Surface-Exposed Proteins ofEhrlichia chaffeensis

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Abstract
The surface proteins ofEhrlichia chaffeensisprovide an important interface for pathogen-host interactions. To investigate the surface proteins ofE. chaffeensis, membrane-impermeable, cleavable Sulfo-NHS-SS-Biotin was used to label intact bacteria. The biotinylated bacterial surface proteins were isolated by streptavidin-agarose affinity purification. The affinity-captured proteins were separated by electrophoresis, and five relatively abundant protein bands containing immunoreactive proteins were subjected to capillary-liquid chromatography-nanospray tandem mass spectrometry analysis. Nineteen out of 22 OMP-1/P28 family proteins, including P28 (which previously was shown to be surface exposed), were detected inE. chaffeensiscultured in human monocytic leukemia THP-1 cells. For the first time, with the exception of P28 and P28-1, 17 OMP-1/P28 family proteins were demonstrated to be expressed at the protein level. The surface exposure of OMP-1A and OMP-1N was verified by immunofluorescence microscopy. OMP-1B was undetectable either by surface biotinylation or by Western blotting of the whole bacterial lysate, suggesting that it is not expressed byE. chaffeensiscultured in THP-1 cells. AdditionalE. chaffeensissurface proteins detected were OMP85, hypothetical protein ECH_0525 (here named Esp73), immunodominant surface protein gp47, and 11 other proteins. The identification ofE. chaffeensissurface-exposed proteins provides novel insights into theE. chaffeensissurface and lays the foundation for rational studies on pathogen-host interactions and vaccine development.