Bacterial secrets of secretion: EuroConference on the biology of type IV secretion processes

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Abstract
Type IV secretion systems (TFSS) mediate secretion or direct cell‐to‐cell transfer of virulence factors (proteins or protein–DNA complexes) from many Gram‐negative animal, human and plant pathogens, such as Agrobacterium tumefaciens, Bartonella tribocorum, Bordetella pertussis, Brucella suis, Helicobacter pylori, Legionella pneumophila and Rickettsia prowazekii, into eukaryotic cells. Bacterial conjugation is also classified as a TFSS‐like process mediating the spread of broad‐host‐range plasmids between Gram‐negative bacteria such as RP4 and R388, which carry antibiotic resistance genes. Genetic, biochemical, cell biological and structural biology experiments led to significant progress in the understanding of several aspects of TFSS processes. X‐ray crystallography revealed that homologues of the A. tumefaciens inner membrane‐associated proteins VirB11 and VirD4 from H. pylori and R388, respectively, may form channels for substrate translocation or assembly of the transmembrane TFSS machinery. Biochemical and cell biological experiments revealed interactions between components of the periplasmic core components VirB8, VirB9 and VirB10, which may form the translocation channel. Analysis of A. tumefaciens virulence proteins VirE2 and VirF suggested that the periplasmic translocation route of the pertussis toxin from B. pertussis may be more generally valid than previously anticipated. Secretion and modification of toxins from H. pylori and L. pneumophila profoundly affect host cell metabolism, thus entering the discipline of cellular microbiology. Finally, results from genome sequencing projects revealed the presence of up to three TFSS in a single organism, and the analysis of their interplay and adaptation to different functions will be a future challenge. TFSS‐carrying plasmids were discovered in different ecosystems, suggesting that genetic exchange may speed up their evolution and adaptation to different cell–cell interactions.