Science 333, 453–456 (2011)

Credit: AAAS

The intracellular pathogen Legionella pneumophila exploits host-cell trafficking machinery during infection by modulating the activity of the GTPase Rab1; the bacterial protein SidM recruits Rab1 to L. pneumophila–containing vacuoles (LCVs) and adds AMP to Tyr77, transiently locking Rab1 in an active, GTP-bound state. Release of Rab1 from LCVs, however, requires its inactivation. Now Neunuebel et al. have identified SidD, a bacterially encoded enzyme that de-AMPylates Rab1. The authors noted a conserved open reading frame adjacent to sidM called sidD; deletion of sidD eliminated Rab1 de-AMPylase activity from L. pneumophila lysate. Recombinant SidD catalyzed removal of [32P]AMP from AMPylated Rab1 in vitro in a concentration-dependent manner and, together with SidM, catalyzed an AMPylation-deAMPylation cycle for Rab1, the products of which were validated by MS. In cells, expression of SidD counteracted Golgi fragmentation phenotypes associated with ectopic SidM expression and restored interaction between GTP-Rab1 and regulatory GTPase-activating proteins. During infection, ΔsidD LCVs showed prolonged colocalization with Rab1 compared to wild-type LCVs, suggesting delayed release of Rab1 from vesicles in the absence of SidD activity. Together, these data support a model in which L. pneumophila AMPylates—and thereby stably activates—host Rab1 to promote docking and fusion of secretory vesicles with LCVs until bacterially encoded SidD de-AMPylates Rab1, allowing Rab1 inactivation and membrane extraction.