Analysis of SdeA modification by SidJ

Legionella pneumophila is a pathogenic gram-negative bacterium that evades host defense pathways to establish its intracellular replicative niche1. SidE family (SidEs) effectors mediate phosphoribosyl (PR)-dependent ubiquitination of host substrates and promote bacterial infection. Legionella effector, SidJ shares the genetic locus with the SidEs and opposes their toxicity in yeast and mammalian cells through an unknown mechanism. Deletion of SidJ alone leads to a significant defect in the growth of Legionella in both its natural host amoeba and in murine macrophages. Here, we show that SidJ is a glutamylase that modifies the catalytic residue in the mono-ADPribosyl transferase (mART) domain of SidEs thus blocking their ubiquitin ligase activity. This reaction requires SidJ binding to calmodulin (CaM), which acts as an essential mammalian co-factor for SidJ. Using quantitative proteomics, we also uncovered multiple host proteins as potential targets of SidJ-mediated glutamylation.