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The cullin-RING ubiquitin E3 ligase (CRL) family comprises over 200 members in humans. The COP9 signalosome complex (CSN) regulates CRLs by removing their ubiquitin-like activator NEDD8. The CUL4A-RBX1-DDB1-DDB2 complex (CRL4A.sup.DDB2) monitors the genome for ultraviolet-light-induced DNA damage. CRL4A.sup.DBB2 is inactive in the absence of damaged DNA and requires CSN to regulate the repair process. The structural basis of CSN binding to CRL4A.sup.DDB2 and the principles of CSN activation are poorly understood. Here we present cryo-electron microscopy structures for CSN in complex with neddylated CRL4A ligases to 6.4 Å resolution. The CSN conformers defined by cryo-electron microscopy and a novel apo-CSN crystal structure indicate an induced-fit mechanism that drives CSN activation by neddylated CRLs. We find that CSN and a substrate cannot bind simultaneously to CRL4A, favouring a deneddylated, inactive state for substrate-free CRL4 complexes. These architectural and regulatory principles appear conserved across CRL families, allowing global regulation by CSN. Much of the intracellular protein degradation in eukaryotes is controlled by cullin-RING ubiquitin ligases (CRLs), a vast class of enzymes which are regulated by the COP9 signalosome (CSN); structural characterization of CSN-N8CRL4A complexes by cryo-electron microscopy reveals an induced-fit mechanism of CSN activation triggered only by catalytically activated CRLs without bound substrate, explaining how CSN acts as a global regulator of CRLs. Control of intracellular protein degradation Much of the intracellular protein degradation in eukaryotes is controlled by cullin-RING ubiquitin ligases (CRLs). The structure of these enzymes and their substrates vary greatly, yet all are regulated by a single complex -- the COP9 signalosome (CSN). What enables CSN to be a master regulator of diverse CRLs? Nicolas Thomä and colleagues present biochemical data and cryo-electron microscopy of CSN-CRL4 complexes revealing an induced-fit mechanism that activates CSN only in the presence of a catalytically activated CRL not bound to a substrate. The authors identify both unique and less-specific CSN-CRL contacts., Author(s): Simone Cavadini [sup.1] [sup.2] , Eric S. Fischer [sup.1] [sup.2] [sup.3] [sup.4] , Richard D. Bunker [sup.1] [sup.2] , Alessandro Potenza [sup.1] [sup.2] , Gondichatnahalli M. Lingaraju [sup.1] [sup.2] [...]

In the 1950s, the drug thalidomide, administered as a sedative to pregnant women, led to the birth of thousands of children with multiple defects. Despite the teratogenicity of thalidomide and its derivatives lenalidomide and pomalidomide, these immunomodulatory drugs (IMiDs) recently emerged as effective treatments for multiple myeloma and 5q-deletion-associated dysplasia. IMiDs target the E3 ubiquitin ligase CUL4-RBX1-DDB1-CRBN (known as [CRL4.sup.crbn]) and promote the ubiquitination of the IKAROS family transcription factors IKZF1 and IKZF3 by CRL4cRbn. Here we present crystal structures of the DDB1-CRBN complex bound to thalidomide, lenalidomide and pomalidomide. The structure establishes that CRBN is a substrate receptor within [CRL4.sup.crbn] and enantioselectively binds IMiDs. Using an unbiased screen, we identified the homeobox transcription factor MEIS2 as an endogenous substrate of [CRL4.sup.crbn]. Our studies suggest that IMiDs block endogenous substrates (MEIS2) from binding to [CRL4.sup.crbn] while the ligase complex is recruiting IKZF1 or IKZF3 for degradation. This dual activity implies that small molecules can modulate an E3 ubiquitin ligase and thereby upregulate or downregulate the ubiquitination of proteins., Thalidomide (α-(N-phthalimido)glutarimide) was introduced to the market in 1954 by the company Chemie Griinenthal. Initially promoted as a sedative with anti-emetic properties (1,2), it became popular for treating 'morning sickness' [...]

In the 1950s the drug thalidomide administered as a sedative to pregnant women led to the birth of thousands of children with multiple defects. Despite its teratogenicity, thalidomide and its derivatives lenalidomide and pomalidomide (together known as Immunomodulatory Drugs: IMiDs) recently emerged as effective treatments for multiple myeloma and 5q-dysplasia. IMiDs target the CUL4-RBX1-DDB1-CRBN (CRL4CRBN) E3 ubiquitin ligase and promote the ubiquitination of Ikaros/Aiolos transcription factors by CRL4CRBN. Here we present the crystal structure of the DDB1-CRBN complex bound to thalidomide, lenalidomide and pomalidomide. The structure establishes CRBN as a CRL4CRBN substrate receptor, which enantioselectively binds IMiDs. Through an unbiased screen we identify the homeobox transcription factor MEIS2 as an endogenous substrate of CRL4CRBN. Our studies suggest that IMiDs block endogenous substrates (MEIS2) from binding to CRL4CRBN when recruiting Ikaros/Aiolos for degradation. This dual activity implies that small molecules can principally modulate a ligase to up- or down-regulate the ubiquitination of proteins.