Septins arrange F-actin-containing fibers on the Chlamydia trachomatis inclusion and are required for normal release of the inclusion by extrusion.
Bottom Line: RNAi against SEPT9 or against the combination of SEPT2/7/9 substantially reduced the number of extrusions from a culture of infected HeLa cells.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.
Affiliation: Institute for Medical Microbiology and Hygiene, University Medical Center Freiburg, Freiburg, Germany.Show MeSH
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Mentions: To understand the contribution of septins to the cytoskeletal embedding of the inclusion, we costained infected cells for septins and microtubules or actin. There appeared to be no appreciable colocalization of SEPT2 and tubulin (see Fig. S2A in the supplemental material). However, simultaneous staining for SEPT2 and actin filaments (using phalloidin) produced a very similar fibrillar picture and clear colocalization of the two structures (Fig. 3). Because of cross-reactivity of the secondary antibodies, we were unable to stain simultaneously for two septin species. However, staining for SEPT9 or SEPT11 also indicated colocalization of these septins with F actin, similar to the case with SEPT2 (Fig. 3; in some images, the bundles containing F actin and septins around the nucleus described earlier  are also visible). These results are strongly suggestive of the formation of fibers that contain actin together with SEPT2, -9, and -11 (and possibly also SEPT7), encasing/coating the chlamydial inclusion.
Affiliation: Institute for Medical Microbiology and Hygiene, University Medical Center Freiburg, Freiburg, Germany.