title: Septins arrange F-actin-containing fibers on the Chlamydia trachomatis inclusion and are required for normal release of the inclusion by extrusion.
abstract: Chlamydia trachomatis is an obligate intracellular human pathogen that grows inside a membranous, cytosolic vacuole termed an inclusion. Septins are a group of 13 GTP-binding proteins that assemble into oligomeric complexes and that can form higher-order filaments. We report here that the septins SEPT2, -9, -11, and probably -7 form fibrillar structures around the chlamydial inclusion. Colocalization studies suggest that these septins combine with F actin into fibers that encase the inclusion. Targeting the expression of individual septins by RNA interference (RNAi) prevented the formation of septin fibers as well as the recruitment of actin to the inclusion. At the end of the developmental cycle of C. trachomatis, newly formed, infectious elementary bodies are released, and this release occurs at least in part through the organized extrusion of intact inclusions. RNAi against SEPT9 or against the combination of SEPT2/7/9 substantially reduced the number of extrusions from a culture of infected HeLa cells. The data suggest that a higher-order structure of four septins is involved in the recruitment or stabilization of the actin coat around the chlamydial inclusion and that this actin recruitment by septins is instrumental for the coordinated egress of C. trachomatis from human cells. The organization of F actin around parasite-containing vacuoles may be a broader response mechanism of mammalian cells to the infection by intracellular, vacuole-dwelling pathogens. Importance: Chlamydia trachomatis is a frequent bacterial pathogen throughout the world, causing mostly eye and genital infections. C. trachomatis can develop only inside host cells; it multiplies inside a membranous vacuole in the cytosol, termed an inclusion. The inclusion is covered by cytoskeletal "coats" or "cages," whose organization and function are poorly understood. We here report that a relatively little-characterized group of proteins, septins, is required to organize actin fibers on the inclusion and probably through actin the release of the inclusion. Septins are a group of GTP-binding proteins that can organize into heteromeric complexes and then into large filaments. Septins have previously been found to be involved in the interaction of the cell with bacteria in the cytosol. Our observation that they also organize a reaction to bacteria living in vacuoles suggests that they have a function in the recognition of foreign compartments by a parasitized human cell.
authors.affiliation: Institute for Medical Microbiology and Hygiene, University Medical Center Freiburg, Freiburg, Germany.
authors.forename: Larisa
authors.initials: L
authors.lastname: Volceanov
authors.affiliation: Institute for Medical Microbiology and Hygiene, University Medical Center Freiburg, Freiburg, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Herbst
authors.affiliation: Institute of Molecular Medicine and Cell Research, University of Freiburg, Freiburg, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Biniossek
authors.forename: Oliver
authors.initials: O
authors.lastname: Schilling
authors.affiliation: Technische Universität München, Chair of Nutrition and Immunology, ZIEL, Research Center for Nutrition and Food Sciences, Biofunctionality Unit, Freising-Weihenstephan, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Haller
authors.affiliation: Institute of Experimental and Clinical Pharmacology and Toxicology, University of Freiburg, Freiburg, Germany.
authors.forename: Thilo
authors.initials: T
authors.lastname: Nölke
authors.affiliation: Department of Microbiology, University of Würzburg, Biocenter, Wuerzburg, Germany.
authors.forename: Prema
authors.initials: P
authors.lastname: Subbarayal
authors.affiliation: Department of Microbiology, University of Würzburg, Biocenter, Wuerzburg, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Rudel
authors.affiliation: Department of Pediatrics and Adolescent Medicine, University Medical Center, Freiburg, Freiburg, Germany.
authors.forename: Barbara
authors.initials: B
authors.lastname: Zieger
authors.affiliation: Institute for Medical Microbiology and Hygiene, University Medical Center Freiburg, Freiburg, Germany georg.haecker@uniklinik-freiburg.de.
authors.forename: Georg
authors.initials: G
authors.lastname: Häcker
mesh_terms: D000199:Actins / Q000378:metabolism*; D002692:Chlamydia trachomatis / Q000502:physiology*; D004847:Epithelial Cells / Q000382:microbiology*; D005089:Exocytosis; D006367:HeLa Cells; D006801:Humans; D002479:Inclusion Bodies / Q000382:microbiology*; D055503:Protein Multimerization; D058112:Septins / Q000378:metabolism*
chemical_list: D000199:Actins; C496549:SEPTIN11 protein, human; C000602252:SEPTIN2 protein, human; C474315:SEPTIN9 protein, human; D058112:Septins