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Characterization of surface layer proteins from Clostridium difficile by liquid chromatography/electrospray ionization mass spectrometry.

Surface layers (S-layers) are regularly ordered protein subunits found as the outermost cell envelope component of many bacteria. Most S-layers are composed of a single protein or glycoprotein species with a molecular weight varying between 40 and 200 kDa. Clostridium difficile is the most common cause of antibiotic associated diarrhea (AAD) and pseudomembranous colitis (PMC) in humans. Detection of the S-layer in some C. difficile strains, and preliminary characterization of two glycoproteins, P36 and P47, involved in the composition of the S-layer of one of these strains (C. difficile C253), led us to investigate the most appropriate conditions for purification and chemical characterization of these proteins. This work describes the results obtained when liquid chromatography (LC) coupled to mass spectrometry (MS) using electrospray ionization was applied to the analysis of C. difficile S-layer proteins (SLPs). In this way the molecular weights of the two SLP components, P36 and P47, were detected to be 34,258 +/- 2 and 39,545 +/- 3 Da, respectively. These data deviate from sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) results by 1.85 and 7.5 kDa. To confirm the LC-MS results, an alternative molecular weight analysis was performed: the two S-layer proteins were isolated by semipreparative high performance liquid chromatography (HPLC), concentrated, and analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF). The two SLP subunits were digested with protease V8, and the peptide maps were determined by LC-MS using a C18 column. Finally, preliminary results about peptide glycosylation were obtained.