Sebastian Guettler

The β-catenin destruction complex (BDC) regulates WNT-β-catenin signaling and is a prime therapeutic target in colorectal cancer, yet its biochemical complexity has hindered mechanistic understanding. We mapped the sequence-function landscape of the BDC using tiled base editor screens across its components CTNNB1, AXIN1, APC and GSK3B. Amongst ~150 previously unreported mutations that affected WNT signaling, we discovered gain-of-function and separation-of-function alleles that reveal mechanisms of complex assembly, including a β-catenin region regulating TCF/LEF transcription factor binding. Critically, we found that the AXIN1-β-catenin interface controls signaling flux through the oncogenic BDC found in APC-mutant cancers. In cells expressing truncated APC, β-catenin itself scaffolds BDC assembly, establishing a substrate-assisted autoregulatory mechanism. This architecture represents an unexploited therapeutic vulnerability: strengthening the AXIN1-β-catenin interaction restores destruction complex function and impairs the growth of colorectal cancer cells. Our mutational resource provides a foundation for mechanistic understanding and therapeutic targeting of the WNT pathway.

TRF1 is a subunit of the shelterin complex that binds to and protects the linear ends of chromosomes known as telomeres. Both genetic deletion and chemical inhibition of TRF1 have been shown to block the growth of lung carcinoma, glioblastoma, and renal cell carcinoma in mice without affecting mouse survival or tissue function, making TRF1 a potential therapeutic target in cancer ^1–3 . Here, we report the discovery of a series of fragment hits that bind at the interface between the TRFH domain of TRF1 (TRF1 _TRFH ) and a peptide of TIN2 (TIN2 _TBM ), an interaction essential for the recruitment of TRF1 to shelterin, using X-ray crystallography (XChem) and ligand-observed NMR (LO-NMR) fragment screening. We discovered a first-in-class inhibitor of the TRF1:TIN2 interaction (compound 40 ) that binds to TRF1 _TRFH with a K _D of 29 µM (95% CI: 20–41 µM), displaces a TIN2 probe with an IC ₅₀ of 67 µM (95% CI: 10–120 µM), and expels TRF1 from purified shelterin. Aided by a novel crystal system of TRF1 _TRFH , we characterised fragments binding in a hotspot at the TRF1:TIN2 interface; these will serve as a starting point for the structure-guided development of potent inhibitors of TRF1 protein:protein interactions to disrupt shelterin complex assembly.

The β-catenin destruction complex (BDC) is a central node in WNT/β-catenin signaling, governing embryonic development and adult tissue homeostasis. Although recognized as a prime therapeutic target in colorectal cancer (CRC) for three decades, its dynamic architecture and biochemical complexity have hindered mechanistic understanding. Here, we systematically mapped the sequence-function landscape of the BDC using tiled base editor screens across four endogenous components— CTNNB1 , AXIN1 , APC , and GSK3B . Validation studies identified ∼150 previously unreported mutations across these genes that affected WNT/β-catenin signaling. In addition to known cancer-associated mutations, we discovered rare gain-of-function and separation-of-function alleles of AXIN1 and CTNNB1 that provide mechanistic insights into complex assembly and regulation. We describe a region in β-catenin that regulates its binding to TCF/LEF transcription factors and demonstrate that the AXIN1–β-catenin interface is critical for controlling signaling flux through the oncogenic BDC. Mechanistic studies revealed that assembly of the oncogenic BDC is scaffolded by its own substrate β-catenin, establishing an autoregulatory mechanism that represents an unexploited vulnerability in cancers harboring common APC truncations. Our comprehensive mutational resource provides a foundation for understanding WNT/β-catenin signaling mechanisms in health and disease, while revealing strategies for therapeutic intervention in WNT-driven cancers.

The ADP-ribosyltransferase tankyrase (with two paralogues, TNKS and TNKS2) plays pivotal roles in diverse cellular processes that encompass signal transduction, including Wnt/β-catenin, Hippo and toll-like receptor (TLR) signalling, mitotic spindle assembly, glucose homeostasis and telomere maintenance, among many other functions. Tankyrase recruits its effectors (substrates and binders) via a degenerate tankyrase-binding motif (TBM) and exerts its activities by subsequent substrate ADP-ribosylation and/or scaffolding. Variants of the TBM, found in diverse proteins, engage the ankyrin repeat cluster (ARC) domains of tankyrase. Yet, whether effector recruitment to tankyrase can be regulated has remained unknown. In this study, we propose that phosphorylation at position eight of the TBM enhances the affinity of effectors for the ARC domains of tankyrase. Using isolated TBM peptides, we demonstrate that phosphorylation of serine, but not tyrosine, strengthens ARC binding by up to an order of magnitude. Interrogation of proteome-wide phosphorylation data reveals that phosphorylation at position eight in the TBM is enriched in proteins that support centrosome function/localization. Our findings suggest that TBM phosphorylation may serve as an effector-specific mechanism for tankyrase recruitment/retention, providing an additional layer of regulation to control tankyrase.

During classical non-homologous end joining (cNHEJ), DNA-dependent protein kinase (DNA-PK) encapsulates free DNA ends, forming a recruitment platform for downstream end-joining factors including ligase 4 (LIG4)¹. DNA-PK can also bind telomeres and regulate their resection^2–4, but does not initiate cNHEJ at this position. How the end-joining process is regulated in this context-specific manner is currently unclear. Here we show that the shelterin components TRF2 and RAP1 form a complex with DNA-PK that directly represses its end-joining function at telomeres. Biochemical experiments and cryo-electron microscopy reveal that when bound to TRF2, RAP1 establishes a network of interactions with KU and DNA that prevents DNA-PK from recruiting LIG4. In mouse and human cells, RAP1 is redundant with the Apollo nuclease in repressing cNHEJ at chromosome ends, demonstrating that the inhibition of DNA-PK prevents telomere fusions in parallel with overhang-dependent mechanisms. Our experiments show that the end-joining function of DNA-PK is directly and specifically repressed at telomeres, establishing a molecular mechanism for how individual linear chromosomes are maintained in mammalian cells.

TRF1 is a subunit of the shelterin complex that binds to and protects the linear ends of chromosomes known as telomeres. Both genetic deletion and chemical inhibition of TRF1 have been shown to block the growth of lung carcinoma, glioblastoma, and renal cell carcinoma in mice without affecting mouse survival or tissue function, making TRF1 a potential therapeutic target in cancer ^1–3 . Here, we report the discovery of a series of fragment hits that bind at the interface between the TRFH domain of TRF1 (TRF1 _TRFH ) and a peptide of TIN2 (TIN2 _TBM ), an interaction essential for the recruitment of TRF1 to shelterin, using X-ray crystallography (XChem) and ligand-observed NMR (LO-NMR) fragment screening. We discovered a first-in-class inhibitor of the TRF1-TIN2 interaction (compound 40 ) that binds to TRF1 _TRFH with a K _D of 29 μM (95% CI: 20 – 41 μM), displaces a TIN2 probe with an IC ₅₀ of 67 ± 28 μM, and expels TRF1 from purified shelterin. Aided by a novel crystal system of TRF1 _TRFH , we characterised fragments binding in a hotspot at the TRF1-TIN2 interface which will serve as a starting point for the structure-guided development of potent inhibitors of TRF1 protein-protein interactions to disrupt shelterin complex assembly.

ADP-ribosylation is a prominent and versatile post-translational modification, which regulates a diverse set of cellular processes. Poly-ADP-ribose (PAR) is synthesised by the poly-ADP-ribosyltransferases PARP1, PARP2, tankyrase (TNKS), and tankyrase 2 (TNKS2), all of which are linked to human disease. PARP1/2 inhibitors have entered the clinic to target cancers with deficiencies in DNA damage repair. Conversely, tankyrase inhibitors have continued to face obstacles on their way to clinical use, largely owing to our limited knowledge of their molecular impacts on tankyrase and effector pathways, and linked concerns around their tolerability. Whilst detailed structure-function studies have revealed a comprehensive picture of PARP1/2 regulation, our mechanistic understanding of the tankyrases lags behind, and thereby our appreciation of the molecular consequences of tankyrase inhibition. Despite large differences in their architecture and cellular contexts, recent structure-function work has revealed striking parallels in the regulatory principles that govern these enzymes. This includes low basal activity, activation by intra- or inter-molecular assembly, negative feedback regulation by auto-PARylation, and allosteric communication. Here we compare these poly-ADP-ribosyltransferases and point towards emerging parallels and open questions, whose pursuit will inform future drug development efforts.

The poly-ADP-ribosyltransferase tankyrase (TNKS, TNKS2) controls a wide range of disease-relevant cellular processes, including WNT–β-catenin signalling, telomere length maintenance, Hippo signalling, DNA damage repair and glucose homeostasis^1,2. This has incentivized the development of tankyrase inhibitors. Notwithstanding, our knowledge of the mechanisms that control tankyrase activity has remained limited. Both catalytic and non-catalytic functions of tankyrase depend on its filamentous polymerization^3–5. Here we report the cryo-electron microscopy reconstruction of a filament formed by a minimal active unit of tankyrase, comprising the polymerizing sterile alpha motif (SAM) domain and its adjacent catalytic domain. The SAM domain forms a novel antiparallel double helix, positioning the protruding catalytic domains for recurring head-to-head and tail-to-tail interactions. The head interactions are highly conserved among tankyrases and induce an allosteric switch in the active site within the catalytic domain to promote catalysis. Although the tail interactions have a limited effect on catalysis, they are essential to tankyrase function in WNT–β-catenin signalling. This work reveals a novel SAM domain polymerization mode, illustrates how supramolecular assembly controls catalytic and non-catalytic functions, provides important structural insights into the regulation of a non-DNA-dependent poly-ADP-ribosyltransferase and will guide future efforts to modulate tankyrase and decipher its contribution to disease mechanisms.

This animated movie presents the mechanism of macroautophagy, hereafter autophagy, by showing the molecular features of the formation of autophagosomes, the hallmark organelle of this intracellular catabolic pathway. It is based on our current knowledge and it also illustrates how autophagosomes can recognize and eliminate selected cargoes.