A RAD18 SAP domain PIP motif enables PCNA mono-ubiquitination and USP1-BRCA1 synthetic lethality

This study identifies a critical SAP domain PIP motif within RAD18 that mediates its interaction with PCNA to drive mono-ubiquitination, establishing this structural interface as a key determinant of USP1-BRCA1 synthetic lethality and a potential mechanism of resistance to USP1-targeted therapies.

The Gist

The Big Picture: Fixing a Broken Assembly Line

Imagine your cell is a massive, high-speed factory. The most important machine in this factory is a conveyor belt called PCNA. This belt carries the workers (DNA polymerases) who build new copies of your genetic blueprint (DNA).

Sometimes, the factory floor gets messy. There are obstacles on the belt (DNA damage) that stop the workers. If the belt stops, the whole factory halts, and the cell can die or become cancerous.

To keep things moving, the cell has a "fix-it crew." One of the most important members of this crew is a protein called RAD18. RAD18's job is to slap a special "sticker" (a ubiquitin tag) onto the conveyor belt (PCNA). This sticker tells the factory: "Hey, there's a problem here! Bring in the specialized repair workers to get past the obstacle."

However, the factory also has a strict "Quality Control Manager" called USP1. USP1's job is to peel off those stickers once the repair is done, so the conveyor belt can go back to normal speed.

The Problem: When the Manager Goes on Strike

The researchers discovered something fascinating about cancer cells that are missing a key safety gene called BRCA1 (common in breast and ovarian cancers).

In these BRCA1-missing cells, the factory is already running on shaky ground. If you stop the Quality Control Manager (USP1) using a drug, the "sticker" (RAD18's tag) stays on the conveyor belt forever.

• The Result: The conveyor belt gets clogged with stickers. The specialized repair workers get confused, the belt slows down, and the factory collapses. This kills the cancer cell. This is called Synthetic Lethality—two broken things together (BRCA1 loss + USP1 blocked) cause total failure, whereas one broken thing alone is survivable.

The Discovery: Finding the "Handshake"

The big mystery for scientists was: How does RAD18 actually grab onto the conveyor belt (PCNA) to put the sticker on?

For years, they knew RAD18 did the job, but they didn't know how it held on. It was like knowing a mechanic could fix a car, but not knowing which wrench they used.

The authors of this paper found the answer. They discovered a specific "grip" or "handshake" on RAD18.

• The Grip: It's a tiny, specific shape on a part of RAD18 called the SAP domain.
• The Metaphor: Imagine RAD18 is a person trying to grab a moving train (PCNA). Most people grab the train with a strong, standard handhold (a "consensus motif"). But RAD18 is a bit different; it uses a weird, awkward, but very specific grip (a "non-consensus PIP motif") that fits perfectly into a specific slot on the train.

The researchers used advanced imaging (like a molecular MRI) to prove that if you break this specific grip (by changing a few letters in the protein's code), RAD18 can no longer hold onto the conveyor belt.

The Experiment: Breaking the Grip

To prove this grip was essential, the scientists played a game of "what if":

1. The Setup: They took cancer cells that were missing BRCA1.
2. The Attack: They tried to kill the cells with a drug that stops USP1 (the Quality Control Manager).
3. The Twist: They created a version of RAD18 where they "glued over" that special grip (the SAP domain PIP motif).
4. The Result:
   • Normal RAD18: The drug worked. The conveyor belt got clogged with stickers, and the cancer cells died.
   • "Glued" RAD18: The drug failed. Because RAD18 couldn't grab the belt, it couldn't put the sticker on. The conveyor belt kept running smoothly, and the cancer cells survived.

This proved that the "grip" is the critical link. Without it, the drug that kills BRCA1-deficient cancers doesn't work.

The Plot Twist: How Cancer Fights Back

The researchers also looked at what happens when cancer cells get used to the drug. They grew cancer cells in a dish with the drug for months, slowly increasing the dose until the cells survived.

They found that the surviving cancer cells had a secret weapon: They stopped making RAD18.

• The Analogy: It's like a factory that keeps getting shut down because the manager keeps peeling off stickers. The workers (cancer cells) eventually realize, "If we just fire the guy who puts the stickers on (RAD18), the manager (USP1) won't have anything to peel off, and we can keep working!"
• By lowering their levels of RAD18, the cancer cells became resistant to the drug.

Why This Matters

This paper is a roadmap for the future of cancer treatment.

1. Understanding the Mechanism: We now know exactly how the drug works. It relies on RAD18 grabbing the conveyor belt.
2. Predicting Resistance: If a patient's cancer stops responding to the drug, doctors might check if the cancer has lowered its RAD18 levels.
3. New Strategies: Since we know the "grip" is essential, scientists might be able to design new drugs that specifically jam that grip, or find ways to stop the cancer from hiding its RAD18.

In short: The researchers found the specific "handshake" RAD18 uses to fix DNA. They proved that breaking this handshake stops a promising cancer drug from working, and they discovered that cancer cells can cheat the system by simply turning off the protein that does the handshake. This knowledge helps us understand why some treatments work and how to stop cancer from fighting back.

Technical Summary

1. Problem Statement

The proliferating cell nuclear antigen (PCNA) is a central sliding clamp in DNA replication and repair. In response to DNA damage, PCNA is mono-ubiquitinated at lysine 164 (K164) by the E2-E3 ligase complex RAD6-RAD18. This modification recruits translesion synthesis (TLS) polymerases. However, the precise structural mechanism by which RAD18 engages PCNA to direct this mono-ubiquitination has remained poorly defined.

Furthermore, the deubiquitinating enzyme USP1 (with its cofactor UAF1) reverses PCNA ubiquitination. Inhibition of USP1 leads to the accumulation of mono-ubiquitinated PCNA and subsequent degradation of total PCNA, causing replication defects. This phenotype is synthetically lethal with BRCA1 deficiency, making USP1 inhibitors a promising therapeutic strategy for BRCA1-deficient breast and ovarian cancers. Despite this, the molecular determinants of response and resistance to USP1 inhibitors are not fully understood. Specifically, it is unclear how RAD18's interaction with PCNA drives the synthetic lethality observed upon USP1 inhibition.

2. Methodology

The authors employed a multidisciplinary approach combining structural biology, biochemistry, and cell biology:

• Structural Modeling & Prediction: Utilized AlphaFold (and AlphaFold3) to model the RAD18 SAP domain and the full RAD6-RAD18₂ complex to identify potential PCNA-binding interfaces.
• Nuclear Magnetic Resonance (NMR) Spectroscopy: Performed solution NMR titration experiments using $^{15}$N/$^{2}$H-labeled PCNA and unlabeled RAD18 peptides (spanning the SAP domain and specific PIP motifs). Chemical shift perturbations (CSPs) were mapped to identify binding surfaces and estimate binding affinity ($K_d$).
• Cell Line Engineering: Generated CRISPR-Cas9 knockout (KO) cell lines (OVCAR8, RPE-1, UWB1.289) for USP1 and RAD18. Created stable reconstitution lines expressing Wild-Type (WT) or mutant RAD18 (specifically the SAP domain PIP motif mutant "3A" and a C-terminal motif mutant).
• Biochemical Assays:
  • Immunoblotting: Analyzed levels of total PCNA, mono-ubiquitinated PCNA, and poly-ubiquitinated PCNA following UV-C irradiation or USP1 inhibitor treatment.
  • SIRF Assay (Proximity Ligation): Used in situ analysis to detect fork-associated PCNA and its ubiquitination status at replication forks.
  • DNA Fiber Assay: Quantified single-stranded DNA (ssDNA) gaps using S1 nuclease digestion to measure replication defects.
• Drug Resistance Models: Cultured BRCA1-deficient cells (UWB1.289) in progressively increasing concentrations of USP1 inhibitors (I-138 and KSQ-4279) to generate resistant clones, followed by proteomic analysis.
• Clonogenic Survival Assays: Measured cell viability and colony formation in response to USP1 inhibitors in various genetic backgrounds.

3. Key Contributions

• Identification of a Non-Consensus PIP Motif: The study identifies a previously unknown, non-canonical PCNA-interacting peptide (PIP) motif located within the SAP domain of RAD18 (residues K241-R254).
• Structural Characterization: Demonstrates that this SAP domain PIP motif binds to the "universal binding site" on PCNA, albeit with low affinity ($K_d > 1,000 \mu M$), which is distinct from the high-affinity binding of canonical PIP motifs found in polymerases.
• Mechanism of Synthetic Lethality: Establishes that the RAD18-PCNA interaction via this SAP PIP motif is the critical driver of USP1 inhibitor-induced synthetic lethality in BRCA1-deficient cells.
• Resistance Mechanism: Identifies that downregulation of RAD18 protein levels is a primary mechanism of acquired resistance to USP1 inhibitors.

4. Key Results

A. RAD18 Drives PCNA Turnover via Mono-ubiquitination

• Loss of USP1 leads to increased mono- and poly-ubiquitinated PCNA and a ~70% reduction in total PCNA levels.
• In USP1 KO cells, loss of RAD18 restores total PCNA levels and reduces ubiquitination, confirming that RAD18 promotes PCNA turnover (via mono-ubiquitination followed by poly-ubiquitination and degradation) which is normally suppressed by USP1.

B. The SAP Domain PIP Motif is Essential for Binding and Ubiquitination

• NMR titration confirmed that the RAD18 SAP domain peptide binds the universal binding site of PCNA.
• Mutating key hydrophobic residues in the SAP PIP motif (V247A, L250A, L251A; the "3A" mutant) abolished binding to PCNA in NMR assays.
• In cells, the 3A mutant failed to rescue UV-induced PCNA mono-ubiquitination in RAD18 KO cells, whereas WT RAD18 did.
• In USP1/RAD18 double KO cells, expression of WT RAD18 restored PCNA turnover (reduced total PCNA), but the 3A mutant had no effect.

C. The Interaction is Required for USP1 Inhibitor Sensitivity

• In BRCA1-deficient (UWB1.289) cells, RAD18 KO conferred resistance to USP1 inhibitors (I-138, KSQ-4279).
• Re-expression of WT RAD18 restored sensitivity, but the 3A mutant (defective in PCNA binding) did not.
• Mechanistic Link: USP1 inhibitors caused rapid accumulation of ssDNA gaps and loss of fork-associated PCNA in WT and WT-reconstituted cells. These defects were absent in cells expressing the 3A mutant, indicating that RAD18-mediated PCNA mono-ubiquitination is the upstream trigger for replication collapse and cell death.

D. RAD18 Downregulation as a Resistance Mechanism

• Cell lines adapted to long-term USP1 inhibitor treatment (resistant clones) exhibited significantly reduced RAD18 protein levels.
• Re-introducing WT RAD18 (but not the 3A mutant) into resistant clones partially restored sensitivity to the inhibitors, confirming that loss of RAD18 is a biologically relevant resistance mechanism.

5. Significance

• Structural Insight: This work defines the first structural interface (the SAP domain PIP motif) required for RAD18 to engage PCNA, resolving a long-standing gap in understanding how the RAD6-RAD18 complex functions.
• Therapeutic Implications: It establishes that the RAD18-PCNA interaction is the "Achilles' heel" in BRCA1-deficient cancers treated with USP1 inhibitors. Disrupting this interface (genetically or pharmacologically) suppresses the therapeutic efficacy.
• Biomarker Development: The findings suggest that RAD18 expression levels could serve as a biomarker for predicting response to USP1 inhibitors. Low RAD18 levels may indicate intrinsic or acquired resistance.
• Resistance Mechanisms: The discovery that cells adapt to USP1 inhibition by downregulating RAD18 provides a clear target for combination therapies (e.g., combining USP1 inhibitors with agents that prevent RAD18 downregulation) to overcome resistance in clinical settings.

In summary, the paper elucidates a critical, weak, transient interaction between RAD18 and PCNA that is essential for the synthetic lethality of USP1 inhibition in BRCA1-deficient cancers, while also revealing a novel mechanism of drug resistance.