The Yersinia pestis proteome was studied as a function of temperature and calcium by two-dimensional differential gel electrophoresis. Over 4,100 individual protein spots were detected, of which hundreds were differentially expressed. A total of 43 differentially expressed protein spots, representing 24 unique proteins, were identified by mass spectrometry. Differences in expression were observed for several virulence-associated factors, including catalase-peroxidase (KatY), murine toxin (Ymt), plasminogen activator (Pla), and F1 capsule antigen (Caf1), as well as several putative virulence factors and membrane-bound and metabolic proteins. Differentially expressed proteins not previously reported to contribute to virulence are candidates for more detailed mechanistic studies, representing potential new virulence determinants.Yersinia pestis, the etiological agent of plague, is a gram-negative bacterium that is both a natural environmental pathogen and a biothreat agent (4,8,32). Early studies of Yersinia physiology uncovered the low calcium response (LCR), whereby bacterial cultures grown in rich medium at an elevated temperature (37°C) exhibit a growth defect upon chelation of calcium ions. The growth arrest was shown to be a result of one of the two type III secretion systems (TTSSs) in Y. pestis, the Ysc TTSS, and is responsible for the secretion of virulence factors known as Yersinia outer proteins, or Yops (21, 29; for a review, see reference 61). This TTSS can be activated in vitro and virulence factors can be released into the medium when Y. pestis is grown at 37°C with submillimolar calcium (for a review, see reference 16). Upon interaction with the host, the TTSS enables virulence factors to enter the host cell through a specialized apparatus, the injectisome (15). Once inside the host cell, Yops affect a variety of host pathways, with detectable expression changes in the pathogen as well as the host (14,52,82).The Y. pestis proteome was previously examined using twodimensional electrophoresis (57,60,71,72). These studies demonstrated that virulence factors were not induced at 26°C or 37°C in the presence of calcium concentrations similar to that found in mammalian plasma (2.5 mM) (71). More recently, the introduction of two-dimensional differential gel electrophoresis (2-D DIGE) has significantly improved the quality of gel-based proteomics through fluorescence-based multiplex analyses providing relative quantitation of expression differences and improved gel-to-gel comparisons (75). Several examples of 2-D DIGE bacterial proteomics have been reported (23), including characterizations of the gram-negative bacterium Escherichia coli (1, 76, 81). Here we report the characterization of the soluble cell-associated proteome of Y. pestis as a function of temperature and calcium, which were used to effect virulence induction. Differentially expressed proteins include virulence-associated factors, membrane-bound proteins, metabolic and housekeeping proteins, and potential new virulence determinants.Bacterial ...