Two-partner secretion in Gram-negative bacteria: a thrifty, specific pathway for large virulence proteins

A collection of large virulence exoproteins, including Ca2+-independent cytolysins, an iron acquisition protein and several adhesins, are secreted by the two-partner secretion (TPS) pathway in various Gram-negative bacteria. The hallmarks of the TPS pathway are the presence of an N-proximal module called the 'secretion domain' in the exoproteins that we have named the TpsA family, and the channel-forming beta-barrel transporter proteins we refer to as the TpsB family. The genes for cognate exoprotein and transporter protein are usually organized in an operon. Specific secretion signals are present in a highly conserved region of the secretion domain of TpsAs. TpsBs probably serve as specific receptors of the TpsA secretion signals and as channels for the translocation of the exoproteins across the outer membrane. A subfamily of transporters also mediates activation of their cognate cytolysins upon secretion. The exoproteins are synthesized as precursors with an N-terminal cleavable signal peptide, and a subset of them carries an extended signal peptide of unknown function. According to our current model, the exoproteins are probably translocated across the cytoplasmic membrane in a Sec-dependent fashion, and their signal peptide is probably processed by a LepB-type signal peptidase. The N-proximal secretion domain directs the exoproteins towards their transporters early, so that translocation across both membranes is coupled. The exoproteins transit through the periplasm in an extended conformation and fold progressively at the cell surface before eventually being released into the extracellular milieu. Several adhesins also undergo extensive proteolytic processing upon secretion. The genes of many new TpsAs and TpsBs are found in recently sequenced genomes, suggesting that the TPS pathway is widespread.