Quantitative proteome analysis of the 20S proteasome of apoptotic Jurkat T cells

Regulated proteolysis plays important roles in cell biology and pathological conditions. A crosstalk exists between apoptosis and the ubiquitin–proteasome system, two pathways responsible for regulated proteolysis executed by different proteases. To investigate whether the apoptotic process also affects the 20S proteasome, we performed three independent SILAC-based quantitative proteome approaches: 1-DE/MALDI-MS, small 2-DE/MALDI-MS and large 2-DE/nano-LC–ESI–MS. Taking the results of all experiments together, no quantitative changes were observed for the α- and β-subunits of the 20S proteasome except for subunit α7. This protein was identified in two protein spots with a down-regulation of the more acidic protein species (α7a) and up-regulation of the more basic protein species (α7b) during apoptosis. The difference in these two α7 protein species could be attributed to oxidation of cysteine-41 to cysteine sulfonic acid and phosphorylation at serine-250 near the C terminus in α7a, whereas these modifications were missing in α7b. These results pointed to the biological significance of posttranslational modifications of proteasome subunit α7 after induction of apoptosis.