Figure 3: Reduction in ATP levels stimulates immunoproteasome activation by the 11S. In β-cells exposed to IFNβ, the 20S proteolytic cores contain a stochastic arrangement of regular and immune proteolytic subunits, and coexist with the 19S and 11S activators. However, the 11S activator stimulates the proteolytic rates only under conditions of ATP depletion. Two mechanisms could explain this observation, as discussed in the text.