Diversity of COP9 signalosome structures and functional consequences

The COP9 signalosome (CSN) is a regulator of the ubiquitin (Ub) proteasome system (UPS). It interacts with hundreds of cullin‐RING ubiquitin E3 ligases (CRLs) and regulates their activity by removing the Ub‐like protein Nedd8 from cullins. In mammalian cells 7 different cullins exist which form CRLs with adaptor proteins and with a large number of substrate recognition subunits such as F‐box and BTB proteins. This large variety of CRL‐complexes is deneddylated by the CSN. The capacity of the CSN to interact with numerous types of CRL complexes can be explained by its structural diversity, which allows different CSN variants to interact with different binding partners and substrates and enables different subunit expression profiles. Diversity of CSN complexes presumably occurs by: (1) flexibility of CSN holo complex structure; (2) formation of CSN mini complexes and free CSN subunits and (3) generation of CSN variantsvia integration of CSN subunit isoforms. In this review we will discuss the structural diversity of the CSN complex and possible functional consequences.