Propilin is the precursor of the pilus subunit, pilin, that forms conjugative pili, the filamentous surface appendages required for cell-to-cell contact during the earlier stages of bacterial conjugation, and that retract after contact is established. Mature pilin is assembled with the help of TraQ and TraX (By similarity). Monomer. Interacts with itself to form filaments; also interacts with TraQ (By similarity). Propilin is directed to the inner membrane, where it is cleaved and acetylated. Mature pilin forms filaments that are secreted to form the conjugative pilus (By similarity). Belongs to the TraA family.