Study of the whole cell lysate of two Coxiella burnetii strains using N-terminomics

The etiological agent of Q fever is Coxiella burnetii , an obligate intracellular Gram-negative bacterium and the only bacterium known to date that survives and replicates within a vacuole of phagolysosomal characteristics. In humans, Q fever is characterized by a wide spectrum of clinical manifestations. Of note is that genetic diversity among C. burnetii strains has been reported. To further investigate C. burnetii's diversity, but now at the proteome level, we compared the proteomes of whole cell lysates from two reference strains, Nine Mile and Q212. Proteomes were isolated from each strain and subjected MS-driven combined fractional diagonal chromatography (COFRADIC), a peptide-centered proteomics technique, with a total of 322 proteins that were unambiguously identified. On the basis of their identified neo-N-terminal peptides that are highly likely generated upon in vivo processing by proteases, the most proteolytical sensitive proteins in these strains were identified, and a consensus cleavage pattern was obtained. Further, with the use of differential proteomics based on the here-identified N-terminal peptides, 44 proteins were found to be differentially expressed between the two C. burnetii strains, representing 13.6% of the here-identified C. burnetii proteome. Among these proteins, 10 proteins were found uniquely expressed in the NM strain including proteins with unknown functions as well as housekeeping enzymes, suggesting that strain-related proteins might be present among such uncharacterized proteins.