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

Legionella pneumophila is a facultative intracellular Gram-negative bacterium that causes Legionnaires disease, one of the most severe manifestations of atypical pneumonia. L. pneumophila naturally thrives within protozoan hosts in aquatic environments, which serve as reservoirs for bacterial replication and transmission. Inhalation of contaminated aerosols delivers L. pneumophila to the lungs where it is able to infect human alveolar macrophages and multiply intracellularly inside a membrane-bound compartment, the Legionella-containing vacuole (LCV). The main virulence factor of the bacteria is the Icm/Dot type IVb secretion system, responsible for the translocation of over 300 bacterial effector proteins into host cells. These effectors allow the remodelation of the LCV into a replication-competent compartment that seggregates from the phagolysosomal pathway. After being translocated into host cells, L. pneumophila effector VipA associates with early endosomes and F-actin. VipA increases the polymerization of actin filaments in vitro by promoting the nucleation step without the requirement of additional partners, and impairs vesicle trafficking in Saccharomyces cerevisiae. In this work, we sought the interacting partners of VipA in early endosomes and explored the effects of this association. We found that VipA interacts directly to two key components of these organelles, the membrane lipid PI3P and the small GTPase Rab5. Binding to Rab5 requires the N-terminal region of VipA but not the C-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 VipAs mode of action and underscore the intricate interactions between L. pneumophila effectors and their host cell targets, namely the endocytic pathway.