Proteomic Screening for Possible Effector Molecules Secreted by the Obligate Intracellular Pathogen Coxiella burnetii

Abstract

Coxiella burnetii is a Gram-negative, gamma-proteobacteria with nearly worldwide distribution, and it is the pathogenic agent of Q-fever in man. It is an obligate intracellular parasite that is highly adapted to reside within the eukaryotic phagolysosome. In fact, it is the only known intracellular bacterium that manages to survive and replicate within a fully formed, acidic phagolysosome. C. burnetti possesses a functional Type 4 Secretion System (T4SS), similar to the Dot/Icm system of Legionella pneumophila. Up to date there have been no reports for the effector molecules secreted by Coxiella's T4SS. These are speculated to have quite different roles than the effectors of other intracellular pathogens, since there is no need for phagosomal arrest or escape in the case of Coxiella. In this study, we have investigated the cytoplasm of Vero cells infected with C. burnetti strain Nine Mile Phase II. We have identified by mass spectrometry (ESI-MS/MS) several C. burnetti proteins that bear typical characteristics of effector molecules. Most of the identified proteins were also very alkaline, something which is supportive for a protective strategy that has evolved in this bizarre pathogen against acidic environments.

Proteomic comparison of virulent phase I and avirulent phase II of Coxiella burnetii, the causative agent of Q fever

“…Additionally, 150 proteins not reported in previous proteomic studies of C. burnetii [40][41][42][43][44][45][46] were identified in this study (Supplemental Table 3). Among these are a number of proteins potentially associated with pathogenicity and virulence.…”

“…The last, (Q83B72) was not detected in any previous study of C. burnetii. The DotD protein is an essential component of the C. burnetii type IV secretion system, and is mechanistically related to the Legionella Dot/Icm apparatus [45,[79][80][81]. Recently it has been suggested that the Dot/Icmrelated secretion system has an important role during phagosome establishment [80].…”

“…To date nearly 200 C. burnetii proteins have been documented in the literature, including some from both phase I and II cells [40][41][42][43][44][45][46]. Unfortunately analysis of the two phases has typically been done under different conditions, complicating comparisons.…”

TheCoxiella burnetiiCryptic Plasmid Is Enriched in Genes Encoding Type IV Secretion System Substrates

“…Thus, CpeD and potentially other Dot/Icm substrates may influence ER function. It should be noted that CpeD was also identified in a proteomic screen of C. burnetii secreted proteins (54).…”

Large-scale identification and translocation of type IV secretion substrates by Coxiella burnetii

“…The use of L. pneumophila as a surrogate host led to the identification of 32 proteins that were translocated into host cells in a Dot/Icm-dependent manner. Interestingly, two of the identified Dot/Icm substrates (CBU1314 and CBU1823) are suggested to localize in the cytoplasm of C. burnetii infected Vero cells (by mass spectrometry) (35). Although the biological significance of the interaction between protein substrates and components of the transport system is largely undefined, such interactions have been very useful in identifying novel substrates (5,18,36).…”