No abstract

Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: tissue-type plasminogen activator (tPA); plasminogen (Plg); middle cerebral artery occlusion (MCAO); blood-brain barrier (BBB); LDL receptor-related protein (LRP); urokinase-type plasminogen activator (uPA); receptor-associated protein (RAP); N-methyl-Daspartate (NMDA); protease-activated receptor (PAR).

The type II secretion pathway or the main terminal branch of the general secretion pathway, as it has also been referred to, is widely distributed among Proteobacteria, in which it is responsible for the extracellular secretion of toxins and hydrolytic enzymes, many of which contribute to pathogenesis in both plants and animals. Secretion through this pathway differs from most other membrane transport systems, in that its substrates consist of folded proteins. The type II secretion apparatus is composed of at least 12 different gene products that are thought to form a multiprotein complex, which spans the periplasmic compartment and is specifically required for translocation of the secreted proteins across the outer membrane. This pathway shares many features with the type IV pilus biogenesis system, including the ability to assemble a pilus‐like structure. This review discusses recent findings on the organization of the secretion apparatus and the role of its various components in secretion. Different models for pilus‐mediated secretion through the gated pore in the outer membrane are also presented, as are the possible properties that determine whether a protein is recognized and secreted by the type II pathway.

Extracellular secretion of proteins is regarded as a major virulence mechanism in bacterial infection. Proteins destined for the extracellular environment of gram-negative bacteria have to cross two membranes during their journey across the bacterial cell envelope. This involves translocation across the cytoplasmic membrane and the outer membrane, which are separated by the periplasmic compartment and the peptidoglycan layer. Several highly specialized pathways have evolved for this purpose. The type II secretion pathway is one such system that is encoded by at least 12 genes and specifically supports the transport of a group of seemingly unrelated proteins across the outer membrane. In order for these proteins to enter the type II secretion pathway, they have to first translocate across the cytoplasmic membrane via the Sec system and then fold into a translocation competent conformation in the periplasm. Proteins secreted by the type II pathway include proteases, cellulases, pectinases, phospholipases, lipases, and toxins. In general, these proteins are associated with destruction of various tissues, which contributes to cell damage and disease. Expression of the genes for these proteins and, in some cases, the secretion genes themselves are under quorum-sensing control or are strictly regulated by the environment at the site of colonization. This type of regulation results in controlled secretion of virulence factors that will only occur when the bacteria have reached their correct location and obtained a critial mass. Species identified to date as harboring type II secretion genes are members of proteobacteria, and most of them appear to be extracellular pathogens. Genome sequencing of bacterial pathogens continues to identify additional species that contain DNA highly homologous to the type II secretion genes, suggesting that this pathway is widely distributed within the proteobacterial family. This review discusses the identification of additional type II secretion genes and the expansion of the type II secretion gene family. The regulation of type II secretion and its role in disease are also addressed. Finally, the finding that a component of the type II secretion pathway in Vibrio cholerae can also support bacteriophage extrusion is discussed.

Vibrio cholerae secretes a number of proteins important for virulence, including cholera toxin. This process requires the products of the eps genes which have homologues in genera such as Aeromonas, Klebsiella and Pseudomonas and are thought to form a membrane‐associated multiprotein complex. Here we show that the putative nucleotide‐binding protein EpsE is associated with and stabilized by the cytoplasmic membrane via interaction with EpsL. Analysis of fusion proteins between EpsE and the homologous ExeE from Aeromonas hydrophila demonstrates that the N‐terminus of EpsE contains the EpsL binding domain and determines species specificity. An intact Walker A box, commonly found in ATP‐binding proteins, is required for activity of EpsE in vivo and for autophosphorylation of purified EpsE in vitro. These results indicate that both the kinase activity of EpsE as well as its ability to interact with the putative cytoplasmic membrane protein EpsL are required for translocation of toxin across the outer membrane in Vibrio cholerae.