Legionella pneumophila is an intracellular pathogen that can avoid delivery to lysosomes — and certain death — after uptake by a host cell, by modifying the vacuole that it is constrained within to provide a safe place for bacterial replication. Research published in the Journal of Experimental Medicine reveals that L. pneumophila recruits the host trafficking proteins Rab1 and Sec22b to aid in the maturation of the vacuole into a replicative organelle.

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.

Sec22b, a SNARE protein that participates in vesicular fusion and is recruited by Rab proteins, was also recruited to the LCV, but again only if the Dot/Icm system was functional. In contrast with Rab1 recruitment, Sec22b recruitment required early ER vesicular transport, so it seems that Sec22b is recruited after Rab1. Kagan et al. disrupted Rab1 function, and used immunogold microscopy to visualize Sec22b, to show that Rab1 recruitment promotes the transport to, and fusion of, ER-derived vesicles that contain Sec22b with the LCV.

Dissecting the LCV-maturation process further to delineate the recruitment and vesicular fusion mechanisms will shed light not only on Legionella pathogenesis, but on the basic cell biology of phagosome maturation. This study clearly demonstrates the utility of bacterial model systems for cell biology research.