The small molecules thalidomide, lenalidomide, and pomalidomide induce the ubiquitination and proteasomal degradation of the transcription factors Ikaros (IKZF1) and Aiolos (IKZF3) by recruiting a Cys2-His2 (C2H2) zinc finger domain to Cereblon (CRBN), the substrate receptor of the CRL4CRBN E3 ubiquitin ligase. Here we screened the human C2H2 zinc finger proteome for degradation in the presence of thalidomide analogs, identifying 11 zinc finger degrons. Structural and functional characterization of the C2H2 zinc finger degrons demonstrates how diverse zinc finger domains bind the permissive drug-CRBN interface. Computational zinc finger docking and biochemical analysis predict that more than 150 zinc fingers bind the drug-CRBN complex in vitro, a larger number than previously anticipated, and we show that selective zinc finger degradation can be achieved through compound modifications. Our results provide a rationale for therapeutically targeting transcription factors that were previously considered undruggable.
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(DDB2)) monitors the genome for ultraviolet-light-induced DNA damage. CRL4A(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(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.
Repair of DNA double-strand breaks via homologous recombination can produce double Holliday junctions (dHJs) that require enzymatic separation. Topoisomerase IIIα (TopIIIα) together with RMI1 disentangles the final hemicatene intermediate obtained once dHJs have converged. How binding of RMI1 to TopIIIα influences it to behave as a hemicatenane dissolvase, rather than as an enzyme that relaxes DNA topology, is unknown. Here, we present the crystal structure of human TopIIIα complexed to the first oligonucleotide binding domain (OB-fold) of RMI1. TopIIIα assumes a toroidal type 1A topoisomerase fold. RMI1 attaches to the edge of the gate in TopIIIα through which DNA passes. RMI1 projects a 23-residue loop into the TopIIIα gate, thereby influencing the dynamics of its opening and closing. Our results provide a mechanistic rationale for how RMI1 stabilizes TopIIIα gate opening to enable dissolution and illustrate how binding partners modulate topoisomerase function.
The xeroderma pigmentosum group D (XPD) helicase is a subunit of transcription/DNA repair factor, transcription factor II H (TFIIH) that catalyzes the unwinding of a damaged DNA duplex during nucleotide excision repair. Apart from two canonical helicase domains, XPD is composed of a 4Fe-S cluster domain involved in DNA damage recognition and a module of uncharacterized function termed the "ARCH domain." By investigating the consequences of a mutation found in a patient with trichothiodystrophy, we show that the ARCH domain is critical for the recruitment of the cyclin-dependent kinase (CDK)-activating kinase (CAK) complex. Indeed, this mutation not only affects the interaction with the MAT1 CAK subunit, thereby decreasing the in vitro basal transcription activity of TFIIH itself and impeding the efficient recruitment of the transcription machinery on the promoter of an activated gene, but also impairs the DNA unwinding activity of XPD and the nucleotide excision repair activity of TFIIH. We further demonstrate the role of CAK in downregulating the XPD helicase activity within TFIIH. Taken together, our results identify the ARCH domain of XPD as a platform for the recruitment of CAK and as a potential molecular switch that might control TFIIH composition and play a key role in the conversion of TFIIH from a factor active in transcription to a factor involved in DNA repair.rare disease | regulation of gene expression T he xeroderma pigmentosum group D (XPD) gene encodes a 5′-3′ helicase (XPD) that harbors mutations in patients suffering from three rare autosomal recessive diseases, xeroderma pigmentosum (XP), trichothiodystrophy (TTD), and Cockayne syndrome (CS) (1, 2). XP is characterized by a deficit of the nucleotide excision repair (NER) pathway, leading to sun sensitivity and susceptibility to skin cancer. TTD is characterized by sulfur-deficient brittle hair and a variety of neuroectodermal symptoms (3). XPD is the founding member of a family of DNA helicases conserved in archaea and eukaryotes. All family members share a four-domain organization including a conserved (Fe-S) cluster-binding domain that is essential for the helicase activity and a module of uncharacterized function named the ARCH domain by its arch-shape structure (4-7). Although archeal XPD homologs are monomers and have no known stable interactors, eukaryotic XPD homologs are part of the general transcription/DNA repair factor transcription factor II H (TFIIH), a multisubunit complex made up of 10 subunits (reviewed in ref. 8). Low-resolution models for TFIIH have been obtained for the complex in yeast (9, 10) and for the human complex (11), showing an overall conservation of shape. Human TFIIH can be resolved into two functional and structural entities bridged by XPD: the core-TFIIH consists of XPB, p62, p52, p44, p34, and p8, whereas the cyclin-dependent kinase (CDK)-activating kinase (CAK) subcomplex contains CDK7, cyclin H, and ménage a trois 1 (MAT1). XPD interacts with the p44 core-TFIIH subunit and with MAT1, a subunit of CAK involve...
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