Domain organization of Legionella effector SetA.

Legionella pneumophila is a human pathogen causing severe pneumonia called Legionnaires' disease. Multiple Legionella effectors are type IV-secreted into the host cell to establish a specific vesicular compartment for pathogen replication. Recently, it has been reported that the Legionella effector SetA shares sequence similarity with glycosyltransferases and interferes with vesicular trafficking of host cells. Here we show that SetA possesses glycohydrolase and mono-O-glucosyltransferase activity by using UDP-glucose as a donor substrate. Whereas the catalytic activity is located at the N terminus of SetA, the C terminus (amino acids 401-644) is essential for guidance of SetA to vesicular compartments of host cells. EGFP-SetA expressed in HeLa cells localizes to early endosomes by interacting with phosphatidylinositol 3-phosphate. EGFP-SetA, transiently expressed in RAW 264.7 macrophages, associates with early phagosomes after infection with Escherichia coli and L. pneumophila. Only the combined expression of the C- and N-terminal domains induces growth defects in yeast similar to full-length SetA. The data indicate that SetA is a multidomain protein with an N-terminal glucosyltransferase domain and a C-terminal phosphatidylinositol 3-phosphate-binding domain, which guides the Legionella effector to the surface of the Legionella-containing vacuole. Both, the localization and the glucosyltransferase domains of SetA are crucial for cellular functions.