Dot/Icm-Translocated Proteins Important for Biogenesis of the Coxiella burnetii-Containing Vacuole Identified by Screening of an Effector Mutant Sublibrary

Coxiella burnetii is an intracellular pathogen that replicates in a lysosome-derived vacuole. A determinant necessary for C. burnetii virulence is the Dot/Icm type IVB secretion system (T4SS). The Dot/Icm system delivers more than 100 proteins, called type IV effectors (T4Es), across the vacuolar membrane into the host cell cytosol. Several T4Es have been shown to be important for vacuolar biogenesis. Here, transposon (Tn) insertion sequencing technology (INSeq) was used to identify C. burnetii Nine Mile phase II mutants in an arrayed library, which facilitated the identification and clonal isolation of mutants deficient in 70 different T4E proteins. These effector mutants were screened in HeLa cells for deficiencies in Coxiella-containing vacuole (CCV) biogenesis. This screen identified and validated seven new T4Es that were important for vacuole biogenesis. Loss-of-function mutations in cbu0414 (coxH1), cbu0513, cbu0978 (cem3), cbu1387 (cem6), cbu1524 (caeA), cbu1752, or cbu2028 resulted in a small-vacuole phenotype. These seven mutant strains produced small CCVs in all cells tested, which included macrophage-like cells. The cbu2028::Tn mutant, though unable to develop large CCVs, had intracellular replication rates similar to the rate of the parental strain of C. burnetii, whereas the other six effector mutants defective in CCV biogenesis displayed significant reductions in intracellular replication. Vacuoles created by the cbu0513::Tn mutant did not accumulate lipidated microtubule-associated protein 1A/1B light chain 3 (LC3-II), suggesting a failure in fusion of the CCV with autophagosomes. These seven T4E proteins add to the growing repertoire of C. burnetii factors that contribute to CCV biogenesis.