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Deciphering the modes of human separase inhibition by securin and CDK1-CCNB1.
Mol Cell Oncol, 2021; 8 (4): 1975473
Accurate chromosome segregation depends on tight regulation of the protease separase, which cleaves the ring-shaped cohesin complex that entraps the two sister chromatids. We recently reported structures of human separase bound to its inhibitors securin or the cyclin-dependent kinase 1 (CDK1)-cyclin B1 (CCNB1)-cyclin-dependent kinases regulatory subunit 1 (CKS1) complex and discovered an array of molecular mechanisms that block cohesin-cleavage.
Structural basis of human separase regulation by securin and CDK1-cyclin B1.
Nature, 2021 Aug; 596 (7870): 138-142
In early mitosis, the duplicated chromosomes are held together by the ring-shaped cohesin complex. Separation of chromosomes during anaphase is triggered by separase-a large cysteine endopeptidase that cleaves the cohesin subunit SCC1 (also known as RAD21). Separase is activated by degradation of its inhibitors, securin and cyclin B, but the molecular mechanisms of separase regulation are not clear. Here we used cryogenic electron microscopy to determine the structures of human separase in complex with either securin or CDK1-cyclin B1-CKS1. In both complexes, separase is inhibited by pseudosubstrate motifs that block substrate binding at the catalytic site and at nearby docking sites. As in Caenorhabditis elegans and yeast, human securin contains its own pseudosubstrate motifs. By contrast, CDK1-cyclin B1 inhibits separase by deploying pseudosubstrate motifs from intrinsically disordered loops in separase itself. One autoinhibitory loop is oriented by CDK1-cyclin B1 to block the catalytic sites of both separase and CDK1. Another autoinhibitory loop blocks substrate docking in a cleft adjacent to the separase catalytic site. A third separase loop contains a phosphoserine that promotes complex assembly by binding to a conserved phosphate-binding pocket in cyclin B1. Our study reveals the diverse array of mechanisms by which securin and CDK1-cyclin B1 bind and inhibit separase, providing the molecular basis for the robust control of chromosome segregation.
The CryoEM structure of the Saccharomyces cerevisiae ribosome maturation factor Rea1.
Elife, 2018 Nov 26; 7 :
The biogenesis of 60S ribosomal subunits is initiated in the nucleus where rRNAs and proteins form pre-60S particles. These pre-60S particles mature by transiently interacting with various assembly factors. The ~5000 amino-acid AAA+ ATPase Rea1 (or Midasin) generates force to mechanically remove assembly factors from pre-60S particles, which promotes their export to the cytosol. Here we present three Rea1 cryoEM structures. We visualise the Rea1 engine, a hexameric ring of AAA+ domains, and identify an α-helical bundle of AAA2 as a major ATPase activity regulator. The α-helical bundle interferes with nucleotide-induced conformational changes that create a docking site for the substrate binding MIDAS domain on the AAA +ring. Furthermore, we reveal the architecture of the Rea1 linker, which is involved in force generation and extends from the AAA+ ring. The data presented here provide insights into the mechanism of one of the most complex ribosome maturation factors.
The potential of cryo-electron microscopy for structure-based drug design.
Essays Biochem, 2017 Nov 8; 61 (5): 543-560
Structure-based drug design plays a central role in therapeutic development. Until recently, protein crystallography and NMR have dominated experimental approaches to obtain structural information of biological molecules. However, in recent years rapid technical developments in single particle cryo-electron microscopy (cryo-EM) have enabled the determination to near-atomic resolution of macromolecules ranging from large multi-subunit molecular machines to proteins as small as 64 kDa. These advances have revolutionized structural biology by hugely expanding both the range of macromolecules whose structures can be determined, and by providing a description of macromolecular dynamics. Cryo-EM is now poised to similarly transform the discipline of structure-based drug discovery. This article reviews the potential of cryo-EM for drug discovery with reference to protein ligand complex structures determined using this technique.
Cryo-EM structure of a metazoan separase-securin complex at near-atomic resolution.
Nat Struct Mol Biol, 2017 Apr; 24 (4): 414-418
Separase is a caspase-family protease that initiates chromatid segregation by cleaving the kleisin subunits (Scc1 and Rec8) of cohesin, and regulates centrosome duplication and mitotic spindle function through cleavage of kendrin and Slk19. To understand the mechanisms of securin regulation of separase, we used single-particle cryo-electron microscopy (cryo-EM) to determine a near-atomic-resolution structure of the Caenorhabditis elegans separase-securin complex. Separase adopts a triangular-shaped bilobal architecture comprising an N-terminal tetratricopeptide repeat (TPR)-like α-solenoid domain docked onto the conserved C-terminal protease domain. Securin engages separase in an extended antiparallel conformation, interacting with both lobes. It inhibits separase by interacting with the catalytic site through a pseudosubstrate mechanism, thus revealing that in the inhibited separase-securin complex, the catalytic site adopts a conformation compatible with substrate binding. Securin is protected from cleavage because an aliphatic side chain at the P1 position represses protease activity by disrupting the organization of catalytic site residues.
A DDX6-CNOT1 complex and W-binding pockets in CNOT9 reveal direct links between miRNA target recognition and silencing.
Mol Cell, 2014 Jun 5; 54 (5): 737-750
CCR4-NOT is a major effector complex in miRNA-mediated gene silencing. It is recruited to miRNA targets through interactions with tryptophan (W)-containing motifs in TNRC6/GW182 proteins and is required for both translational repression and degradation of miRNA targets. Here, we elucidate the structural basis for the repressive activity of CCR4-NOT and its interaction with TNRC6/GW182s. We show that the conserved CNOT9 subunit attaches to a domain of unknown function (DUF3819) in the CNOT1 scaffold. The resulting complex provides binding sites for TNRC6/GW182, and its crystal structure reveals tandem W-binding pockets located in CNOT9. We further show that the CNOT1 MIF4G domain interacts with the C-terminal RecA domain of DDX6, a translational repressor and decapping activator. The crystal structure of this complex demonstrates striking similarity to the eIF4G-eIF4A complex. Together, our data provide the missing physical links in a molecular pathway that connects miRNA target recognition with translational repression, deadenylation, and decapping.
Structure and assembly of the NOT module of the human CCR4-NOT complex.
Nat Struct Mol Biol, 2013 Nov; 20 (11): 1289-1297
The CCR4-NOT deadenylase complex is a master regulator of translation and mRNA stability. Its NOT module orchestrates recruitment of the catalytic subunits to target mRNAs. We report the crystal structure of the human NOT module formed by the CNOT1, CNOT2 and CNOT3 C-terminal (-C) regions. CNOT1-C provides a rigid scaffold consisting of two perpendicular stacks of HEAT-like repeats. CNOT2-C and CNOT3-C heterodimerize through their SH3-like NOT-box domains. The heterodimer is stabilized and tightly anchored to the surface of CNOT1 through an unexpected intertwined arrangement of peptide regions lacking defined secondary structure. These assembly peptides mold onto their respective binding surfaces and form extensive interfaces. Mutagenesis of individual interfaces and perturbation of endogenous protein ratios cause defects in complex assembly and mRNA decay. Our studies provide a structural framework for understanding the recruitment of the CCR4-NOT complex to mRNA targets.
Crystal structure of the MID-PIWI lobe of a eukaryotic Argonaute protein.
Proc Natl Acad Sci U S A, 2011 Jun 28; 108 (26): 10466-10471
Argonaute proteins (AGOs) are essential effectors in RNA-mediated gene silencing pathways. They are characterized by a bilobal architecture, in which one lobe contains the N-terminal and PAZ domains and the other contains the MID and PIWI domains. Here, we present the first crystal structure of the MID-PIWI lobe from a eukaryotic AGO, the Neurospora crassa QDE-2 protein. Compared to prokaryotic AGOs, the domain orientation is conserved, indicating a conserved mode of nucleic acid binding. The PIWI domain shows an adaptable surface loop next to a eukaryote-specific α-helical insertion, which are both likely to contact the PAZ domain in a conformation-dependent manner to sense the functional state of the protein. The MID-PIWI interface is hydrophilic and buries residues that were previously thought to participate directly in the allosteric regulation of guide RNA binding. The interface includes the binding pocket for the guide RNA 5' end, and residues from both domains contribute to binding. Accordingly, micro-RNA (miRNA) binding is particularly sensitive to alteration in the MID-PIWI interface in Drosophila melanogaster AGO1 in vivo. The structure of the QDE-2 MID-PIWI lobe provides molecular and mechanistic insight into eukaryotic AGOs and has significant implications for understanding the role of these proteins in silencing.
Crystal structure and ligand binding of the MID domain of a eukaryotic Argonaute protein.
EMBO Rep, 2010 Jul; 11 (7): 522-527
Argonaute (AGO) proteins are core components of RNA-induced silencing complexes and have essential roles in RNA-mediated gene silencing. They are characterized by a bilobal architecture, consisting of one lobe containing the amino-terminal and PAZ domains and another containing the MID and PIWI domains. Except for the PAZ domain, structural information on eukaryotic AGO domains is not yet available. In this study, we report the crystal structure of the MID domain of the eukaryotic AGO protein QDE-2 from Neurospora crassa. This domain adopts a Rossmann-like fold and recognizes the 5'-terminal nucleotide of a guide RNA in a manner similar to its prokaryotic counterparts. The 5'-nucleotide-binding site shares common residues with a second, adjacent ligand-binding site, suggesting a mechanism for the cooperative binding of ligands to the MID domain of eukaryotic AGOs.