Legionella Hijacks Rab Legionella pneumophila can infect eukaryotic cells and takes up residence within intracellular vacuoles, where it multiplies. In order to produce and maintain this intracellular niche, the pathogen must manipulate membrane trafficking within the host cell. Now, Müller et al. (p. 946 , published online 22 July) describe the ability of Legionella pneumophila to manipulate vesicular trafficking by the covalent modification of the small guanosine triphosphatase (GTPase) Rab1, which normally regulates the transport of endoplasmic reticulum–derived vesicles in eukaryotic cells. The Legionella protein DrrA is released into the cytosol of infected cells, where it specifically AMPylates a tyrosine residue of one of the regulating regions of Rab1. The modification renders the Rab protein inaccessible to GTPase-activating proteins and thus locks it in its active guanosine triphosphate–bound state.