Biology subjects

Diffley, J. F. X.

Publications and source records attributed to Diffley, J. F. X..

10 recordsLinked to original sources

bioRxiv2026

Structure of the Pre-Initiation Complex Explains CMGE Biogenesis

Summary paragraphWhen cells enter S phase, bidirectional DNA replication is initiated through the kinase-regulated recruitment of three activators (Cdc45, GINS and Pol epsilon) to a duplex DNA-loaded double hexamer of MCM ATPases. Together these proteins form two CMGE helicases that establish divergent replication forks as they become separated1. To understand CMGE biogenesis, we reconstituted the pre-Initiation Complex with purified yeast proteins. The cryo-EM structure shows a set of firing factors caught in the act of assembling two symmetric CMGEs. We show how stepwise complex formation reshapes MCM in preparation for DNA opening and we explain how ATP promotes firing-factor ejection and CMGE maturation. While we find that Sld2 promotes GINS recruitment to MCM as expected, it also aids efficient separation of the CMGE dimer, and it is essential for lagging strand ejection from MCM. These findings have direct implications for our understanding of the metazoan Sld2 ortholog, RECQL4, pointing to a replication-fork establishment mechanism conserved across eukaryotes.

biochemistry↗

bioRxiv2026

Mcm10 braces converging replicative helicases to pull apart origin DNA

Eukaryotic DNA replication initiation requires controlled, temporally separated steps to preserve genome stability. The MCM replicative helicase is loaded on duplex DNA as an inactive double hexamer, which nucleates the assembly of dimeric Cdc45-MCM-GINS-Pol epsilon (dCMGE) replisomes. Mcm10 splits dCMGE into two, generating divergent replication forks, but the mechanism is unknown. Using ATPase-defective yeast MCM variants that slow origin melting, we captured five reaction intermediates that explain the structural mechanism. Two Mcm10 molecules bridge across the CMGE dimer, bracing two converging helicases. The restrained MCM motors pull apart the two DNA filaments, such that each lagging strand becomes ejected through the Mcm2-5 gate. Our reconstituted structures resemble the double CMG stabilized by metazoan DONSON, pointing to an origin DNA melting mechanism conserved across evolution.

biochemistry↗

bioRxiv2026

Selective Elimination of TP53 Mutant Cells by Transcript-Activated Chromatin Shredding

Genetic mutations that drive cancer often occur in tumor suppressor proteins, including the p53 transcription factor which is altered in [~]40-50% of cases1,2. However, current therapies fail to target most such mutations because the mutant proteins typically lack defined drug-binding pockets, and restoring the endogenous function has proven challenging. Here, we programmed CRISPR-Cas12a2, an RNA-guided nuclease with trans-nucleolytic cleavage activities3,4, to selectively kill cancer cells by targeting cancer-specific transcripts. This approach eliminates cells by inducing trans chromatin cleavage, triggering DNA damage and cell death. Unlike existing methods, RNA-guided Cas12a2 senses cellular RNA signatures to shred chromatin, enabling precise targeting of undruggable mutations. Transcript-activated chromatin shredding provides an innovative paradigm to develop precision disease treatments for undruggable targets.

cell biology↗

bioRxiv2024

DNA replication fork stabilization and restart mechanisms revealed by biochemical reconstitution

Understanding how DNA replication forks stall and restart and how the DNA damage checkpoint prevents irreversible fork collapse in molecular detail are crucial for understanding how cells maintain stable genomes and how they prevent the genetic instability that drives cancer. Here we describe the reconstitution of fork stalling and restart with purified budding yeast proteins. After nucleotide depletion, leading strand DNA synthesis quickly stops but CMG helicase continues to unwind and Okazaki fragments continue to initiate on the lagging strand. Incomplete Okazaki fragments sequester PCNA, RFC and DNA polymerases {delta} and {varepsilon} which prevents normal DNA synthesis restart and exposes nascent DNA to nuclease attack. The DNA damage checkpoint limits this sequestration by restraining fork progression, which protects stalled forks from collapse and ensures restart.

biochemistry↗

bioRxiv2024

The mechanism of checkpoint-dependent DNA replication fork stabilization in human cells

The DNA damage checkpoint is crucial for maintaining genome stability after genotoxic stress; without it, excess DNA replication origins are activated, stalled DNA replication forks cannot restart normally, high levels of DNA damage and single-stranded DNA (ssDNA) accumulate and cells cannot complete S phase. Preventing excess origin firing suppresses all these effects. Here we show that when replication is not restrained by a functional checkpoint, excess DNA synthesis sequesters the processivity factor PCNA and its loader RFC, preventing normal fork restart. Nascent DNA ends unprotected by RFC/PCNA are attacked by the Helicase-Like Transcription Factor (HLTF), causing irreversible replication fork collapse and hyperaccumulation of single-stranded DNA. This explains how the checkpoint stabilizes stalled replication forks and has implications for how origin firing is normally coordinated with fork progression. Loss of HLTF suppresses fork collapse and cell lethality in checkpoint-deficient cells, which has implications for how resistance to anti-checkpoint therapies may arise.

cell biology↗

bioRxiv2024

MCM Double Hexamer Loading Visualised with Human Proteins

Eukaryotic DNA replication begins with the loading of the MCM replicative DNA helicase as a head-to-head double hexamer (DH) at origins of DNA replication1-3. Our current understanding of how DH is assembled by the Origin Recognition Complex (ORC), CDC6 and CDT1 comes mostly from budding yeast. Here we characterise human DH (hDH) loading using biochemical reconstitution and cryo-electron microscopy with purified proteins. We show that hDH engages DNA differently from yeast (yDH), and generates [~]5 base pairs of unwound DNA at the interface between hexamers, as seen in hDH isolated from cells4. We identify several differences from yeast in the order of factor recruitment and dependencies during hDH assembly. Unlike yeast5-8, the ORC6 subunit of ORC is not essential for initial MCM recruitment or hDH loading, but contributes to an alternative hDH assembly pathway requiring an intrinsically disordered region (IDR) in ORC1, which may work through a novel MCM-ORC (hMO*) intermediate. Our work presents a detailed view of how DHs are assembled in an organism utilising sequence-independent replication origins, it provides further evidence for diversity in eukaryotic DH assembly mechanisms9, and it represents the first step toward reconstitution of DNA replication initiation with purified human proteins.

biochemistry↗

bioRxiv2023

A chromatinized origin reduces ORC mobility and spatially constrains MCM

Chromatin replication involves the assembly and activity of the replisome within the nucleosomal landscape. At the core of the replisome is the Mcm2-7 complex (MCM), which is loaded onto DNA after binding to the Origin Recognition Complex (ORC). In yeast, ORC is a dynamic protein that diffuses rapidly along DNA, unless halted by origin recognition sequences. However, less is known about the dynamics of ORC proteins in the presence of nucleosomes and attendant consequences for MCM loading. To address this, we harnessed an in vitro single-molecule approach to interrogate a chromatinized origin of replication. We find that ORC binds the origin of replication with similar efficiency independently of whether the origin is chromatinized, despite ORC mobility being reduced by the presence of nucleosomes. Recruitment of MCM also proceeds efficiently on a chromatinized origin, but subsequent movement of MCM away from the origin is severely constrained. These findings suggest that chromatinized origins in yeast are essential for the local retention of MCM, which may facilitate subsequent assembly of the replisome.

biophysics↗

bioRxiv2022

Nucleotide binding halts diffusion of the eukaryotic replicative helicase during activation

The eukaryotic replicative helicase CMG centrally orchestrates the replisome and leads the way at the front of replication forks1. Understanding the motion of CMG on the DNA is therefore key to our understanding of DNA replication. In vivo, CMG is assembled and activated through a cell-cycle-regulated mechanism involving 36 polypeptides that has been reconstituted from purified proteins in ensemble biochemical studies2,3. Conversely, single-molecule studies of CMG motion have thus far4-6 relied on pre-formed CMG assembled through an unknown mechanism upon overexpression of individual constituents7,8. Here, we report the first activation at the single-molecule level of CMG fully reconstituted from purified yeast proteins and the quantification of its motion. We observe that CMG can move on DNA in two ways: by unidirectional translocation and by diffusion. We demonstrate that CMG preferentially exhibits unidirectional translocation in the presence of ATP, whereas it preferentially exhibits diffusive motion in the absence of ATP. We also demonstrate that nucleotide binding halts diffusive CMG. Taken together, our findings support a mechanism by which nucleotide binding allows newly assembled CMG to engage with the DNA within its central channel without melting it, halting its diffusion and facilitating the initial DNA melting required to initiate DNA replication.

biophysics↗

bioRxiv2021

Cdc6 is sequentially regulated by PP2A-Cdc55, Cdc14 and Sic1 for origin licensing in S. cerevisiae

Cdc6, a subunit of the pre-replicative complex, contains multiple regulatory Cdk1 consensus sites, SP or TP motifs. In S. cerevisiae, Cdk1 phosphorylates Cdc6-T7 to recruit Cks1, the Cdk1 phospho-adaptor in S-phase, for subsequent multisite phosphorylation and protein degradation. Cdc6 accumulates in mitosis and is tightly bound by Clb2 through N-terminal phosphorylation in order to prevent premature origin licensing and degradation. It has been extensively studied how Cdc6 phosphorylation is regulated by the Cyclin-Cdk1 complex. However, a detailed mechanism on how Cdc6 phosphorylation is reversed by phosphatases has not been elucidated. Here, we show that PP2ACdc55 dephosphorylates Cdc6 N-terminal sites to release Clb2. Cdc14 dephosphorylates the C-terminal phospho-degron, leading to Cdc6 stabilization in mitosis. In addition, the Cdk1 inhibitor, Sic1, releases Clb2{middle dot}Cdk1{middle dot}Cks1 from Cdc6 to load Mcm2-7 on the chromatin upon mitotic exit. Thus, pre-RC assembly and origin licensing is promoted by the attenuation of distinct CDK-dependent Cdc6 inhibitory mechanisms.

cell biology↗

bioRxiv2020

Can a nick promote partial genome re-replication?

Single-stranded DNA breaks, including simple nicks, are amongst the most common forms of DNA damage in cells. They can be readily repaired by ligation; however, if a nick occurs just ahead of an approaching replisome, the outcome is a collapsed replication fork in which the nick is converted into a single-ended double-strand DNA break. Attention has largely focused on the processes by which this broken end is used to prime replication restart. We realized that in eukaryotic cells, where replication initiates from multiple replication origins, a second fork converging on the collapsed fork offers additional opportunities for repair, but also generates a substrate that can promote localized re-replication. We have modelled this with purified proteins in vitro and have found that there is, indeed, an additional hazard that eukaryotic replisomes face. We discuss how this problem might be mitigated.

biochemistry↗