GSP-dependent protein secretion in gram-negative bacteria: the Xcp system of Pseudomonas aeruginosa

FEMS Microbiol Rev. 1998 Sep;22(3):177-98. doi: 10.1111/j.1574-6976.1998.tb00366.x.


Bacteria have evolved several secretory pathways to release proteins into the extracellular medium. In Gram-negative bacteria, the exoproteins cross a cell envelope composed of two successive hydrophobic barriers, the cytoplasmic and outer membranes. In some cases, the protein is translocated in a single step across the cell envelope, directly from the cytoplasm to the extracellular medium. In other cases, outer membrane translocation involves an extension of the signal peptide-dependent pathway for translocation across the cytoplasmic membrane via the Sec machinery. By analogy with the so-called general export pathway (GEP), this latter route, including two separate steps across the inner and the outer membrane, was designated as the general secretory pathway (GSP) and is widely conserved among Gram-negative bacteria. In their great majority, exoproteins use the main terminal branch (MTB) of the GSP, namely the Xcp machinery in Pseudomonas aeruginosa, to reach the extracellular medium. In this review, we will use the P. aeruginosa Xcp system as a basis to discuss multiple aspects of the GSP mechanism, including machinery assembly, exoprotein recognition, energy requirement and pore formation for driving through the outer membrane.

Publication types

  • Review

MeSH terms

  • Amino Acid Sequence
  • Aspartic Acid Endopeptidases / metabolism*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Endopeptidases*
  • Fimbriae Proteins*
  • Gram-Negative Bacteria / metabolism*
  • Membrane Glycoproteins / genetics
  • Membrane Proteins / metabolism
  • Membrane Transport Proteins*
  • Molecular Sequence Data
  • Pseudomonas aeruginosa / enzymology*
  • Sequence Alignment
  • Serine Endopeptidases*


  • Bacterial Proteins
  • Membrane Glycoproteins
  • Membrane Proteins
  • Membrane Transport Proteins
  • XcpP protein, Pseudomonas aeruginosa
  • XcpQ protein, Pseudomonas aeruginosa
  • XcpX protein, Pseudomonas aeruginosa
  • pilE protein, Neisseria gonorrhoeae
  • Fimbriae Proteins
  • Endopeptidases
  • prepilin peptidase protein, Bacteria
  • Serine Endopeptidases
  • xanthomonapepsin
  • Aspartic Acid Endopeptidases