After entering the host cell (usually a macrophage) L. pneumophila is confined in a Legionella-containing vacuole (LCV), which later matures into a replicative organelle. LCV maturation depends on the expression of the Dot/Icm bacterial secretion system, which injects bacterial proteins into the host cell cytosol. Microscopy and bacterial genetics have revealed that vesicles that exit the endoplasmic reticulum (ER), and are destined for transport to the Golgi, are redirected to fuse with the LCV. Subsequent remodelling of the LCV results in a protected niche — the replicative organelle — in which the bacteria multiply. Mechanisms that mediate the LCV maturation process, including the transport pathways that the bacterium affects, were investigated in this study.
Rab proteins function to transport vesicles between the ER and the Golgi, so Kagan et al. used immunofluorescence microscopy to investigate whether Rab proteins were recruited to the LCV. Of the three Rab proteins (Rab1, 2 and 6) that function in vesicular transport between the ER and the Golgi only Rab1 was localized to the LCV. This process depended on functioning of the Dot/Icm secretion system, but not on the exit of early secretory vesicles from the ER, so bacterial factors injected into the host cell presumably recruit Rab1. Since treatment of host cells to prevent vesicular transport did not prevent Rab1 recruitment to the LCV, Rab1 recruitment is independent of ER vesicle-mediated LCV-remodelling. Abrogation of cellular Rab1 function reduced the intracellular replication of L. pneumophila so Rab1 clearly has a role in conversion of the LCV into a competent replication organelle.
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