CSNAP, the smallest CSN subunit, modulates proteostasis through cullin-RING ubiquitin ligases

Abstract

The cullin-RING ubiquitin E3 ligase (CRL) family consists of ~250 complexes that catalyze ubiquitylation of proteins to achieve cellular regulation. All CRLs are inhibited by the COP9 signalosome complex (CSN) through both enzymatic (deneddylation) and non-enzymatic (steric) mechanisms. The relative contribution of these two mechanisms is unclear. Here, we decouple the mechanisms using CSNAP, the recently discovered ninth subunit of the CSN. We find that CSNAP reduces the affinity of CSN toward CRL complexes. Removing CSNAP does not affect deneddylation, but leads to global effects on the CRL, causing altered reproductive capacity, suppressed DNA damage response, decreased viability, and delayed cell cycle progression. Thus, although CSNAP is only 2% of the CSN mass, it plays a critical role in the steric regulation of CRLs by the CSN.

Structural dynamics of the human COP9 signalosome revealed by cross-linking mass spectrometry and integrative modeling

“…As canonical CSN subunits (CSN1-8) have a oneto-one correspondence to the subunits of the 19S proteasome lid subcomplex (3,28), CSN9 is homologous to DSS1, the smallest component of the 19S lid. While CSN9 is not essential for the assembly and catalytic activity of CSN (27), a recent study has suggested that CSN9 reduces the affinity of CSN-CRL interactions, contributing to steric regulation of CRLs (14). The depletion of CSN9 appears to have a global impact on CRL-associated activities, leading to altered reproductive capacity, suppressed DNA damage response, decreased viability, and delayed cell cycle progression (14).…”

“…While CSN9 is not essential for the assembly and catalytic activity of CSN (27), a recent study has suggested that CSN9 reduces the affinity of CSN-CRL interactions, contributing to steric regulation of CRLs (14). The depletion of CSN9 appears to have a global impact on CRL-associated activities, leading to altered reproductive capacity, suppressed DNA damage response, decreased viability, and delayed cell cycle progression (14). It has also been suggested that the C terminus of CSN9 is important in its incorporation within the CSN complex, likely through interactions with CSN3, CSN5, and CSN6 (27).…”

“…The dynamic cycle of neddylation and deneddylation of cullins is a critical step in regulating the assembly and activity of CRLs (6,9,10). In addition to enzymatic regulation of CRLs, the CSN can inactivate CRLs noncatalytically by direct binding, preventing their association with E2 enzymes and ubiquitination substrates (11)(12)(13)(14). While abnormal CRL activity is frequently associated with various human diseases, multiple studies have also identified the CSN as a positive regulator of oncogenes and negative regulator of tumor suppressors (15)(16)(17)(18)(19).…”

Role of NEDD8 and neddylation dynamics in DNA damage response

“…Therefore, the activity of CSN is also regulated by both phosphorylation and translocation. It was also found that the loss of CSNAP, the ninth subunit of CSN, causes attenuated DNA damage response after treatment with UV irradiation (Fuzesi-Levi et al, 2020;Rozen et al, 2015). Although CSNAP does not regulate the neddylation of CRLs, it has steric effects on CRLs so that the affinity of CSN for CRLs is promoted when CSNAP is abolished.…”

The COP9 Signalosome: A Multi-DUB Complex

“…Thus, phosphorylation/dephosphorylation in response to signaling processes produces a dynamic heterogeneity of CSN complexes. Moreover, it has recently been recognized that a fraction of cellular CSN contains a non-essential, non-canonical component called CSNAP [ 23 , 24 , 25 ]. A further unexplored source of heterogeneity is provided by the fact that several CSN core subunits occur as paralogs/isoforms [ 26 , 27 ].…”