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Obtaining protein crystals suitable for characterizing ligands and understanding different protein conformations remains a bottleneck for structural studies. Using nanobodies as crystallization chaperones is one strategy to address the problem, but its reliability is uncharacterized and in this study, we observed it to have a poor success rate. We therefore set out to engineer robust crystallization behaviour into the nanobody scaffold by exploring the nanobody-nanobody interface predominant in the PDB using >200 combinations of surface mutations. This yielded multiple polymorphs, all mediated by this interface, with one providing far superior resolution and reliability of diffraction. We therefore refer to the modified nanobody scaffolds as “Gluebodies” and propose at least 2 variants that should be routinely generated for chaperone experiments. We furthermore show that Gluedodies cannot rescue intrinsically non-crystallizing proteins, but are a powerful approach to improve the packing and resolution limit of poorly diffracting crystals.

Forkhead box N1 (FOXN1) is a member of the forkhead box family of transcription factors and plays an important role in thymic epithelial cell differentiation and development. FOXN1 mutations in humans and mice give rise to the “nude” phenotype, which is marked by athymia. FOXN1 belongs to a subset of the FOX family that recognizes an alternative forkhead-like (FHL) consensus sequence (GACGC) that is different from the more widely recognized forkhead (FKH) sequence RYAAAYA (where R is purine, and Y is pyrimidine). Here, we present the FOXN1 structure in complex with DNA containing an FHL motif at 1.6 Å resolution, in which the DNA sequence is recognized by a mixture of direct and water-mediated contacts provided by residues in an α-helix inserted in the DNA major groove (the recognition helix). Comparisons with the structure of other FOX family members revealed that the FKH and FHL DNA sequences are bound in two distinct modes, with partially different registers for the protein DNA contacts. We identified a single alternative rotamer within the recognition helix itself as an important determinant of DNA specificity and found protein sequence features in the recognition helix that could be used to predict the specificity of other FOX family members. Finally, we demonstrate that the C-terminal region of FOXN1 is required for high-affinity DNA binding and that FOXN1 has a significantly reduced affinity for DNA that contains 5′-methylcytosine, which may have implications for the role of FOXN1 in thymic involution. ISSN:0021-9258 ISSN:1083-351X

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common reason for chronic kidney disease in children. Although more than 30 monogenic causes have been implicated in isolated forms of human CAKUT so far, the vast majority remains elusive. To identify novel monogenic causes of CAKUT we applied homozygosity mapping, together with whole exome sequencing, in a patient from consanguineous descent with isolated CAKUT. We identified a homozygous missense mutation (p.Arg415His) of the Ets Translocation Variant Gene 4 (ETV4). The transcription factor ETV4 is a downstream target of the GDNF/RET signaling pathway that plays a crucial role in kidney development. We show by means of electrophoretic mobility shift assay that the Arg415His mutant causes loss of the DNA binding affinity of ETV4 and fails to activate transcription in a cell-based luciferase reporter assay. We furthermore investigated the impact of the mutant protein on cell migration rate. Unlike wildtype ETV4, the Arg415His mutant failed to rescue cell migration defects observed in two ETV4 knock-down cell-lines. We therefore identified and functionally characterized a recessive mutation in ETV4 in a human patient with CAKUT. We hypothesize that the pathomechanism of this mutation could be via loss of the transcriptional function of ETV4, and a resulting abrogation of GDNF/RET/ETV4 signaling pathway.

Background: Transcription factors of the Ets family regulate development and cancer. Results: We show the details of DNA sequence recognition and identify structural changes that drastically affect DNA binding. Conclusion: The activity of Ets transcription factors is regulated by their oxidation state. Significance: Ets transcription factors may control the response of cells to oxygen levels., Ets transcription factors, which share the conserved Ets DNA-binding domain, number nearly 30 members in humans and are particularly involved in developmental processes. Their deregulation following changes in expression, transcriptional activity, or by chromosomal translocation plays a critical role in carcinogenesis. Ets DNA binding, selectivity, and regulation have been extensively studied; however, questions still arise regarding binding specificity outside the core GGA recognition sequence and the mode of action of Ets post-translational modifications. Here, we report the crystal structures of Etv1, Etv4, Etv5, and Fev, alone and in complex with DNA. We identify previously unrecognized features of the protein-DNA interface. Interactions with the DNA backbone account for most of the binding affinity. We describe a highly coordinated network of water molecules acting in base selection upstream of the GGAA core and the structural features that may account for discrimination against methylated cytidine residues. Unexpectedly, all proteins crystallized as disulfide-linked dimers, exhibiting a novel interface (distant to the DNA recognition helix). Homodimers of Etv1, Etv4, and Etv5 could be reduced to monomers, leading to a 40–200-fold increase in DNA binding affinity. Hence, we present the first indication of a redox-dependent regulatory mechanism that may control the activity of this subset of oncogenic Ets transcription factors.

Summary The cytotoxicity of DNA-protein crosslinks (DPCs) is largely ascribed to their ability to block the progression of DNA replication. DPCs frequently occur in cells, either as a consequence of metabolism or exogenous agents, but the mechanism of DPC repair is not completely understood. Here, we characterize SPRTN as a specialized DNA-dependent and DNA replication-coupled metalloprotease for DPC repair. SPRTN cleaves various DNA binding substrates during S-phase progression and thus protects proliferative cells from DPC toxicity. Ruijs-Aalfs syndrome (RJALS) patient cells with monogenic and biallelic mutations in SPRTN are hypersensitive to DPC-inducing agents due to a defect in DNA replication fork progression and the inability to eliminate DPCs. We propose that SPRTN protease represents a specialized DNA replication-coupled DPC repair pathway essential for DNA replication progression and genome stability. Defective SPRTN-dependent clearance of DPCs is the molecular mechanism underlying RJALS, and DPCs are contributing to accelerated aging and cancer., Graphical Abstract, Highlights • DNA-protein crosslinks (DPCs) stall DNA replication and induce genomic instability • SPARTAN (SPRTN) is a DNA replication-coupled metalloprotease which proteolyses DPCs • SPRTN metalloprotease is a fundamental enzyme in DPC repair pathway • Ruijs-Aalfs syndrome is caused by a defect in DPC repair due to mutations in SPRTN, Monogenic mutations in SPRTN cause genomic instability, premature aging, and hepatocellular carcinoma. The molecular mechanism of how SPRTN protects genome stability and prevents accelerated aging and cancer is not clear. Vaz, Popovic, et al. show that SPRTN is a DNA replication-coupled metalloprotease for DNA-protein crosslink repair in proliferative human cells.

The MAGE (melanoma associated antigen) protein family are tumour-associated proteins normally present only in reproductive tissues such as germ cells of the testis. The human genome encodes over 60 MAGE genes of which one class (containing MAGE-A3 and MAGE-A4) are exclusively expressed in tumours, making them an attractive target for the development of targeted and immunotherapeutic cancer treatments. Some MAGE proteins are thought to play an active role in driving cancer, modulating the activity of E3 ubiquitin ligases on targets related to apoptosis. Here we determined the crystal structures of MAGE- A3 and MAGE-A4. Both proteins crystallized with a terminal peptide bound in a deep cleft between two tandem-arranged winged helix domains. MAGE-A3 (but not MAGE-A4), is pre- dominantly dimeric in solution. Comparison of MAGE-A3 and MAGE-A3 with a structure of an effector-bound MAGE-G1 suggests that a major conformational rearrangement is required for binding, and that this conformational plasticity may be targeted by allosteric binders.

Summary DNA polymerase theta (Polθ) has been identified as a crucial alternative non-homologous end-joining factor in mammalian cells. Polθ is upregulated in a range of cancer cell types defective in homologous recombination, and knockdown has been shown to inhibit cell survival in a subset of these, making it an attractive target for cancer treatment. We present crystal structures of the helicase domain of human Polθ in the presence and absence of bound nucleotides, and a characterization of its DNA-binding and DNA-stimulated ATPase activities. Comparisons with related helicases from the Hel308 family identify several unique features. Polθ exists as a tetramer both in the crystals and in solution. We propose a model for DNA binding to the Polθ helicase domain in the context of the Polθ tetramer, which suggests a role for the helicase domain in strand annealing of DNA templates for subsequent processing by the polymerase domain., Graphical Abstract, Highlights • DNA polymerase theta (Polθ) is unique in having a DNA helicase-like domain • Polθ has a role in the repair of DNA double-strand breaks and in cancer cell survival • We solved the structure of the tetrameric helicase-like domain • The tetramer may align broken DNA ends for microhomology-mediated end joining, We describe a structure of an unusual helicase domain of DNA polymerase theta (Polθ). The helicase-like domain of Polθ forms tetramers and this tetrameric structure might bring together broken DNA ends for microhomology-mediated end joining.

Background: ETS transcription factors regulate expression of genes involved in development and cancer. Results: Crystal structure of the Ets-2 DNA binding domain reveals how closely spaced binding sites are recognized. Conclusion: A significant difference in binding regulation is observed between the closely related Ets-1 and Ets-2 transcription factors. Significance: This may explain the different physiological roles of different Ets proteins., Ets-2, like its closely related homologue Ets-1, is a member of the Ets family of DNA binding transcription factors. Both proteins are subject to multiple levels of regulation of their DNA binding and transactivation properties. One such regulatory mechanism is the presence of an autoinhibitory module, which in Ets-1 allosterically inhibits the DNA binding activity. This inhibition can be relieved by interaction with protein partners or cooperative binding to closely separated Ets binding sites in a palindromic arrangement. In this study we describe the 2.5 Å resolution crystal structure of a DNA complex of the Ets-2 Ets domain. The Ets domain crystallized with two distinct species in the asymmetric unit, which closely resemble the autoinhibited and DNA bound forms of Ets-1. This discovery prompted us to re-evaluate the current model for the autoinhibitory mechanism and the structural basis for cooperative DNA binding. In contrast to Ets-1, in which the autoinhibition is caused by a combination of allosteric and steric mechanisms, we were unable to find clear evidence for the allosteric mechanism in Ets-2. We also demonstrated two possibly distinct types of cooperative binding to substrates with Ets binding motifs separated by four and six base pairs and suggest possible molecular mechanisms for this behavior.

RING ubiquitin ligases (E3) recruit ubiquitin-conjugate enzymes (E2) charged with ubiquitin (Ub) to catalyze ubiquitination. Non-covalent Ub binding to the backside of certain E2s promotes processive polyUb formation, but the mechanism remains elusive. Here, we show that backside bound Ub (Ub(B)) enhances both RING-independent and RING-dependent UbcH5B-catalyzed donor Ub (Ub(D)) transfer, but with a more prominent effect in RING-dependent transfer. Ub(B) enhances RING E3s' affinities for UbcH5B-Ub, and RING E3-UbcH5B-Ub complex improves Ub(B)'s affinity for UbcH5B. A comparison of the crystal structures of a RING E3, RNF38, bound to UbcH5B-Ub in the absence and presence of Ub(B), together with molecular dynamics simulation and biochemical analyses, suggests Ub(B) restricts the flexibility of UbcH5B's α1 and α1β1 loop. Ub(B) supports E3 function by stabilizing the RING E3-UbcH5B-Ub complex, thereby improving the catalytic efficiency of Ub transfer. Thus, Ub(B) serves as an allosteric activator of RING E3-mediated Ub transfer.