitching motility by cycles of extension and retraction, and they facilitate horizontal gene transfer through conjugative pili and natural transformation via DNA uptake. Minor pilins and adhesin-like tip proteins often determine host specificity and binding strength.

Diversity among pilins reflects species-specific roles. While major pilins define shaft properties, minor pilins, pilin-like proteins, and tip adhesins tailor interactions with hosts and environments. In addition to Type IV pili in Gram-negatives, some Gram-positive bacteria use sortase-assembled pili with functionally analogous roles.