The small GTPase Rab5 inhibits actin polymerization mediated by the Legionella pneumophila effector VipA.

Legionella pneumophila is a facultative intracellular bacterium that causes Legionnaires’ disease, one of the most severe manifestations of atypical pneumonia. The capacity of L. pneumophila to cause disease relates with its ability to survive and multiply inside a membrane-bound compartment, the Legionella-containing vacuole (LCV). The remodeling of the LCV into a replication-competent compartment requires the Icm/Dot type 4 secretion system, that translocates over 300 bacterial effector proteins into host cells. Their action is concerted spatially and timely in order to modulate a multitude of host cell targets and thus ensure LCV segregation from the phagolysosomal pathway. The L. pneumophila effector VipA associates with early endosomes and promotes the nucleation of new actin filaments in vitro. Additionally, ectopically produced VipA impairs vesicle trafficking in Saccharomyces cerevisiae. However, the mechanisms underlying its association with the endocytic pathway have remained elusive. In this work, we sought the molecular interacting partners of VipA in early endosomes and explored the effects of this association. We found that VipA interacts directly with two key components of these organelles, the membrane lipid phosphoinositide 3-phosphate (PI3P) and the small GTPase Rab5. Binding to Rab5 requires the NH2-terminal region of VipA but not the COOH-terminal/actin-binding region. Furthermore, binding to Rab5 inhibits VipA-mediated actin polymerization by preventing de novo actin filament formation. These findings offer new insights into the mode of action of VipA and underscore the intricate network of interactions between L. pneumophila effectors and their host cell targets, namely in the endocytic pathway.