GSP-DEPENDENT PROTEIN SECRETION IN GRAM-NEGATIVE BACTERIA - THE XCP SYSTEM OF PSEUDOMONAS-AERUGINOSA

Bacteria have evolved several secretory pathways to release proteins i nto the extracellular medium. In Gram-negative bacteria, the exoprotei ns cross a cell envelope composed of two successive hydrophobic barrie rs, 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 memb rane translocation involves an extension of the signal peptide-depende nt pathway for translocation across the cytoplasmic membrane via the S ec machinery. By analogy with the so-called general export pathway (GE P), this latter route, including two separate steps across the inner a nd 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 Xc p system as a basis to discuss multiple aspects of the GSP mechanism, including machinery assembly, exoprotein recognition, energy requireme nt and pore formation for driving through the outer membrane. (C) 1998 Federation of European Microbiological Societies. Published by Elsevi er Science B.V. All rights reserved.