The gram-negative bacterium Legionella pneumophila is a facultative pathogen that causes Legionnaires’ disease, a severe form of pneumonia. L. pneumophila translocates over 350 effectors to coerce host cellular processes and promote bacterial survival. Of note, five effectors translocated by Legionella have been identified to target actin or actin regulators and modulate actin organization in host cells. These studies reveal that Legionella employs multiple effectors with distinct mechanisms to manipulate actin cytoskeleton. Despite accumulating evidence, the exact mechanism and the biological significance of actin hijacking during Legionella infection are still largely unclear.My dissertation research focuses on characterization of two Legionella effectors which are actin regulators to promote actin assembly in host cells. We found that the Legionella effector MavH contains a WH2 like domain that binds actin, a lipid binding domain associating with PI(3)P and a CPI motif recruiting capping proteins (CP). And we demonstrated that MavH promotes actin assembly dependent on the WH2 like domain and the PI(3)P binding domain both in vivo and in vitro. In addition, MavH fine-tunes actin cytoskeleton arrangement by modulating CP activities. Another Legionella effector RavH, we found it directly interacts with actin via a repeat of three WH2 like domains. Similar to MavH, it also contains a lipid binding domain that prefers binding with phosphatidylinositol phosphorylated at the 3 hydroxyl position. We found that RavH promotes actin assembly on yeast vacuoles and promotes actin nucleation efficiently in vitro. Taken together we found the Legionella effectors MavH and RavH facilitate actin assembly around PI3P positive organelles. MavH promotes actin assembly and fine-tunes actin organization via the CPI motif, however, RavH is a strong actin nucleator that greatly enhances actin assembly. These studies expand our understanding of the importance of controlling actin cytoskeleton during Legionella infection and assist the elucidation of pathogenic mechanisms of Legionella infection.