Type IV pilus protein PilA

Type IV pilus protein PilA is the major structural subunit of type IV pili (T4P), which are long, flexible, and dynamic fibers found on the surface of many Gram-negative and some Gram-positive bacteria. These pili play crucial roles in bacterial motility (twitching), adherence to surfaces, biofilm formation, pathogenesis, DNA uptake (natural competence), and secretion processes. PilA consists of a periplasmic domain and a single transmembrane helix at its N-terminus, adopting a lollipop-like shape. Thousands of PilA subunits polymerize to form helical filaments, providing mechanical strength and flexibility to T4P fibers. PilA is essential for forming T4P fibers and mediates attachment to host cells or abiotic surfaces—key steps in colonization and infection. The assembly/disassembly cycle of PilA involves extension driven by ATPase activity (PilF) and retraction powered by another ATPase (PilT or homologs). Inner membrane platform complexes involving proteins such as PilM/N/O interact with incoming monomeric PilA units during biogenesis.