Coxiella burnetii is an obligate intracellular bacterial pathogen that causes the zoonotic disease Q fever in humans. C. burnetii occupies a unique autolysosomal niche termed the Coxiella containing vacuole (CCV). The establishment of the CCV is necessary for C. burnetii survival and replication within host cells. C. burnetii utilises an arsenal of over 130 type IV effector proteins (T4Es) some of which modulate host cell trafficking pathways allowing for the formation of the CCV. Cig57 (Cbu1751), Cbu1752, and Cbu1754 are three novel Coxiella T4Es implicated in CCV biogenesis through the subversion of host clathrin-mediated transport. Cig57, Cbu1752, and Cbu1754 are thought to act synergistically to hijack clathrin transport via interactions with the clathrin heavy chain and through associations with FCHO2, a clathrin nucleating protein. Although Cig57, Cbu1752, and Cbu1754 have been identified as essential T4Es implicated in CCV expansion, their mechanistic function and relationship with each other and the clathrin mediated transport system as a whole remains unknown.
The basic structural characteristics of Cig57, Cbu1752, and Cbu1754 were determined using a combination of analytical ultracentrifugation and small angle x-ray scattering experiments. Effector interactions were probed with microscale thermophoresis and structures were solved with X-ray crystallography. Cig57 and Cbu1754 were found to be monomeric and roughly globular in solution, whereas Cbu1752 adopted an extended conformation and displayed low levels of self-association. The crystal structure of the Cig57 central domain (residues 178-307) was solved with a resolution of 1.75 Å. Cig57 displays weak binding to the host protein FCHO2, whereas Cbu1752 was found to interact directly with clathrin heavy chain.
Q fever is an endemic and potentially life-threatening disease in Australia which displays a particular risk to those working with animals. This works increases our understanding of C. burnetii infection and may lead to better treatment of disease.