The bacterial pathogen Legionella pneumophila replicates in the vacuoles of infected host cells (see Itzen and Goody). To form and maintain this vacuole, the bacterium subverts membrane trafficking by targeting the small guanosine triphosphatase (GTPase) Rab1, which modulates the transport of vesicles derived from the endoplasmic reticulum. Previous work showed that the Legionella effector protein DrrA acts as a guanine nucleotide exchange factor (GEF) for Rab1, thus activating this GTPase. Furthermore, DrrA maintains the active state of Rab1 by adding an adenosine 5'-monophosphate (AMP) to Tyr77, a posttranslational modification called ampylation. This modification requires the FIC (filamentation induced by cAMP) domain of DrrA and renders Rab1 inaccessible to GTPase-activating proteins. Mukherjee et al. found that Rab1 is also subject to an additional posttranslational modification at a serine residue adjacent to the ampylated tyrosine residue during Legionella infection. The Legionella protein AnkX added a phosphocholine moiety to Ser76, and this process also required a functional FIC domain. The Rab1 family member Rab35, which is involved in cargo sorting from early endosomes, was also phosphocholinated. Enlarged endosomes (a phenotype that is consistent with disruption of Rab35 function) and impaired secretion of alkaline phosphatase were detected in eukaryotic cells ectopically expressing AnkX, but not a form of AnkX lacking a functional FIC domain [AnkX(H229A)]. Furthermore, secretion of alkaline phosphatase was also attenuated in cells expressing a form of Rab1 that could not be phosphocholinated. Phosphocholination disrupted the binding of Rab35 to the GEF connecdenn but did not prevent the interaction of Rab1 with the GEF domain of DrrA. Thus, phosphocholination disrupts the function of Rab3, but the effect of phosphocholination on Rab1 remains to be determined. The authors note that other examples of phosphocholination in eukaryotic organisms have been reported.
S. Mukherjee, X. Liu, K. Arasaki, J. McDonough, J. E. Galán, C. R. Roy, Modulation of Rab GTPase function by a protein phosphocholine transferase. Nature 477, 103–106 (2011). [PubMed]
A. Itzen, R. S. Goody, Covalent coercion by Legionella pneumophila. Cell Host Microbe 10, 89–91 (2011). [PubMed]