Conjugative pili of IncP plasmids, and the Ti plasmid T pilus are composedof cyclic subunits

TrbC propilin is the precursor of the pilin subunit TrbC of IncP conjugativ e pill in Escherichia coli. Likewise, its homologue, VirB2 propilin, is pro cessed into T pilin of the Ti plasmid T pilus in Agrobacterium tumefaciens. TrbC and VirB2 propilin are truncated post-translationally at the N termin us by the removal of a 36/47-residue leader peptide, respectively. TrbC pro pilin undergoes a second processing step by the removal of 27 residues at t he C terminus by host-encoded functions followed by the excision of four ad ditional C-terminal residues by a plasmid-borne serine protease. The final product TrbC of 78 residues is cyclized via an intramolecular covalent head -to-tail peptide bond. The T pilin does not undergo additional truncation b ut is likewise cyclized. The circular structures of these pilins, as verifi ed by mass spectrometry, represent novel primary configurations that confor m and assemble into the conjugative apparatus.