Phospholipid-independent biogenesis of a functional RP4 conjugation pilus

Conjugation of the RP4 plasmid is mediated by short, non-retractable, rigid mating pili. Here, we report the cryo-EM structure of the RP4 pilus at 2.75 Å resolution. Uniquely, this revealed that the cyclic TrbC pilin subunit is not lipidated; the only known example of a mating pilus made in the absence of phospholipids. This discovery challenges the prevailing model that the lipid moiety is essential for pili biogenesis and conjugation.