Mass spectrometry generated a list of 105 C. burnetii proteins in ACCM culture supernatants. Immunoblotting

of culture supernatants following growth of C. burnetii transformants expressing individual epitope-tagged versions of identified proteins confirmed secretion of 27 of these proteins. Secretion of epitope-tagged proteins also occurred during growth of C. burnetii in Vero host cells. An intact N-terminal signal sequence was required for secretion, indicating secreted proteins have a transient periplasmic location. Results Coxiella burnetii proteins are present in growth medium supernatant The Dot/Icm type IVB secretion system BIX 1294 of C. burnetii has been extensively studied [9, 10, 39]. However, little is known about other secretion systems of C. burnetii that are presumably important for intracellular parasitism. To determine if C. burnetii secretes proteins during axenic growth, bacteria were cultivated in ACCM-2 without neopeptone to eliminate media proteins. Following 7 days of growth, supernatant was concentrated and analyzed

by SDS-PAGE and silver staining (Figure 1). Many proteins were detected, with the majority GDC-0449 having a molecular weight below 20 kDa. In a discovery experiment to generate a list of potentially secreted proteins to further investigate, SDS-PAGE was conducted again and proteins stained with Coomassie G-250 to allow analysis by microcapillary reverse-phase HPLC nano-electrospray tandem mass spectrometry (μLC/MS/MS). A list of 105 proteins was generated (Additional files 1 and 2) with functions assigned based on the annotated genome of the C. burnetii Nine Mile RSA493 reference strain [18]. Sixteen proteins were annotated as hypothetical exported proteins, which represents 36% of the total proteins with this annotation in the predicted C. burnetii proteome [18]. Twenty-nine proteins,

such as translation initiation factor 1 (InfA) and ribosomal protein subunit L31P (RpmE), were predicted as cytoplasmic using the PSORTb v3.0.2 bacterial protein subcellular localization prediction program [40]. This result could be explained by a small amount of bacterial lysis releasing abundant cytoplasmic proteins that are then detected by highly sensitive mass spectrometry. The only Dot/Icm type IVB secretion Bay 11-7085 system substrate detected was CBU0937 [39]. However, type IVB-dependent secretion of CBU0937 was demonstrated using L. pneumophila as a surrogate host, and the protein contains a predicted signal sequence, which are typically not associated with Dot/Icm type IVB effectors [41]. Thus, CBU0937 may represent a false positive type IVB effector. Nonetheless, the lack of identified C. burnetii Dot/Icm type IVB secretion system substrates in culture supernatants indicates secretion via this mechanism requires host cell-derived signals. Figure 1 Multiple Coxiella burnetii proteins are present in growth medium supernatant. C.