Maturation of IncP pilin precursors resembles the catalytic Dyad-like mechanism of leader peptidases.
The pilus subunit, the pilin, of conjugative IncP pili is encoded by the trbC gene. IncP pilin is composed of 78 amino acids forming a ring structure (R. Eisenbrandt, M. Kalkum, E.-M. Lai, C. I. Kado, and E. Lanka, J. Biol. Chem. 274:22548-22555, 1999). Three enzymes are involved in maturation of the pilin: LepB of Escherichia coli for signal peptide removal and a yet-unidentified protease for removal of 27 C-terminal residues. Both enzymes are chromosome encoded. Finally, the inner membrane-associated IncP TraF replaces a four-amino-acid C-terminal peptide with the truncated N terminus, yielding the cyclic polypeptide. We refer to the latter process as "prepilin cyclization." We have used site-directed mutagenesis of trbC and traF to unravel the pilin maturation process. Each of the mutants was analyzed for its phenotypes of prepilin cyclization, pilus formation, donor-specific phage adsorption, and conjugative DNA transfer abilities. Effective prepilin cyclization was determined by matrix-assisted laser desorption-ionization-mass spectrometry using an optimized sample preparation technique of whole cells and trans-3-indolyl acrylic acid as a matrix. We found that several amino acid exchanges in the TrbC core sequence allow prepilin cyclization but disable the succeeding pilus assembly. We propose a mechanism explaining how the signal peptidase homologue TraF attacks a C-terminal section of the TrbC core sequence via an activated serine residue. Rather than cleaving and releasing hydrolyzed peptides, TraF presumably reacts as a peptidyl transferase, involving the N terminus of TrbC in the aminolysis of a postulated TraF-acetyl-TrbC intermediate. Under formal loss of a C-terminal tetrapeptide, a new peptide bond is formed in a concerted action, connecting serine 37 with glycine 114 of TrbC.
Eisenbrandt
R
R
Max-Planck-Institut für Molekulare Genetik, Dahlem, D-14195 Berlin, Germany.
Kalkum
M
M
Lurz
R
R
Lanka
E
E
D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001435:Bacteriophages / Q000235:genetics; D001665:Binding Sites; D002384:Catalysis; D003227:Conjugation, Genetic; D003546:Cysteine Endopeptidases / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli; D036781:Fimbriae Proteins; D005822:Genetic Vectors; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D033903:Periplasmic Proteins; D010641:Phenotype; D018933:Pili, Sex / Q000502:physiology*; D011498:Protein Precursors / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D021382:Protein Sorting Signals; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases / Q000235:genetics / Q000378:metabolism*
D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D008565:Membrane Proteins; D033903:Periplasmic Proteins; D011498:Protein Precursors; D021382:Protein Sorting Signals; D036781:Fimbriae Proteins; D012697:Serine Endopeptidases; C026515:type I signal peptidase; D003546:Cysteine Endopeptidases