ATR, short for ataxia-telangiectasia and Rad3-related, belongs to the PIKK family of giant kinases that also includes ATM, DNA-PKcs, mTOR and SMG1. Like its siblings, ATR is an enormous HEAT-repeat scaffold carrying a kinase domain buried deep within, and it operates as a dimer paired with its obligate partner ATRIP. Since the first cryo-EM structures of human ATR–ATRIP appeared in 2018, researchers have known the complex’s overall architecture but not the physical choreography of its activation. The central puzzle has been phosphorylation: ATR autophosphorylates itself, and this modification is considered a hallmark of the active kinase, yet nobody had seen structurally what phosphorylation does to the machine or how the activator protein TopBP1 drives the process. The new study addresses that gap directly by trapping the complex at successive stages of its catalytic cycle.

The team began with precatalytic snapshots. In two structures determined with the non-hydrolysable ATP analogue ATPγS wedged in the active site, they found that nucleotide binding alone accomplishes something remarkable: it primes the catalytic center for chemistry without wholesale rearrangement of the kinase domain. When the researchers then added a fragment of Chk1, the key downstream effector of the ATR pathway, the substrate docked neatly onto the primed enzyme. In other words, ATP binding and substrate positioning can proceed as relatively independent, modest steps rather than requiring a dramatic conformational detonation. This finding revises the textbook expectation that PIKK kinases need massive global movements to become catalytically competent, and it suggests that the rate-limiting event in ATR activation lies elsewhere — specifically, in the regulatory rearrangements triggered by TopBP1.

That TopBP1-dependent rearrangement is the structural centerpiece of the paper. In the activated structure, two molecules of TopBP1 bind simultaneously to the ATR–ATRIP dimer, one engaging each protomer from opposite faces. TopBP1 latches onto the HEAT-repeat scaffold far from the active site, and in doing so it locks the N-HEAT and M-HEAT domains of ATR together in both protomers. In the precatalytic and substrate-bound states, these scaffold elements make intimate contact in one protomer while hovering apart in the other — an inherent asymmetry. TopBP1 binding abolishes that asymmetry, snapping the dimer into a near-symmetric conformation that promotes ATR–ATRIP autophosphorylation and drives efficient phosphorylation of Chk1. Detailed atomic contacts reveal how this is achieved: an intensive network of hydrogen bonds welds a TopBP1 beta strand to the M-HEAT domain, the tryptophan residue W1145 of TopBP1 buries into a hydrophobic pocket at the N-HEAT/M-HEAT junction, and salt-bridge interactions fasten a TopBP1 helix onto the N-HEAT superhelix.

Perhaps the most provocative observation concerns what happens after activation. The researchers purified ATR–ATRIP complexes that had been fully autophosphorylated in the presence of TopBP1, then stripped the activator away and determined the structure of the phosphorylated kinase on its own. Remarkably, the phosphorylated complex remained fully competent to phosphorylate Chk1, and it did so regardless of how much TopBP1 was added back. Structurally, the released enzyme adopted an asymmetric conformation, yet its kinase domain was essentially indistinguishable from that of the TopBP1-bound, near-symmetric activated state. The authors conclude that catalytic activity is compatible with both near-symmetric and asymmetric architectures, meaning that phosphorylation — not a rigid, TopBP1-imposed symmetry — is the true durable switch. Once ATR has phosphorylated itself, the checkpoint engine keeps running even if its ignition key is removed, a property with clear implications for how sustained checkpoint signaling is maintained at damaged replication forks.

The autophosphorylation story was reinforced biochemically. Mass spectrometry mapped dozens of newly phosphorylated sites on ATR and ATRIP, and phosphatase treatment confirmed that the mobility shifts seen in gels were genuinely phosphorylation-dependent. Activity assays showed a clean dose-response: non-phosphorylated ATR–ATRIP phosphorylated Chk1 only when TopBP1 was titrated in, whereas the pre-phosphorylated complex catalyzed Chk1 modification at TopBP1 concentrations approaching zero. Interestingly, an isolated ATR head domain displayed only basal activity with or without TopBP1, underscoring that the full dimeric scaffold — and its TopBP1-driven reorganization — is required to reach full catalytic power. Together these experiments elevate ATR autophosphorylation from a correlative biomarker to a mechanistically demonstrated molecular switch for checkpoint activation.

The paper’s second act concerns therapy. ATR inhibitors such as berzosertib (VX-970/M6620), ceralasertib (AZD6738), gartisertib (VX-803/M4344) and elimusertib (BAY 1895344) are in clinical development as sensitizers that push tumor cells with pre-existing DNA repair defects over the edge. The team determined four separate inhibitor-bound structures of ATR–ATRIP, one for each compound, providing the first side-by-side atomic views of how this pharmacological class occupies the ATR active site. The nucleotide-binding pocket turns out to be divisible into four sub-compartments: an adenine-binding pocket lined by residues including M2325, Y2365, I2377, W2379 and V2380; an inner hydrophobic pocket; a ribose-binding site; and a triphosphate-binding site. All four inhibitors anchor themselves in the adenine pocket, but each then exploits the pocket’s flexibility differently, extending flanking chemical groups into the hydrophobic cavity, the ribose site, or the triphosphate region.

These structures explain both potency and selectivity. ATR’s adenine and hydrophobic sub-pockets are shifted toward the C-lobe relative to the corresponding sites in DNA-PKcs, mTOR and ATM, a displacement caused by the bulky side chains of M2325 and V2493 in ATR. Inhibitors can therefore nestle into an ATR-specific geometry that the closely related PIKK kinases cannot accommodate. Superimposition with structures of DNA-PKcs, mTOR and ATM further showed that a key hydrogen bond formed by berzosertib with glycine 2385 of ATR would be sterically blocked by the larger residues occupying the equivalent positions in the sibling kinases. In effect, the drugs exploit the conformational plasticity of the ATR active site — the very same flexibility that allows ATPγS to prime the kinase — to achieve multipocket engagement that favors ATR over its family members. For medicinal chemists, the four structures amount to a comparative map of which sub-pockets each clinical candidate touches, a starting grid for designing next-generation inhibitors with improved selectivity and resistance profiles.

Taken together, the study delivers a structural framework in which ATR regulation can finally be read as a sequence of physically defined events. ATP binding primes the catalytic cleft and positions Chk1; TopBP1 binding to the HEAT scaffold reorganizes the dimer and licenses autophosphorylation; phosphorylation converts the enzyme into a durably active state that survives TopBP1 dissociation; and small molecules shut the whole system down by wedging into the plastic ATP pocket. The work also resonates with parallel findings on ATM, where asymmetric activation of the dimeric kinase has recently been visualized, hinting at shared regulatory logic across the PIKK family. For cancer biology, the implications are tangible: tumors rely obsessively on ATR to survive their own replication chaos, and a structure-level understanding of both the activation switch and the drug-binding pocket should accelerate the design of inhibitors that hit ATR harder and more selectively. As ATR inhibitors advance through clinical trials, the molecular movie assembled by Wang, Cheng, Qiao and colleagues provides the kind of mechanistic blueprint on which the next generation of checkpoint-targeted therapies can be drawn.

Subject of Research: Cryo-EM structural mechanisms of phosphorylation-driven activation and inhibitor binding of the human ATR–ATRIP checkpoint kinase complex

Article Title: Mechanistic insights into phosphorylation-driven activation and therapeutic inhibition of human ATR–ATRIP

Article References: Wang, L., Wang, M., Zhao, L., Rao, Q., Wu, H., Ma, B., Wang, J., Zheng, J., Li, Y., Xu, Y., Guo, J., Cheng, J., & Qiao, S. (2026). Mechanistic insights into phosphorylation-driven activation and therapeutic inhibition of human ATR–ATRIP. Nature Structural & Molecular Biology. https://doi.org/10.1038/s41594-026-01887-4

Image Credits: AI Generated

DOI: 10.1038/s41594-026-01887-4

Keywords: ATR, ATRIP, TopBP1, Chk1, cryo-electron microscopy, DNA damage response, replication stress, autophosphorylation, kinase activation, ATR inhibitors, berzosertib, cancer therapy

The new study, published in the British Journal of Cancer, focuses on a kinase called ATR, short for ataxia telangiectasia and Rad3-related protein. ATR sits at the heart of the cellular response to replication stress, the potentially lethal situation in which the molecular machinery that copies DNA stalls or breaks down. When chemotherapy drugs such as mitomycin C damage DNA, dividing cells rely heavily on ATR signalling to pause the cell cycle, stabilise stalled replication forks and coordinate repair. Block ATR pharmacologically, and cells exposed to DNA-damaging agents lose their safety net: replication forks collapse, DNA double-strand breaks accumulate, and the cell is pushed toward catastrophe. This concept, often described as exploiting a vulnerability created by the tumour’s own dependence on DNA damage checkpoints, has already shown promise in clinical trials of ATR inhibitors such as berzosertib in combination with platinum chemotherapy for advanced solid tumours.

What makes the Utrecht study distinctive is its model system. Rather than relying on immortalised cancer cell lines grown in two dimensions, which often fail to capture the biology of real tumours, the team used patient-derived organoids, miniature three-dimensional tumour cultures grown from tissue of six patients with non-muscle invasive bladder cancer. Organoids preserve many of the genetic and molecular features of the original tumours, including the expression of urothelial carcinoma markers, making them a far more faithful testing ground for new drug combinations. The researchers confirmed that their organoid lines expressed characteristic bladder cancer markers, validating them as genuine representatives of the disease they were designed to model.

The experimental design cleverly mimicked clinical practice. In patients, mitomycin C is delivered directly into the bladder as an instillation that remains in contact with the tumour tissue for roughly one to two hours before being drained. The researchers therefore exposed the organoids to mitomycin C for just two hours, replicating the transient exposure that tumour cells experience in the bladder, and only afterwards did they add ATR inhibitors, which the cells encountered for a prolonged 72-hour period. Three clinically relevant ATR inhibitors were tested: berzosertib, ceralasertib and tuvusertib, all of which have entered clinical trials in various cancers. The team also examined combinations with gemcitabine and epirubicin, two further agents used in intravesical chemotherapy regimens, in one organoid line.

The results were striking. Organoids treated with mitomycin C alone, or with an ATR inhibitor alone, eventually recovered: when the researchers followed the cultures for six weeks after treatment, the surviving cells proliferated at rates similar to untreated controls, demonstrating that neither agent on its own could eliminate the tumour cell population. In sharp contrast, organoids that received the sequential combination of mitomycin C followed by an ATR inhibitor showed severely impaired viability, and crucially, this effect persisted throughout the six-week observation period. The combination did not merely slow the cancer cells down; it appeared to destroy their capacity to regrow, which is precisely the property needed for a therapy intended to prevent recurrence after tumour resection.

Delving into the mechanism, the researchers showed that berzosertib potently suppressed the ATR signalling that mitomycin C normally triggers. DNA damage induced by the chemotherapy was marked by phosphorylated H2AX, a well-established molecular beacon of DNA double-strand breaks, and blocking ATR prevented the checkpoint response that would normally allow cells to survive this damage. Consistent with catastrophic, irreparable DNA damage, the combination treatment drove the organoid cells into apoptosis, the controlled programme of cell death. Quantitative analysis of the drug interaction using synergy scoring frameworks confirmed that the effect was genuinely synergistic rather than merely additive, meaning the two drugs together killed far more cells than would be predicted from their individual activities.

The implications for patients are considerable. Recurrence after intravesical therapy remains the central clinical challenge in non-muscle invasive bladder cancer, driving repeated surgeries, lifelong surveillance and, in a substantial minority of cases, progression to life-threatening muscle-invasive disease. The economic burden of bladder cancer across Europe is among the highest of any malignancy, largely because of the intensity of monitoring and repeat treatment that recurrence entails. A regimen that converts transient chemotherapy exposure into durable eradication of residual tumour cells could reduce recurrence rates, spare patients repeated interventions and delay or prevent progression. Because ATR inhibitors such as berzosertib, ceralasertib and tuvusertib are already in clinical development, the path from laboratory finding to clinical testing is shorter than for an entirely novel drug class.

There are important caveats. The study is preclinical, conducted in organoids rather than in patients, and although organoids are among the most clinically predictive laboratory models available, they cannot fully reproduce the immune system, the bladder wall architecture or the complex urine environment that shapes drug activity in vivo. The number of organoid lines tested, six for the mitomycin C combinations, is modest, and the gemcitabine and epirubicin experiments were limited to a single line, so the generality of the synergy across the molecular diversity of bladder cancer remains to be established. Questions also remain about the optimal sequencing, dosing and delivery of ATR inhibitors in the bladder, and about whether systemic administration would be needed or whether the inhibitors could themselves be delivered intravesically to limit side effects.

Nevertheless, the study provides a compelling proof of principle that the DNA damage response is a druggable Achilles heel of non-muscle invasive bladder cancer, and it establishes patient-derived organoids as a practical platform for optimising intravesical combination therapies before they are tested in the clinic. The findings build on a growing body of evidence that ATR inhibition sensitises bladder tumours to DNA-targeted agents, including earlier work showing enhanced cisplatin and gemcitabine activity in bladder cancer cell lines and clinical trial data combining berzosertib with platinum chemotherapy in advanced urothelial carcinoma. If the synergy observed in these miniature tumours translates to patients, the humble bladder instillation, a treatment whose basic design has changed little in decades, could be transformed into a precision strike that leaves behind not just damaged cancer cells, but none at all.

Subject of Research: Combining ATR kinase inhibitors with intravesical chemotherapy to prevent recurrence in non-muscle invasive bladder cancer, tested in patient-derived organoids.

Article Title: ATR inhibitors synergise with mitomycin C to enhance cytotoxicity in patient-derived non-muscle invasive bladder cancer organoids

Article References: Zuidema, A., Nijland, L., van Megesen, K., Vosjan, M. M., Viergever, B. J., Kranenburg, O., & Meijer, R. P. (2026). ATR inhibitors synergise with mitomycin C to enhance cytotoxicity in patient-derived non-muscle invasive bladder cancer organoids. British Journal of Cancer. https://doi.org/10.1038/s41416-026-03581-0

Image Credits: AI Generated

DOI: 10.1038/s41416-026-03581-0

Keywords: bladder cancer, ATR inhibitors, mitomycin C, patient-derived organoids, DNA damage response, non-muscle invasive bladder cancer, berzosertib, ceralasertib, tuvusertib, intravesical chemotherapy, drug synergy, cancer recurrence