# Structural basis of CSN-mediated SCF deneddylation

## TL;DR

This study reveals the structural details of how the COP9 signalosome (CSN) regulates Cullin-RING ligases through deneddylation, offering insights for drug design.

## Contribution

The study provides high-resolution cryo-EM structures of CSN-CRL complexes, capturing key intermediates in the deneddylation process.

## Key findings

- Cryo-EM structures reveal autoinhibited and catalytic states of CSN-CRL complexes.
- Four dissociation intermediates define the stepwise release of CSN from its product.
- CSNAP is localized within a CSN3-CSN8 groove, contributing to complex stability.

## Abstract

Cullin-RING ligases (CRLs) are the largest family of E3 ligases, with ubiquitination activity dynamically regulated by neddylation and deneddylation by the COP9 signalosome (CSN). CSN-mediated deneddylation not only deactivates CRLs but also enables substrate receptor exchange. Although CSN is a promising drug target, the structural basis underlying its catalytic mechanism remains unclear. Here, we use cryo-electron microscopy (cryo-EM) to uncover distinct functional states of CSN-CRL (SCF) complexes, capturing key intermediates of the deneddylation cycle. We visualise an autoinhibited docking state and a catalytic intermediate in which CSN5 Ins-1 loop, RBX1 RING and neddylated Cullin WHB domains are repositioned for isopeptide cleavage. We further resolve four dissociation intermediates that define the stepwise release of CSN from its product, with RBX1 RING stabilising key interactions. Additionally, our structures locate CSNAP within a CSN3-CSN8 groove. Together, our study provides a mechanistic model for CSN function and informs the rational design of CSN-targeted therapeutics.

Cullin-RING ligases are regulated by the COP9 signalosome (CSN) through deneddylation. Here, authors report high-resolution cryo-EM structures that capture catalytic and dissociation intermediates, identify CSNAP within the complex, and reveal a stepwise pathway for CSN disengagement.