The voltage-gated potassium channel family (Kv) constitutes the most diverse class of ion channels in the nervous system. Dipeptidyl peptidase 10 (DPP10) is an inactive peptidase that modulates the electrophysiological properties, cell-surface expression and subcellular localization of voltage-gated potassium channels. As a consequence, DPP10 malfunctioning is associated with neurodegenerative conditions like Alzheimer and fronto-temporal dementia, making this protein an attractive drug target. In this work, we report the crystal structure of DPP10 and compare it to that of DPP6 and DPP4. DPP10 belongs to the S9B serine protease subfamily and contains two domains with two distinct folds: a β-propeller and a classical α/β-hydrolase fold. The catalytic serine, however, is replaced by a glycine, rendering the protein enzymatically inactive. Difference in the entrance channels to the active sites between DPP10 and DPP4 provide an additional rationale for the lack of activity. We also characterize the DPP10 dimer interface focusing on the alternative approach for designing drugs able to target protein-protein interactions.

Polycomb repressive complex 2 (PRC2) is an important regulator of cellular differentiation and cell type identity. Overexpression or activating mutations of EZH2, the catalytic component of the PRC2 complex, are linked to hyper-trimethylation of lysine 27 of histone H3 (H3K27me3) in many cancers. Potent EZH2 inhibitors that reduce levels of H3K27me3 kill mutant lymphoma cells and are efficacious in a mouse xenograft model of malignant rhabdoid tumors. Unlike most SET domain methyltransferases, EZH2 requires PRC2 components, SUZ12 and EED, for activity, but the mechanism by which catalysis is promoted in the PRC2 complex is unknown. We solved the 2.0 Å crystal structure of the EZH2 methyltransferase domain revealing that most of the canonical structural features of SET domain methyltransferase structures are conserved. The site of methyl transfer is in a catalytically competent state, and the structure clarifies the structural mechanism underlying oncogenic hyper-trimethylation of H3K27 in tumors harboring mutations at Y641 or A677. On the other hand, the I-SET and post-SET domains occupy atypical positions relative to the core SET domain resulting in incomplete formation of the cofactor binding site and occlusion of the substrate binding groove. A novel CXC domain N-terminal to the SET domain may contribute to the apparent inactive conformation. We propose that protein interactions within the PRC2 complex modulate the trajectory of the post-SET and I-SET domains of EZH2 in favor of a catalytically competent conformation.

Aberrant isoform expression of chromatin-associated proteins can induce epigenetic programs related to disease. The MDS1 and EVI1 complex locus (MECOM) encodes PRDM3, a protein with an N-terminal PR-SET domain, as well as a shorter isoform, EVI1, lacking the N-terminus containing the PR-SET domain (ΔPR). Imbalanced expression of MECOM isoforms is observed in multiple malignancies, implicating EVI1 as an oncogene, while PRDM3 has been suggested to function as a tumor suppressor through an unknown mechanism. To elucidate functional characteristics of these N-terminal residues, we compared the protein interactomes of the full-length and ΔPR isoforms of PRDM3 and its closely related paralog, PRDM16. Unlike the ΔPR isoforms, both full-length isoforms exhibited a significantly enriched association with components of the NuRD chromatin remodeling complex, especially RBBP4. Typically, RBBP4 facilitates chromatin association of the NuRD complex by binding to histone H3 tails. We show that RBBP4 binds to the N-terminal amino acid residues of PRDM3 and PRDM16, with a dissociation constant of 3.0 μM, as measured by isothermal titration calorimetry. Furthermore, high-resolution X-ray crystal structures of PRDM3 and PRDM16 N-terminal peptides in complex with RBBP4 revealed binding to RBBP4 within the conserved histone H3-binding groove. These data support a mechanism of isoform-specific interaction of PRDM3 and PRDM16 with the NuRD chromatin remodeling complex.

The p53 transcription factor is a critical barrier to pancreatic cancer progression. To unravel mechanisms of p53-mediated tumor suppression, which have remained elusive, we analyzed pancreatic cancer development in mice expressing p53 transcriptional activation domain (TAD) mutants. Surprisingly, the p5353,54 TAD2 mutant behaves as a "super-tumor suppressor," with an enhanced capacity to both suppress pancreatic cancer and transactivate select p53 target genes, including Ptpn14. Ptpn14 encodes a negative regulator of the Yap oncoprotein and is necessary and sufficient for pancreatic cancer suppression, like p53. We show that p53 deficiency promotes Yap signaling and that PTPN14 and TP53 mutations are mutually exclusive in human cancers. These studies uncover a p53-Ptpn14-Yap pathway that is integral to p53-mediated tumor suppression.

PRDM9 is a PR domain containing protein which trimethylates histone 3 on lysine 4 and 36. Its normal expression is restricted to germ cells and attenuation of its activity results in altered meiotic gene transcription, impairment of double-stranded breaks and pairing between homologous chromosomes. There is growing evidence for a role of aberrant expression of PRDM9 in oncogenesis and genome instability. Here we report the discovery of MRK-740, a potent (IC50: 80 ± 16 nM), selective and cell-active PRDM9 inhibitor (Chemical Probe). MRK-740 binds in the substrate-binding pocket, with unusually extensive interactions with the cofactor S-adenosylmethionine (SAM), conferring SAM-dependent substrate-competitive inhibition. In cells, MRK-740 specifically and directly inhibits H3K4 methylation at endogenous PRDM9 target loci, whereas the closely related inactive control compound, MRK-740-NC, does not. The discovery of MRK-740 as a chemical probe for the PRDM subfamily of methyltransferases highlights the potential for exploiting SAM in targeting SAM-dependent methyltransferases.

Malaria transmission-blocking vaccines (TBVs) aim to induce antibodies that interrupt malaria parasite development in the mosquito, thereby blocking onward transmission, and provide a much-needed tool for malaria control and elimination. The parasite surface protein Pfs48/45 is a leading TBV candidate. Here, we isolated and characterized a panel of 81 human Pfs48/45-specific monoclonal antibodies (mAbs) from donors naturally exposed to Plasmodium parasites. Genetically diverse mAbs against each of the three domains (D1-D3) of Pfs48/45 were identified. The most potent mAbs targeted D1 and D3 and achieved >80% transmission-reducing activity in standard membrane-feeding assays, at 10 and 2 μg/mL, respectively. Co-crystal structures of D3 in complex with four different mAbs delineated two conserved protective epitopes. Altogether, these Pfs48/45-specific human mAbs provide important insight into protective and non-protective epitopes that can further our understanding of transmission and inform the design of refined malaria transmission-blocking vaccine candidates.

Chromatin compaction mediates progenitor to post-mitotic cell transitions and modulates gene expression programs, yet the mechanisms are poorly defined. Snf2h and Snf2l are ATP-dependent chromatin remodelling proteins that assemble, reposition and space nucleosomes, and are robustly expressed in the brain. Here we show that mice conditionally inactivated for Snf2h in neural progenitors have reduced levels of histone H1 and H2A variants that compromise chromatin fluidity and transcriptional programs within the developing cerebellum. Disorganized chromatin limits Purkinje and granule neuron progenitor expansion, resulting in abnormal post-natal foliation, while deregulated transcriptional programs contribute to altered neural maturation, motor dysfunction and death. However, mice survive to young adulthood, in part from Snf2l compensation that restores Engrailed-1 expression. Similarly, Purkinje-specific Snf2h ablation affects chromatin ultrastructure and dendritic arborization, but alters cognitive skills rather than motor control. Our studies reveal that Snf2h controls chromatin organization and histone H1 dynamics for the establishment of gene expression programs underlying cerebellar morphogenesis and neural maturation.

Breast cancer linked with BRCA1/2 mutations commonly recur and resist current therapies, including PARP inhibitors. Given the lack of effective targeted therapies for BRCA1-mutant cancers, we sought to identify novel targets to selectively kill these cancers. Here, we report that loss of RNF8 significantly protects Brca1-mutant mice against mammary tumorigenesis. RNF8 deficiency in human BRCA1-mutant breast cancer cells was found to promote R-loop accumulation and replication fork instability, leading to increased DNA damage, senescence, and synthetic lethality. Mechanistically, RNF8 interacts with XRN2, which is crucial for transcription termination and R-loop resolution. We report that RNF8 ubiquitylates XRN2 to facilitate its recruitment to R-loop-prone genomic loci and that RNF8 deficiency in BRCA1-mutant breast cancer cells decreases XRN2 occupancy at R-loop-prone sites, thereby promoting R-loop accumulation, transcription-replication collisions, excessive genomic instability, and cancer cell death. Collectively, our work identifies a synthetic lethal interaction between RNF8 and BRCA1, which is mediated by a pathological accumulation of R-loops.

WDR5 (WD40 repeat protein 5) is an essential component of the human trithorax-like family of SET1 [Su(var)3-9 enhancer-of-zeste trithorax 1] methyltransferase complexes that carry out trimethylation of histone 3 Lys4 (H3K4me3), play key roles in development and are abnormally expressed in many cancers. In the present study, we show that the interaction between WDR5 and peptides from the catalytic domain of MLL (mixed-lineage leukaemia protein) (KMT2) can be antagonized with a small molecule. Structural and biophysical analysis show that this antagonist binds in the WDR5 peptide-binding pocket with a Kd of 450 nM and inhibits the catalytic activity of the MLL core complex in vitro. The degree of inhibition was enhanced at lower protein concentrations consistent with a role for WDR5 in directly stabilizing the MLL multiprotein complex. Our data demonstrate inhibition of an important protein-protein interaction and form the basis for further development of inhibitors of WDR5-dependent enzymes implicated in MLL-rearranged leukaemias or other cancers.

Huntington's disease results from expansion of a glutamine-coding CAG tract in the huntingtin (HTT) gene, producing an aberrantly functioning form of HTT. Both wildtype and disease-state HTT form a hetero-dimer with HAP40 of unknown functional relevance. We demonstrate in vivo and in cell models that HTT and HAP40 cellular abundance are coupled. Integrating data from a 2.6 Å cryo-electron microscopy structure, cross-linking mass spectrometry, small-angle X-ray scattering, and modeling, we provide a near-atomic-level view of HTT, its molecular interaction surfaces and compacted domain architecture, orchestrated by HAP40. Native mass spectrometry reveals a remarkably stable hetero-dimer, potentially explaining the cellular inter-dependence of HTT and HAP40. The exon 1 region of HTT is dynamic but shows greater conformational variety in the polyglutamine expanded mutant than wildtype exon 1. Our data provide a foundation for future functional and drug discovery studies targeting Huntington's disease and illuminate the structural consequences of HTT polyglutamine expansion.

UHRF1 is a key mediator of inheritance of epigenetic DNA methylation patterns during cell division and is a putative target for cancer therapy. Recent studies indicate that interdomain interactions critically influence UHRF1's chromatin-binding properties, including allosteric regulation of its histone binding. Here, using an integrative approach that combines small angle X-ray scattering, NMR spectroscopy, and molecular dynamics simulations, we characterized the dynamics of the tandem tudor domain-plant homeodomain (TTD-PHD) histone reader module, including its 20-residue interdomain linker. We found that the apo TTD-PHD module in solution comprises a dynamic ensemble of conformers, approximately half of which are compact conformations, with the linker lying in the TTD peptide-binding groove. These compact conformations are amenable to cooperative, high-affinity histone binding. In the remaining conformations, the linker position was in flux, and the reader adopted both extended and compact states. Using a small-molecule fragment screening approach, we identified a compound, 4-benzylpiperidine-1-carboximidamide, that binds to the TTD groove, competes with linker binding, and promotes open TTD-PHD conformations that are less efficient at H3K9me3 binding. Our work reveals a mechanism by which the dynamic TTD-PHD module can be allosterically targeted with small molecules to modulate its histone reader function for therapeutic or experimental purposes.

We developed and optimized a high-throughput project workflow to generate renewable recombinant antibodies to human proteins involved in epigenetic signalling. Three different strategies to produce phage display compatible protein antigens in bacterial systems were compared, and we found that in vivo biotinylation through the use of an Avi tag was the most productive method. Phage display selections were performed on 265 in vivo biotinylated antigen domains. High-affinity Fabs (<20nM) were obtained for 196. We constructed and optimized a new expression vector to produce in vivo biotinylated Fabs in E. coli. This increased average yields up to 10-fold, with an average yield of 4 mg/L. For 118 antigens, we identified Fabs that could immunoprecipitate their full-length endogenous targets from mammalian cell lysates. One Fab for each antigen was converted to a recombinant IgG and produced in mammalian cells, with an average yield of 15 mg/L. In summary, we have optimized each step of the pipeline to produce recombinant antibodies, significantly increasing both efficiency and yield, and also showed that these Fabs and IgGs can be generally useful for chromatin immunoprecipitation (ChIP) protocols.

Pfs230 is essential for Plasmodium falciparum transmission to mosquitoes and is the protein targeted by the most advanced malaria-transmission-blocking vaccine candidate. Prior understanding of functional epitopes on Pfs230 is based on two monoclonal antibodies (mAbs) with moderate transmission-reducing activity (TRA), elicited from subunit immunization. Here, we screened the B cell repertoire of two naturally exposed individuals possessing serum TRA and identified five potent mAbs from sixteen Pfs230 domain-1-specific mAbs. Structures of three potent and three low-activity antibodies bound to Pfs230 domain 1 revealed four distinct epitopes. Highly potent mAbs from natural infection recognized a common conformational epitope that is highly conserved across P. falciparum field isolates, while antibodies with negligible TRA derived from natural infection or immunization recognized three distinct sites. Our study provides molecular blueprints describing P. falciparum TRA, informed by contrasting potent and non-functional epitopes elicited by natural exposure and vaccination.

Malaria is a global health burden, with Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) responsible for the majority of infections worldwide. Circumsporozoite protein (CSP) is the most abundant protein on the surface of Plasmodium sporozoites, and antibodies targeting the central repeat region of CSP can prevent parasite infection. Although much has been uncovered about the molecular basis of antibody recognition of the PfCSP repeats, data remains scarce for PvCSP. Here, we performed molecular dynamics simulations for peptides comprising the PvCSP repeats from strains VK210 and VK247 to reveal how the PvCSP central repeats are highly disordered, with minor propensities to adopt turn conformations. Next, we solved eight crystal structures to unveil the interactions of two inhibitory monoclonal antibodies (mAbs), 2F2 and 2E10.E9, with PvCSP repeats. Both antibodies can accommodate subtle sequence variances in the repeat motifs and recognize largely coiled peptide conformations that also contain isolated turns. Our structural studies uncover various degrees of Fab-Fab homotypic interactions upon recognition of the PvCSP central repeats by these two inhibitory mAbs, similar to potent mAbs against PfCSP. These findings augment our understanding of host-Plasmodium interactions and contribute molecular details of Pv inhibition by mAbs to unlock structure-based engineering of PvCSP-based vaccines.

The huntingtin (HTT) protein plays critical roles in numerous cellular pathways by functioning as a scaffold for its many interaction partners and HTT knock out is embryonic lethal. Interrogation of HTT function is complicated by the large size of this protein so we studied a suite of structure-rationalized subdomains to investigate the structure-function relationships within the HTT-HAP40 complex. Protein samples derived from the subdomain constructs were validated using biophysical methods and cryo-electron microscopy, revealing they are natively folded and can complex with validated binding partner, HAP40. Derivatized versions of these constructs enable protein-protein interaction assays in vitro, with biotin tags, and in cells, with luciferase two-hybrid assay-based tags, which we use in proof-of-principle analyses to further interrogate the HTT-HAP40 interaction. These open-source biochemical tools enable studies of fundamental HTT biochemistry and biology, will aid the discovery of macromolecular or small-molecule binding partners and help map interaction sites across this large protein.

The cyclic guanosine-3',5'-monophosphate (cGMP)-dependent protein kinase (PKG) was identified >25 y ago; however, efforts to obtain a structure of the entire PKG enzyme or catalytic domain from any species have failed. In malaria parasites, cooperative activation of PKG triggers crucial developmental transitions throughout the complex life cycle. We have determined the cGMP-free crystallographic structures of PKG from Plasmodium falciparum and Plasmodium vivax, revealing how key structural components, including an N-terminal autoinhibitory segment (AIS), four predicted cyclic nucleotide-binding domains (CNBs), and a kinase domain (KD), are arranged when the enzyme is inactive. The four CNBs and the KD are in a pentagonal configuration, with the AIS docked in the substrate site of the KD in a swapped-domain dimeric arrangement. We show that although the protein is predominantly a monomer (the dimer is unlikely to be representative of the physiological form), the binding of the AIS is necessary to keep Plasmodium PKG inactive. A major feature is a helix serving the dual role of the N-terminal helix of the KD as well as the capping helix of the neighboring CNB. A network of connecting helices between neighboring CNBs contributes to maintaining the kinase in its inactive conformation. We propose a scheme in which cooperative binding of cGMP, beginning at the CNB closest to the KD, transmits conformational changes around the pentagonal molecule in a structural relay mechanism, enabling PKG to orchestrate rapid, highly regulated developmental switches in response to dynamic modulation of cGMP levels in the parasite.