Figure 2.

Major intracellular proteolytic systems. The ubiquitin–proteasome system degrades and eliminates specific substrate proteins in an ubiquitin-tagging system consisting of more than 1,000 ubiquitin ligases. The autophagy–lysosome system primarily degrades non-specific cell components, including proteins and micro-organisms, by compartmentalization by isolation membranes. The caspase system functions mainly for apoptosis. In contrast, calpains primarily use proteolytic processing, rather than degradation, to modulate or modify their substrates’ activity, specificity, longevity, localization, and structure.