Legionella pneumophila modulates host cytoskeleton by an effector of transglutaminase activity

The bacterial pathogen Legionella pneumophila delivers more than 330 effector proteins into host cells through its Dot/Icm type IV secretion system (T4SS) to facilitate its intracellular replication. A number of these effectors modulate organelle trafficking pathways to create a membrane-bound niche called the Legionella containing vacuole (LCV). In this study, we found that L. pneumophila induces F-actin accumulation in host cell cortex by its Dot/Icm substrate RavJ (Lpg0944). RavJ harbors an C101H138D170 motif associated with human tissue transglutaminases (TGs). We showed that RavJ catalyzes a covalent linkage between actin and the Motin family proteins Angiomotin (AMOT) and Angiomotin-like 1 (AMOTL1), proteins known to regulate tube formation and cell migration. Further study revealed that RavJ-induced crosslink between actin and AMOT occurs on its Gln354 residue. Crosslink between actin and AMOT significantly reduces the binding between actin and its binding partner cofilin, suggesting that RavJ inhibits actin depolymerization. We also demonstrated that the metaeffector LegL1 directly interacts with RavJ to antagonize its transglutaminase activity, leading to reduced crosslink between actin and Motin proteins. Our results reveal a novel mechanism of modulating the host actin cytoskeleton by L. pneumophila.