NEDD8 (neural precursor cell expressed developmentally downregulated protein 8) is a ubiquitin-like protein that activates the largest ubiquitin E3 ligase family, the cullin-RING ligases. Many ...non-cullin neddylation targets have been proposed in recent years. However, overexpression of exogenous NEDD8 can trigger NEDD8 conjugation through the ubiquitylation machinery, which makes validating potential NEDD8 targets challenging. Here, we re-evaluate studies of non-cullin targets of NEDD8 in light of the current understanding of the neddylation pathway, and suggest criteria for identifying genuine neddylation substrates under homeostatic conditions. We describe the biological processes that might be regulated by non-cullin neddylation, and the utility of neddylation inhibitors for research and as potential therapies. Understanding the biological significance of non-cullin neddylation is an exciting research prospect primed to reveal fundamental insights.
DNA repair by homologous recombination is highly suppressed in G1 cells to ensure that mitotic recombination occurs solely between sister chromatids. Although many homologous recombination factors ...are cell-cycle regulated, the identity of the events that are both necessary and sufficient to suppress recombination in G1 cells is unknown. Here we report that the cell cycle controls the interaction of BRCA1 with PALB2-BRCA2 to constrain BRCA2 function to the S/G2 phases in human cells. We found that the BRCA1-interaction site on PALB2 is targeted by an E3 ubiquitin ligase composed of KEAP1, a PALB2-interacting protein, in complex with cullin-3 (CUL3)-RBX1 (ref. 6). PALB2 ubiquitylation suppresses its interaction with BRCA1 and is counteracted by the deubiquitylase USP11, which is itself under cell cycle control. Restoration of the BRCA1-PALB2 interaction combined with the activation of DNA-end resection is sufficient to induce homologous recombination in G1, as measured by RAD51 recruitment, unscheduled DNA synthesis and a CRISPR-Cas9-based gene-targeting assay. We conclude that the mechanism prohibiting homologous recombination in G1 minimally consists of the suppression of DNA-end resection coupled with a multi-step block of the recruitment of BRCA2 to DNA damage sites that involves the inhibition of BRCA1-PALB2-BRCA2 complex assembly. We speculate that the ability to induce homologous recombination in G1 cells with defined factors could spur the development of gene-targeting applications in non-dividing cells.
The structural determination of biological macromolecules has been transformative for understanding biochemical mechanisms and developing therapeutics. However, the ultimate goal of characterizing ...how structural dynamics underpin biochemical processes has been difficult. This is largely due to significant technical challenges that hinder data collection and analysis on the native timescales of macromolecular dynamics. Single‐particle cryo‐EM provides a powerful platform to approach this challenge, since samples can be frozen faster than the single‐turnover timescales of most biochemical reactions. In order to enable time‐resolved analysis, significant innovations in the handling and preparation of cryo‐EM samples have been implemented, bringing us closer to the goal of the direct observation of protein dynamics in the milliseconds to seconds range. Here, the current state of time‐resolved cryo‐EM is reviewed and the most promising future research directions are discussed.
Time‐resolved cryo‐EM is an emerging technique in structural biology that allows the user to capture structural states which would otherwise be too transient for standard methods. There has been a resurgence in technical advancements in this field in the last five years and this review provides a summary of the technical highlights.
Mechanistic understanding of biochemical reactions requires structural and kinetic characterization of the underlying chemical processes. However, no single experimental technique can provide this ...information in a broadly applicable manner and thus structural studies of static macromolecules are often complemented by biophysical analysis. Moreover, the common strategy of utilizing mutants or crosslinking probes to stabilize intermediates is prone to trapping off-pathway artefacts and precludes determining the order of molecular events. Here we report a time-resolved sample preparation method for cryo-electron microscopy (trEM) using a modular microfluidic device, featuring a 3D-mixing unit and variable delay lines that enables automated, fast, and blot-free sample vitrification. This approach not only preserves high-resolution structural detail but also substantially improves sample integrity and protein distribution across the vitreous ice. We validate the method by visualising reaction intermediates of early RecA filament growth across three orders of magnitude on sub-second timescales. The trEM method reported here is versatile, reproducible, and readily adaptable to a broad spectrum of fundamental questions in biology.
DNA end resection plays a critical function in DNA double-strand break repair pathway choice. Resected DNA ends are refractory to end-joining mechanisms and are instead channeled to homology-directed ...repair. Using biochemical, genetic, and imaging methods, we show that phosphorylation of Saccharomyces cerevisiae Sae2 controls its capacity to promote the Mre11-Rad50-Xrs2 (MRX) nuclease to initiate resection of blocked DNA ends by at least two distinct mechanisms. First, DNA damage and cell cycle-dependent phosphorylation leads to Sae2 tetramerization. Second, and independently, phosphorylation of the conserved C-terminal domain of Sae2 is a prerequisite for its physical interaction with Rad50, which is also crucial to promote the MRX endonuclease. The lack of this interaction explains the phenotype of rad50S mutants defective in the processing of Spo11-bound DNA ends during meiotic recombination. Our results define how phosphorylation controls the initiation of DNA end resection and therefore the choice between the key DNA double-strand break repair mechanisms.
Autophagy is a conserved intracellular degradation pathway exerting various cytoprotective and homeostatic functions by using de novo double-membrane vesicle (autophagosome) formation to target a ...wide range of cytoplasmic material for vacuolar/lysosomal degradation. The Atg1 kinase is one of its key regulators, coordinating a complex signaling program to orchestrate autophagosome formation. Combining in vitro reconstitution and cell-based approaches, we demonstrate that Atg1 is activated by lipidated Atg8 (Atg8-PE), stimulating substrate phosphorylation along the growing autophagosomal membrane. Atg1-dependent phosphorylation of Atg13 triggers Atg1 complex dissociation, enabling rapid turnover of Atg1 complex subunits at the pre-autophagosomal structure (PAS). Moreover, Atg1 recruitment by Atg8-PE self-regulates Atg8-PE levels in the growing autophagosomal membrane by phosphorylating and thus inhibiting the Atg8-specific E2 and E3. Our work uncovers the molecular basis for positive and negative feedback imposed by Atg1 and how opposing phosphorylation and dephosphorylation events underlie the spatiotemporal regulation of autophagy.
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•Lipidated Atg8 activates Atg1, stimulating substrate phosphorylation at phagophores•Phosphorylation of Atg13 by Atg1 dissociates the Atg1 complex at the PAS•Atg1 downregulates Atg8 lipidation at phagophores by inhibiting Atg3 and Atg12•Protein phosphatase activity reverses the inhibitory functions of Atg1
Schreiber et al. show that upon autophagy induction, lipidated Atg8 activates the Atg1 kinase. This downregulates Atg8 lipidation as Atg1 phosphorylates two of the enzymes involved in Atg8 lipidation, inhibiting their activity. Phosphorylation of Atg13 further dissociates the Atg1 complex, regulating Atg1 complex dynamics at the phagophore assembly site.
Skp1-Cul1-Fbox (SCF) E3 ligases are activated by ligation to the ubiquitin-like protein Nedd8, which is reversed by the deneddylating Cop9 signalosome (CSN). However, CSN also promotes SCF substrate ...turnover through unknown mechanisms. Through biochemical and electron microscopy analyses, we determined molecular models of CSN complexes with SCFSkp2/Cks1 and SCFFbw7 and found that CSN occludes both SCF functional sites—the catalytic Rbx1-Cul1 C-terminal domain and the substrate receptor. Indeed, CSN binding prevents SCF interactions with E2 enzymes and a ubiquitination substrate, and it inhibits SCF-catalyzed ubiquitin chain formation independent of deneddylation. Importantly, CSN prevents neddylation of the bound cullin, unless binding of a ubiquitination substrate triggers SCF dissociation and neddylation. Taken together, the results provide a model for how reciprocal regulation sensitizes CSN to the SCF assembly state and inhibits a catalytically competent SCF until a ubiquitination substrate drives its own degradation by displacing CSN, thereby promoting cullin neddylation and substrate ubiquitination.
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► Electron microscopy structures of CSN-SCF complexes are revealed ► Molecular models for CSN and CSN-SCF complexes are presented ► SCF assembly state influences CSN binding and deneddylation activity ► Mechanistic insight into noncatalytic CSN inhibition of SCF activity is gained
Cullin-RING ligases (CRLs), the largest family of ubiquitin E3 ligases, are crucial regulators of many cellular processes. Their specificity and activity are regulated by assembly with different substrate receptors and ligation to the ubiquitin-like protein Nedd8. CSN deconjugates Nedd8 but stably associates with CRLs, suggesting additional levels of regulation. Schulman and colleagues reveal structural data on CRL-SCF complexes and provide mechanistic insight into noncatalytic CSN functions. Surprisingly, they find that CRLs reciprocally regulate CSN activity and that cognate ubiquitination substrates dissociate CSN-CRL complexes.
Modification of proteins by ubiquitin (Ub) and Ub‐like (Ubl) modifiers regulates a variety of cellular functions. The ability of Ub to form chains of eight structurally and functionally distinct ...types adds further complexity to the system. Ub‐specific proteases (USPs) hydrolyse polyUb chains, and some have been suggested to be cross‐reactive with Ubl modifiers, such as neural precursor cell expressed, developmentally downregulated 8 (NEDD8) and interferon‐stimulated gene 15 (ISG15). Here, we report that USP21 cleaves Ub polymers, and with reduced activity also targets ISG15, but is inactive against NEDD8. A crystal structure of USP21 in complex with linear diUb aldehyde shows how USP21 interacts with polyUb through a previously unidentified second Ub‐ and ISG15‐binding surface on the USP domain core. We also rationalize the inability of USP21 to target NEDD8 and identify differences that allow USPs to distinguish between structurally related modifications.
The crystal structure of the ubiquitin‐specific protease USP21 bound to linear diubiquitin‐aldehyde reveals a second S2 ubiquitin/ISG15‐binding site. The structure reveals why USP21 cannot hydrolyze NEDD8 modifications.
Homologous recombination (HR) is an essential DNA double-strand break (DSB) repair mechanism, which is frequently inactivated in cancer. During HR, RAD51 forms nucleoprotein filaments on RPA-coated, ...resected DNA and catalyzes strand invasion into homologous duplex DNA. How RAD51 displaces RPA and assembles into long HR-proficient filaments remains uncertain. Here, we employed single-molecule imaging to investigate the mechanism of nematode RAD-51 filament growth in the presence of BRC-2 (BRCA2) and RAD-51 paralogs, RFS-1/RIP-1. BRC-2 nucleates RAD-51 on RPA-coated DNA, whereas RFS-1/RIP-1 acts as a “chaperone” to promote 3′ to 5′ filament growth via highly dynamic engagement with 5′ filament ends. Inhibiting ATPase or mutation in the RFS-1 Walker box leads to RFS-1/RIP-1 retention on RAD-51 filaments and hinders growth. The rfs-1 Walker box mutants display sensitivity to DNA damage and accumulate RAD-51 complexes non-functional for HR in vivo. Our work reveals the mechanism of RAD-51 nucleation and filament growth in the presence of recombination mediators.
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•C. elegans BRCA2 and Rad51 paralogs synergistically promote Rad51 filament assembly•Rad51 paralogs dynamically engage with the 5′ Rad51 filament ends•Rad51 paralogs stimulate Rad51 filament growth in a 3′–5′ direction•Retention of Rad51 paralogs at filament ends hinders growth and blocks HR in vivo
Belan et al. exploit single-molecule approaches to reveal the mechanism of C. elegans Rad51 filament assembly in the presence of recombination mediators. BRCA2 primarily enhances Rad51 nucleation on ssDNA, whereas Rad51 paralogs dynamically engage with 5′ Rad51 filament ends and stimulate filament growth in a 3′–5′ direction.
Human CtIP is a decisive factor in DNA double-strand break repair pathway choice by enabling DNA-end resection, the first step that differentiates homologous recombination (HR) from non-homologous ...end-joining (NHEJ). To coordinate appropriate and timely execution of DNA-end resection, CtIP function is tightly controlled by multiple protein-protein interactions and post-translational modifications. Here, we identify the Cullin3 E3 ligase substrate adaptor Kelch-like protein 15 (KLHL15) as a new interaction partner of CtIP and show that KLHL15 promotes CtIP protein turnover via the ubiquitin-proteasome pathway. A tripeptide motif (FRY) conserved across vertebrate CtIP proteins is essential for KLHL15-binding; its mutation blocks KLHL15-dependent CtIP ubiquitination and degradation. Consequently, DNA-end resection is strongly attenuated in cells overexpressing KLHL15 but amplified in cells either expressing a CtIP-FRY mutant or lacking KLHL15, thus impacting the balance between HR and NHEJ. Collectively, our findings underline the key importance and high complexity of CtIP modulation for genome integrity.
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