Coxiella burnetii is an intracellular bacterium that causes the human disease Q fever. C. burnetii infects alveolar macrophages, and replicates within a spacious, lysosome-derived vacuole. This vacuole matures to feature an acidic internal environment characteristic of a lysosome. At this stage, the Dot/Icm type IV secretion system is activated and over 100 effector proteins are translocated into the host cytoplasm to modulate intracellular survival. One of the effector proteins is Cig57. Mutational analysis demonstrated that Cig57 is essential for replication of C. burnetii, however the potential properties and function of this effector have not yet been determined.
Here, we have discovered that Cig57 has a role in exploiting clathrin-mediated endocytosis (CME) of the eukaryotic host. Little is known about vesicular trafficking that contributes to vacuole formation, but the clathrin-mediated pathway may aide overall success of C. burnetii inside human cells.
Important endocytic sorting motifs, recognised by adapter complexes during CME, are found in Cig57. We have established that these motifs are important for normal growth of C. burnetii. Immunoprecipitations and immunofluorescence microscopy were utilised to examine the interaction and co-localisation of Cig57 to FCHO1 and FCHO2, two important proteins involved in CME. We also report that depletion of FCHO2 or clathrin, through siRNA silencing, are important for normal vacuole formation. Further, uptake of transferrin by CME is impaired in the presence of Cig57.
Ultimately, these results validate the importance of Cig57 during infection, highlight a link between Cig57 and CME, and provide further insight into how C. burnetii may manipulate normal host cell function for intracellular survival.