Structural and functional characterization of the Legionella pneumophila effector CegC2

The human pathogen Legionella pneumophila is the etiological agent of a severe form of pneumonia known as Legionnaires’ disease. Successful infection is achieved through a specialized type IVB secretion system that injects over 330 proteins, called effectors, into the host cell cytosol. These effectors modulate various host cell processes, such as vesicle trafficking, lipid metabolism, and ubiquitination. However, the biological functions of more than two-thirds of effectors have not been determined. In this study, we characterized the hypothetical effector Lpg0126 (CegC2). We found that CegC2 is a multidomain protein. The N-terminal region of CegC2 harbors a cysteine protease domain that is critical for its toxic effect on mammalian cells. We determined the crystal structure of this region at a final resolution of 3.18 Å. According to the structure, CegC2 forms a dimer, and the cysteine protease active site is located at the dimer interface. Interestingly, we observed copper bound near the active site in our structure. Additionally, we found that CegC2 binds to the lipid PI3P on its C-terminal end and localizes to the Legionella-containing vacuole (LCV) during infection. Taken together, our results provide insights into the functional role of CegC2 during infection, and we have identified a novel copper-binding phenotype that has not been observed with other effectors.