Biology subjects

Arkinson, C.

Publications and source records attributed to Arkinson, C..

11 recordsLinked to original sources

bioRxiv2025

Faf1 accelerates p97-mediated protein unfolding by promoting ubiquitin engagement

P97/VCP is a protein unfoldase of the AAA+ ATPase family that plays essential roles in numerous processes, including ER-associated degradation and DNA replication. For unfolding of proteins that are modified with K48-linked ubiquitin chains, p97 works with the heterodimeric cofactor Ufd1-Npl4, and the cofactor Faf1 was shown to enhance this activity during replisome disassembly, yet the mechanisms remain unknown. Here, we employ an in vitro reconstituted system with human components for biochemical experiments, FRET-based assays, and cryo-EM structure determination to reveal that Faf1 plays a generic role in accelerating ubiquitin-dependent substrate processing by promoting the unfolding of an initiator ubiquitin and its engagement by the ATPase. Faf1 thereby uses its p97-bound C-terminal UBX domain to anchor a long helix that braces the UT3 domain of Ufd1 and stabilizes the Ufd1-Npl4 cofactor for ubiquitin unfolding. Our findings demonstrate how p97 works simultaneously with several cofactors to facilitate the unfolding of ubiquitinated proteins, indicating more complex regulatory mechanisms for substrate selection than for the simpler yeast Cdc48.

biochemistry↗

bioRxiv2024

Structural landscape of AAA+ ATPase motor states in the substrate-degrading human 26S proteasome reveals conformation-specific binding of TXNL1

The 26S proteasome targets many cellular proteins for degradation during general homeostasis, protein quality control, and the regulation of vital processes. A broad range of proteasome-interacting cofactors thereby modulates these functions and aids in substrate degradation. Here, we solved several high-resolution structures of the redox active cofactor TXNL1 bound to the human 26S proteasome at saturating and sub-stoichiometric concentrations by time resolved cryo-EM. We identified distinct binding modes of TXNL1 that depend on the proteasome conformational and ATPase motor states. Together with biophysical and biochemical experiments, our structural studies reveal that the resting-state proteasome prior to substrate engagement with the ATPase motor binds TXNL1 with low affinity and in variable positions on top of the Rpn11 deubiquitinase. In contrast, the actively degrading proteasome shows additional interactions leading to high-affinity TXNL1 binding, whereby TXNL1s C-terminal tail covers the catalytic groove of the Rpn11 deubiquitinase and coordinates the active-site Zn2+. Furthermore, these cryo-EM structures of the degrading proteasome capture the ATPase hexamer in all registers of spiral-staircase arrangements and thus visualize the complete ATP-hydrolysis cycle of the AAA+ motor, indicating temporally asymmetric hydrolysis and conformational changes in bursts during mechanical substrate unfolding and translocation. Remarkably, we catch the proteasome in the act of unfolding the beta-barrel mEos3.2 substrate while the ATPase hexamer is in a particular spiral staircase register. Our findings challenge current models for protein translocation through hexameric AAA+ motors and reveal how the proteasome uses its distinct but broad range of conformational states to coordinate cofactor binding and substrate processing. HighlightsO_LIHigh resolution structures of the redox active cofactor TXNL1 in complex with the human 26S proteasome solved by time-resolved cryo-EM. C_LIO_LITXNL1 binds the catalytic groove of the main proteasomal deubiquitinase Rpn11 and coordinates its active-site Zinc specifically in substrate-degrading states of the proteasome. C_LIO_LIVisualizing a partially unfolded intermediate of the mEos model substrate during processing. C_LIO_LIStructures of the actively degrading human proteasome reveal all spiral-staircase registers of the AAA+ ATPase hexamer with unexpected nucleotide occupancies that indicate asymmetric ATP hydrolysis mechanisms, conformational changes with burst phases, and thus new models for hand-over-hand substrate translocation. C_LI

biochemistry↗

bioRxiv2024

Nub1 traps unfolded FAT10 for ubiquitin-independent degradation by the 26S proteasome

The ubiquitin-like modifier FAT10 targets hundreds of proteins in the mammalian immune system to the 26S proteasome for degradation. This degradation pathway requires the cofactor Nub1, yet the underlying mechanisms remain unknown. Here, we reconstituted a minimal in vitro system and revealed that Nub1 utilizes FAT10s intrinsic instability to trap its N-terminal ubiquitin-like domain in an unfolded state and deliver it to the 26S proteasome for engagement, allowing the degradation of FAT10-ylated substrates in a ubiquitin- and p97-independent manner. Through hydrogen-deuterium exchange, structural modeling, and site-directed mutagenesis, we identified the formation of a peculiar complex with FAT10 that activates Nub1 for docking to the 26S proteasome, and our cryo-EM studies visualized the highly dynamic Nub1 complex bound to the proteasomal Rpn1 subunit during FAT10 delivery and the early stages of ATP-dependent degradation. These studies thus identified a novel mode of cofactor-mediated, ubiquitin-independent substrate delivery to the 26S proteasome that relies on trapping partially unfolded states for engagement by the proteasomal ATPase motor.

biochemistry↗

bioRxiv2024

Structural and biochemical insights on the mechanism of action of the clinical USP1 inhibitor, KSQ-4279

DNA damage triggers cell signalling cascades that mediate repair. This signalling is frequently dysregulated in cancers. The proteins that mediate this signalling are potential targets for therapeutic intervention. Ubiquitin-specific protease 1 (USP1) is one such target, with small molecule inhibitors already in clinical trials. Here we use biochemical assays and cryo-electron microscopy (cryo-EM) to study the clinical USP1 inhibitor, KSQ-4279 (RO7623066), and compare this to the well-established, tool compound, ML323. We find that KSQ-4279 binds to the same cryptic site of USP1 as ML323 but disrupts the protein structure in subtly different ways. Inhibitor binding drives a substantial increase in thermal stability of USP1, which may be mediated through the inhibitors filling a hydrophobic tunnel in USP1. Our results contribute to the understanding of the mechanism of action of USP1 inhibitors at the molecular level.

biochemistry↗

bioRxiv2023

Profiling ubiquitin signaling with UBIMAX reveals DNA damage- and SCFβTRCP-dependent ubiquitylation of the actin-organizing protein Dbn1

Ubiquitin widely modifies proteins, thereby regulating most cellular functions. The complexity of ubiquitin signalling necessitates unbiased methods enabling global detection of dynamic protein ubiquitylation. Here, we describe UBIMAX (UBiquitin target Identification by Mass spectrometry in Xenopus egg extracts), which enriches ubiquitin-conjugated proteins and quantifies regulation of protein ubiquitylation under precise and adaptable conditions. We benchmark UBIMAX by investigating DNA double-strand break-responsive ubiquitylation events, identifying previously known targets and revealing the actin-organising protein Dbn1 as a novel major target of DNA damage-induced ubiquitylation. We find that Dbn1 is targeted for proteasomal degradation by the SCF{beta}-Trcp1 ubiquitin ligase, in a conserved mechanism driven by ATM-mediated phosphorylation of a previously uncharacterized {beta}-Trcp1 degron containing an SQ motif. We further show that this degron is sufficient to induce DNA-damage dependent protein degradation of a model substrate. Collectively, we demonstrate UBIMAXs ability to identify novel targets of stimulus-regulated ubiquitylation and reveal an SCF{beta}-Trcp1-mediated ubiquitylation mechanism controlled directly by the apical DNA damage response kinases.

molecular biology↗

bioRxiv2022

Structural and biochemical basis of FANCI-FANCD2 interdependent ubiquitination

The Fanconi Anaemia pathway operates for the repair of interstrand crosslinks and the maintenance of genomic stability upon replication stalling. Di-monoubiquitination of the FANCI-FANCD2 (ID2) complex is a central and crucial step in this pathway. Evidence suggests that FANCD2 ubiquitination precedes FANCI ubiquitination, and that both the FANCD2-ubiquitinated (ID2Ub) and the di- monoubiquitinated (IUbD2Ub) complex clamp on DNA. However, FANCD2 is deubiquitinated at a faster rate than FANCI, which can result in a FANCI-ubiquitinated ID2 complex (IUbD2). Here, we present a 4.1 [A] cryo-EM structure of IUbD2 complex bound to double-stranded DNA. We show that this complex, like ID2Ub and IUbD2Ub, is also in the closed ID2 conformation and clamps on DNA. While the target lysine of FANCD2 (K561) is partially buried in the non-ubiquitinated ID2-DNA complex, it becomes fully exposed in the IUbD2-DNA structure, and thus can be ubiquitinated at a faster rate. The IUbD2-DNA complex cannot easily revert to the non-ubiquitinated ID2 state, due to USP1-UAF1-resistance, conferred by the presence of DNA and FANCD2. ID2Ub-DNA, on the other hand, can be efficiently deubiquitinated by USP1-UAF1, unless further ubiquitination on FANCI occurs. FANCI ubiquitination also progresses at a faster rate in ID2Ub-DNA over ID2-DNA complex, and results in partial DNA-dependent protection from FANCD2 deubiquitination. Taken together, our results suggest that, while FANCD2 ubiquitination promotes FANCI ubiquitination, FANCI ubiquitination in turn maintains FANCD2 ubiquitination by two mechanisms: it prevents excessive FANCD2 deubiquitination within an IUbD2Ub-DNA complex, and it enables re-ubiquitination of FANCD2 within a transient, closed-on-DNA, IUbD2 complex.

biochemistry↗

bioRxiv2022

Cryo-EM reveals a mechanism of USP1 inhibition through a cryptic binding site

Repair of DNA damage is critical to genomic integrity and frequently disrupted in cancers. USP1, a nucleus-localized deubiquitinase, lies at the interface of multiple DNA repair pathways and is a promising drug target for certain cancers. Although multiple inhibitors of this enzyme, including one in phase I clinical trials, have been established, their binding mode is unknown. Here we use cryo-Electron Microscopy to study an assembled enzyme-substrate-inhibitor complex of USP1 and the well-established inhibitor, ML323. Achieving 2.5 [A] resolution, we discover an unusual binding mode in which the inhibitor displaces part of the hydrophobic core of USP1. The consequent conformational changes in the secondary structure lead to subtle rearrangements in the active site that underlie the mechanism of inhibition. These structures provide a platform for structure-based drug design targeting USP1. One Sentence SummaryUSP1, a cancer target, is inhibited by ML323 displacing part of the protein fold, allosterically disrupting the active site.

biochemistry↗

bioRxiv2020

Structural basis of FANCD2 deubiquitination by USP1-UAF1

Ubiquitin-Specific Protease 1 (USP1), together with the cofactor UAF1, acts during DNA repair processes to specifically to remove mono-ubiquitin signals. The mono-ubiquitinated FANCI-FANCD2 heterodimer is one such substrate and is involved in the repair of DNA interstrand crosslinks via the Fanconi Anemia pathway. Here we determine structures of human USP1-UAF1 with and without ubiquitin, and bound to mono-ubiquitinated FANCI-FANCD2 substrate. Crystal structures of USP1-UAF1 reveal plasticity in USP1 and key differences to USP12-UAF1 and USP46-UAF1. A cryoEM reconstruction of USP1-UAF1 in complex mono-ubiquitinated FANCI-FANCD2, highlights a highly orchestrated deubiquitination process with USP1-UAF1 driving conformational changes in the substrate. An extensive interface between UAF1 and FANCI, confirmed by mutagenesis and biochemical assays, provides a molecular explanation for their requirement despite neither being directly involved in catalysis. Overall, our data provide molecular details of USP1-UAF1 regulation and substrate recognition.

biochemistry↗

bioRxiv2020

Differential functions of FANCI and FANCD2 ubiquitination stabilize ID2 complex on DNA

The Fanconi Anemia (FA) pathway is a dedicated pathway for the repair of DNA interstrand crosslinks, and which is additionally activated in response to other forms of replication stress. A key step in the activation of the FA pathway is the monoubiquitination of each of the two subunits (FANCI and FANCD2) of the ID2 complex on specific lysine residues. However, the molecular function of these modifications has been unknown for nearly two decades. Here we find that ubiquitination of FANCD2 acts to increase ID2s affinity for double stranded DNA via promoting/stabilizing a large-scale conformational change in the complex, resulting in a secondary "Arm" ID2 interphase encircling DNA. Ubiquitination of FANCI, on the other hand, largely protects the ubiquitin on FANCD2 from USP1/UAF deubiquitination, with key hydrophobic residues of FANCIs ubiquitin being important for this protection. In effect, both of these post-translational modifications function to stabilise a conformation in which the ID2 complex encircles DNA.

biochemistry↗

bioRxiv2018

Specificity for deubiquitination of monoubiquitinated FANCD2 is driven by the N-terminus of USP1

The DNA damage response depends on ubiquitin signalling to orchestrate DNA repair. The Fanconi Anemia pathway for interstrand crosslink repair, and the translesion synthesis pathway for DNA damage tolerance, both require cycles of monoubiquitination and deubiquitination. The ubiquitin specific protease USP1 regulates both these pathways by deubiquitinating monoubiquitinated PCNA, FANCD2 and FANCI. Loss of USP1 activity gives rise to chromosomal instability. While many USPs hydrolyse ubiquitin-ubiquitin linkages, USP1 targets ubiquitin-substrate conjugates at specific sites. The molecular basis of USP1s specificity for multiple substrates is poorly understood. Here we show that the molecular determinants for substrate deubiquitination by USP1 reside within the highly conserved and extended N-terminus. We find that the N-terminus of USP1 harbours a FANCD2-specific binding sequence required for deubiquitination of K561 on FANCD2. In contrast, the N-terminus is not required for PCNA or FANCI deubiquitination. Furthermore, we show that the N-terminus of USP1 is sufficient to engineer specificity in a more promiscuous USP.

biochemistry↗

bioRxiv2018

Allosteric network in Ube2T drives specificity for RING E3 catalysed ubiquitin signals.

In eukaryotes, DNA damage repair is implemented by a host of proteins that are coordinated by defined molecular signals. One such signal that transpires during the Fanconi Anemia (FA) - interstrand crosslink (ICL) repair pathway is the site-specific monoubiquitination of FANCD2 and FANCI proteins by a large, multi-protein FA core complex. The mechanics for this exquisitely specific monoubiquitin signal has been elusive. Here we show FANCL, the RING E3 module of the FA core complex, allosterically activates its cognate E2 Ube2T for monoubiquitination by a mechanism distinct from the typical RING-based catalysis. FANCL triggers intricate re-wiring of Ube2Ts intra-residue network thus activating the E2 for precision targeting. This network is intrinsically regulated by conserved gates and loops which can be engineered to yield Ube2T variants that enhance FANCD2 ubiquitination by ~30-fold without compromising on target specificity. Finally, we also uncover allosteric networks in other ubiquitin E2s that can be leveraged by RING E3 ligases to drive specific ubiquitination.

biochemistry↗