Research
We study the molecular mechanisms underlying intracellular survival and replication of the bacterial pathogen Legionella pneumophila. Within infected cells L. pneumophila establishes a replication vacuole by exploiting host cell protein and vesicle transport processes. An essential aspect of intracellular bacterial survival is the delivery of a large repertoire of L. pneumophila effector proteins into the host cell. Our main goal is to determine the molecular function of these translocated effectors and to identify the host cell pathways modulated by these proteins.
Legionella pneumophila is a Gram-negative bacterium that is ubiquitously found in fresh-water environments as a natural parasite of amoebae. When inhaled by humans L. pneumophila can cause a severe pneumonia known as Legionnaires’ disease. A key feature for survival within amoeba as well as alveolar macrophages is the ability of the pathogen to establish a replication vacuole that avoids lysosomal fusion. For this purpose, L. pneumophila hijack cargo vesicles of the infected host cell and transform their phagosome into a specialized membrane compartment that resembles host cell rough endoplasmic reticulum (ER). Within this protective niche L. pneumophila can replicate to high numbers, eventually lyze the spent host cell and infect neighboring cells.
Intracellular survival of L. pneumophila depends on the activity of a type IV secretion system (T4SS) named Dot/Icm. This Dot/Icm translocation apparatus delivers a large number of bacterial effector proteins directly into the host cell cytosol. Little is known about the molecular function of these translocated substrates. However, bacterial mutants with a non-functional T4SS are avirulent, underscoring the importance of the effector proteins for L. pneumophila pathogenesis.
We recently discovered SidM as a new Dot/Icm-translocated substrate from L. pneumophila. Our studies showed that SidM modulates the activity of the host cell GTPase Rab1. Rab1 is the key regulator of vesicle transport from the endoplasmic reticulum (ER) to the Golgi complex and colocalizes with the Legionella-containing vacuole (LCV) during infection. We now demonstrated that recruitment of Rab1 to the LCV surface is mediated by SidM. More precisely, we discovered that SidM exhibits the combined activity of a GDI displacement factor (GDF) and a guanine nucleotide exchange factor (GEF). The GDF activity enables SidM to dissociate the complex of Rab1 and its chaperone GDP dissociation inhibitor (GDI), whereas the GEF activity allows SidM to activate GDI-free Rab1 by catalyzing exchange of GDP in Rab1 against GTP. Bacterial mutants lacking SidM are defective for Rab1 recruitment, indicating that this bifunctional activity of SidM is essential for L. pneumophila to target the pool of GDI-bound Rab1 during infection. Although SidM is the first known example for a protein with both GEF and GDF activity, it is most likely that other pro- and eukaryotic proteins exist that share this ability.
LidA is another L. pneumophila protein that is being investigated in our lab. LidA is translocated by the Dot/Icm system during infection and localizes to the surface of the LCV. We now discovered that LidA also interacts with host cell Rab1. However, we found that in the absence of SidM LidA is not sufficient to recruit Rab1 to the LCV. Consistent with this, we revealed that LidA does not possess GDF activity which may explain why this effector cannot target Rab1 bound by GDI. These findings indicate that a hierarchy in Rab1 binding between SidM and LidA exists most likely to coordinate Rab1 modulation by these effector proteins during infection.
The future goal of our lab is to characterize the molecular function of other L. pneumophila effector proteins, to identify host cell targets for these effectors, and to evaluate the role of these host-pathogen interactions for L. pneumophila virulence.
If you are interested in joining our research team in the Unit of Microbial Pathogenesis see the Open Positions page.