LAUSR

Divergent protein kinase SidJ is a protein produced by Legionella pneumophila that orchestrates this intracellular pathogen's establishment within the host cell. SidJ prevents lysosome fusion with the vacuole, within which the bacterium resides and replicates. SidJ also modulates the toxicity of the SidE family ubiquitin ligases that catalyze phosphoribosyl-linked host protein ubiquitination. Black et al. discovered that SidJ is activated by host calmodulin. Furthermore, although SidJ has a pseudokinase fold, it does not phosphorylate SidE proteins but polyglutamylates them instead. The in vivo relevance of this mechanism for bacterial infectivity was verified in the natural reservoir host Acanthamoeba castellanii. Science, this issue p. 787 A protein kinase fold in a bacterial effector protein catalyzes polyglutamylation, not phosphorylation. Enzymes with a protein kinase fold transfer phosphate from adenosine 5′-triphosphate (ATP) to substrates in a process known as phosphorylation. Here, we show that the Legionella meta-effector SidJ adopts a protein kinase fold, yet unexpectedly catalyzes protein polyglutamylation. SidJ is activated by host-cell calmodulin to polyglutamylate the SidE family of ubiquitin (Ub) ligases. Crystal structures of the SidJ-calmodulin complex reveal a protein kinase fold that catalyzes ATP-dependent isopeptide bond formation between the amino group of free glutamate and the γ-carboxyl group of an active-site glutamate in SidE. We show that SidJ polyglutamylation of SidE, and the consequent inactivation of Ub ligase activity, is required for successful Legionella replication in a viable eukaryotic host cell.