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Ubunit beta (PRKCSH), creatin kinase B (CKB), and apolipoprotein E (Apo-E) were selected for these further investigations. All these proteins showed a considerable regulation as detected using the ICPL technique. Rac1 and clathrin heavy chain (CLH-17) whose regulation was just below the significant threshold as evident after ICPL based analysis (s. above) were additionally quantified by Western bot analysis (Figure 5). Quantitative results obtained for endoplasmin and apolipoprotein E were consistent with the quantitative proteome data as shown in Figure 5A, D. Some discrepancies were observed using anti-glucosidase 2 subunit beta and anti-creatin kinase Btype antibodies (Figure 5B, C). Glucosidase 2 subunit beta seemed to be up-regulated by 2-fold after treatment with mutant rTcdA for 24 h, whereas ICPL data showed no changes in protein expression of glucosidase 2 subunit beta under these conditions. However, in cells treated with rTcdA wt the analyses using antibodies against glucosidase 2 beta fit well to the quantitative mass spectrometry data. The western blot analyses for creatin kinase B-type also indicated differences to the ICPL-based mass spectrometry data for the 5 h time point. Under these conditions creatine kinase B-type was found up-regulated by western blot analysis after treatment with mutant rTcdA but no regulation was detected using ICPL analysis (Figure 5C). The data acquired for apolipoprotein E after 24 h treatment with rTcdA wt and mutant rTcdA corresponded well to the western blot results. After 5 h treatment with rTcdA wt or mutant rTcdA apolipoprotein E was not detected by mass spectrometry; however the protein was clearly detected by western blot analysis and showed no significant changes at protein level after 5 h in PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/6833145 the cytosolic fraction. The proteins clathrin heavy chain 1 and Rac1 wereDiscussion The main virulence factors of C. difficile are the toxins TcdA and TcdB that inactivate Rho proteins in eukaryotic cells. The toxins act in the cytosol of target cells to catalyze the transfer of a glucose moiety onto a crucial threonine residue of Rho proteins. Delivery of the toxins into the cytosol and their effects on Rho proteins and Rho-dependent signaling in host cells is investigated [4,7] but proteins and pathways involved in processes downstream of Rho proteins Nelfinavir (Mesylate) are largely unclear at present. In particular, the question whether large clostridial cytotoxins affect target cells independently of the intrinsic glucosyltransferase activity and thus independently of Rho proteins is controversially discussed [4,16]. Hence we initiated a toxicoproteomic study to analyze cellular effects of large clostridial cytotoxins using recombinant wild type or mutant TcdA. A similar study has been conducted to elucidate the function of the C3 exoenzyme of C. botulinum where several proteins were identified to be responsive to C3 treatment in neuronal cells [17]. Thus, a comprehensive proteome analysis was conducted using the ICPL technique and LC-MALDI-MS. ICPL labeling occurs at lysine residues and prevents trypsin cleavage after these residues. Thus, the tryptic digestion was altered to an ArgC specificity leading to the generation of slightly larger peptides on average. However, quantification was only possible for half of the identified proteins. Although lysine is overrepresented in human proteins (>7 ), several of the identified peptides did not contain lysine residues. Additionally, few lysine residues remained unlab.