Legionella pneumophila (LP) secretes more than 300 effectors into the host cytosol to facilitate intracellular replication. One of these effectors, SidH, 253 kDa in size with no sequence similarity to proteins of known function is toxic when overexpressed in host cells. SidH is regulated by the LP metaeffector LubX which targets SidH for degradation in a temporal manner during LP infection. The mechanism underlying the toxicity of SidH and its role in LP infection are unknown. Here, …