Effector Protein Translocation by the Coxiella burnetii Dot/Icm Type IV Secretion System Requires Endocytic Maturation of the Pathogen-Occupied Vacuole

The human pathogen Coxiella burnetii encodes a type IV secretion system called Dot/Icm that is essential for intracellular replication. The Dot/Icm system delivers bacterial effector proteins into the host cytosol during infection. The effector proteins delivered by C. burnetii are predicted to have important functions during infection, but when these proteins are needed during infection has not been clearly defined. Here, we use a reporter system consisting of fusion proteins that have a beta-lactamase enzyme (BlaM) fused to C. burnetii effector proteins to study protein translocation by the Dot/Icm system. Translocation of BlaM fused to the effector proteins CBU0077, CBU1823 and CBU1524 was not detected until 8-hours after infection of HeLa cells, which are permissive for C. burnetii replication. Translocation of these effector fusion proteins by the Dot/Icm system required acidification of the Coxiella-containing vacuole. Silencing of the host genes encoding the membrane transport regulators Rab5 or Rab7 interfered with effector translocation, which indicates that effectors are not translocated until bacteria traffic to a late endocytic compartment in the host cell. Similar requirements for effector translocation were discerned in bone marrow macrophages derived from C57BL/6 mice, which are primary cells that restrict the intracellular replication of C. burnetii. In addition to requiring endocytic maturation of the vacuole for Dot/Icm-mediated translocation of effectors, bacterial transcription was required for this process. Thus, translocation of effector proteins by the C. burnetii Dot/Icm system occurs after acidification of the CCV and maturation of this specialized organelle to a late endocytic compartment. This indicates that creation of the specialized vacuole in which C. burnetii replicates represents a two-stage process mediated initially by host factors that regulate endocytic maturation and then by bacterial effectors delivered into host cells after bacteria establish residency in a lysosome-derived organelle.