For many years, the ubiquitin-26S proteasome degradation pathway was considered the primary route for proteasomal degradation. However, it is now becoming clear that proteins can also be targeted for degradation by a ubiquitin-independent mechanism mediated by the core 20S proteasome itself.
The COP9 signalosome complex (CSN) regulates protein degradation by controlling the function of cullin-RING-ubiquitin-ligases (CRLs). The complex is conserved throughout evolution and in higher organisms it consists of eight subunits, termed CSN1 to CSN8, in order of decreasing molecular mass.
Protein complexes are dynamic entities that alter their composition according to the tissue type, the nature of the cell itself (e.g., normal vs. diseased), and the intracellular localization. Such variability in subunit composition creates unique functional properties, enabling the complex to respond and adapt to varying cellular conditions.