3.6.1. Stoichiometry of 20S proteasomes and regulators in cellular systems

The stoichiometry of the key regulators of UPS and UIPP, namely, 19S and 11S regulators, is of particular interest to further understand how cross talk of these two proteolytic systems is affected by PA28γ. Since each biochemical process is restricted kinetically by its rate limiting step, the assembly of proteasomal complexes should be a matter of serious debate. Only few studies so far covered this question due to methodological limitations. Therefore, a recent proteomic approach is remarkable, elaborating the stoichiometry of 20S proteasome-associated regulators in detail [86, 87]. Using formaldehyde cross-linking in combinations with 20Scomplex directed affinity purification on MCP21-coupled sepharose, and applying mass spectrometric quantification, proteomic analysis compared the stoichiometry of 20S proteasome complexes in nine different cell lines. Interestingly, 19S and 20S particles reached 1:1 ratio, but PA28γ-associated proteasomes occurred as minor species, occupying about 1–0.2% of proteasomes [86]. All investigated cellular systems had in common that about 20–40% of 20S proteasomes persisted in a regulator-unbound state, establishing a pool of proteasomes directly available for UIPP tasks or for dynamic behavior in the case of expressional alterations in the activator population [86].

As central pathways of protein degradation, UPS and UIPP participate in regulating the delicate balance of pro- and anti-apoptotic proteins and cell cycle regulators [88, 89]. The contribution of proteasomal degradation in controlling crucial steps of the mitochondrial pathway of apoptosis has been reviewed comprehensively [89]. Here, we are focusing on the PA28γ-mediated mechanisms, affecting the balance between pro- and anti-apoptotic regulatory proteins (Figure 2).
