Bacterial virulence: The cycle of success for Legionella

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Two papers have provided an insight into the mechanisms that Legionella pneumophila uses to subvert the host-cell vesicular trafficking pathway to create its replicative niche.

Two recent papers have provided an insight into the mechanisms that Legionella pneumophila uses to subvert the host-cell vesicular trafficking pathway to create its replicative niche.

L. pneumophila must establish a replicative niche within alveolar macrophages. This process involves remodelling of the Legionella-containing vacuole (LCV) by exploiting the ability of the host protein Rab1 to recruit material from the vesicular transport pathway between the endoplasmic reticulum and Golgi apparatus. Rab proteins achieve this by reversibly associating with lipid membranes through a process that is known as Rab membrane cycling. Active GTP-bound prenylated Rab proteins are present in membranes. Once inactivated by a GTPase-activating protein (GAP), GDP–Rabs are removed from the membrane and maintained in an inactive form in the cytosol through association with a guanine nucleotide-dissociation inhibitor (GDI). The GDI can be displaced by a GDI-displacement factor (GDF), and the GDP-bound Rab protein is then recruited back to the membrane and activated by a guanine nucleotide-exchange factor (GEF). After the active Rab has carried out its function, membrane cycling is completed when the Rab is inactivated by a GAP protein.

L. pneumophila uses a type IV secretion system to secrete effector proteins into the host-cell cytoplasm. Previous work had shown that one type IV effector, DrrA/SidM, is required for the recruitment of Rab1 to the LCV and has Rab1-specific GEF activity. Machner and Isberg, and Ingmundson et al. were interested in whether DrrA/SidM also has GDF activity. Both groups purified overexpressed, tagged forms of Rab1 and Rab-GDI from eukaryotic cells to ensure that they were prenylated and then showed that DrrA/SidM interferes with Rab1–Rab-GDI complex formation by displacing the Rab-GDI, thus confirming that DrrA/SidM does have GDF activity. Machner and Isberg went on to look at the effects of the presence of liposomal membranes. They found that the activation of Rab1 by the GEF activity of DrrA/SidM was enhanced in the presence of a lipid bilayer. They were then able to follow the association of the proteins with the membrane during nucleotide exchange, which confirmed that once the GDF activity of DrrA/SidM has displaced the Rab-GDI, the free Rab1 protein is inserted in the membrane and can then be activated by the GEF activity of DrrA/SidM.

Both groups were also interested in other L. pneumophila type IV effectors that might function downstream of DrrA. Machner and Isberg showed that L. pneumophila LidA binds GTP–Rab1 that has been activated by DrrA/SidM and supports the accumulation of activated Rab1 on the LCV. Ingmundson et al. found that LepB is delivered into the host-cell cytoplasm shortly after bacterial uptake, is present on the early LCV membrane and can disrupt secretory transport in a mammalian cell line. They completed their work on LepB by demonstrating that this L. pneumophila effector has GAP activity towards Rab1 and propose that this activity completes the membrane cycling of Rab1 that is begun by DrrA/SidM.

DrrA/SidM is the first protein to be identified that has both GDF and GEF activity. Both groups point out that this raises the possibility that there might be eukaryotic GEFs that have similar dual functions.

Sheilagh Molloy

References

Machner, M. P. & Isberg, R. I. A bifunctional bacterial protein links GDI displacement to Rab1 activation. Science 318, 974–977 (2007) | Article | PubMed |
Ingmundson, A. et al. Legionella pneumophila proteins that regulate Rab1 membrane cycling. Nature 450, 365–369 (2007) | Article | PubMed |