Architecture, function, and substrates of the type II secretion system

The type II secretion system (T2SS) is one of several extracellular secretion systems in Gram-negative bacteria. While highly prevalent in gamma- and betaproteobacteria, the T2SS is also recognized to a lesser extent in members of the delta and alpha classes (¹). It is known for its prolific protease secretion activity. In addition, the T2SS mediates extracellular delivery of a variety of toxins, lipases, and enzymes that break down complex carbohydrates, thus conferring a survival advantage to pathogenic as well as environmental species (^2-4). The T2SS is not restricted to extracellular pathogens, such as Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Vibrio cholerae; it is also present and contributes to growth of intracellular pathogens, including Legionella pneumophila, which replicates in aquatic amoebae, alveolar macrophages, and epithelial cells (^5-7). The obligate intracellular pathogen Chlamydia trachomatis also depends on T2SS components for extracellular secretion; however, its T2SS is atypical, as some components are missing or are too different from homologs in other species to be identified using BLAST algorithms (^{8, 9}).