Allosteric inhibition of hypoxia inducible factor-2 with small molecules.

Hypoxia inducible factors (HIFs) are heterodimeric transcription factors induced in many cancers where they frequently promote the expression of protumorigenic pathways. Though transcription factors are typically considered 'undruggable', the PAS-B domain of the HIF-2α subunit contains a large cavity within its hydrophobic core that offers a unique foothold for small-molecule regulation. Here we identify artificial ligands that bind within this pocket and characterize the resulting structural and functional changes caused by binding. Notably, these ligands antagonize HIF-2 heterodimerization and DNA-binding activity in vitro and in cultured cells, reducing HIF-2 target gene expression. Despite the high sequence identity between HIF-2α and HIF-1α, these ligands are highly selective and do not affect HIF-1 function. These chemical tools establish the molecular basis for selective regulation of HIF-2, providing potential therapeutic opportunities to intervene in HIF-2-driven tumors, such as renal cell carcinomas.

Introduction of the Capsules environment to support further growth of the SBGrid structural biology software collection.

The expansive scientific software ecosystem, characterized by millions of titles across various platforms and formats, poses significant challenges in maintaining reproducibility and provenance in scientific research. The diversity of independently developed applications, evolving versions and heterogeneous components highlights the need for rigorous methodologies to navigate these complexities. In response to these challenges, the SBGrid team builds, installs and configures over 530 specialized software applications for use in the on-premises and cloud-based computing environments of SBGrid Consortium members. To address the intricacies of supporting this diverse application collection, the team has developed the Capsule Software Execution Environment, generally referred to as Capsules. Capsules rely on a collection of programmatically generated bash scripts that work together to isolate the runtime environment of one application from all other applications, thereby providing a transparent cross-platform solution without requiring specialized tools or elevated account privileges for researchers. Capsules facilitate modular, secure software distribution while maintaining a centralized, conflict-free environment. The SBGrid platform, which combines Capsules with the SBGrid collection of structural biology applications, aligns with FAIR goals by enhancing the findability, accessibility, interoperability and reusability of scientific software, ensuring seamless functionality across diverse computing environments. Its adaptability enables application beyond structural biology into other scientific fields.

Principles of ligand binding within a completely buried cavity in HIF2alpha PAS-B.

Hypoxia-inducible factors (HIFs) are heterodimeric transcription factors responsible for the metazoan hypoxia response and promote tumor growth, metastasis, and resistance to cancer treatment. The C-terminal Per-ARNT-Sim (PAS) domain of HIF2alpha (HIF2alpha PAS-B) contains a preformed solvent-inaccessible cavity that binds artificial ligands that allosterically perturb the formation of the HIF heterodimer. To better understand how small molecules bind within this domain, we examined the structures and equilibrium and transition-state thermodynamics of HIF2alpha PAS-B with several artificial ligands using isothermal titration calorimetry, NMR exchange spectroscopy, and X-ray crystallography. Rapid association rates reveal that ligand binding is not dependent upon a slow conformational change in the protein to permit ligand access, despite the closed conformation observed in the NMR and crystal structures. Compensating enthalpic and entropic contributions to the thermodynamic barrier for ligand binding suggest a binding-competent transition state characterized by increased structural disorder. Finally, molecular dynamics simulations reveal conversion between open and closed conformations of the protein and pathways of ligand entry into the binding pocket.

Dynamics of carbon monoxide photodissociation in Bradyrhizobium japonicum FixL probed by picosecond midinfrared spectroscopy.

The FixL proteins are heme-based bacterial oxygen sensors, distinct from globins in structure and ligand binding properties. To better understand the dynamics of ligand dissociation and binding within the PAS domain fold of FixL, we have carried out picosecond visible pump-midinfrared probe spectroscopy on the isolated PAS domain of FixL from Bradyrhizobium japonicum. We employ the diatomic ligand CO as a probe of the ligand-dissociation pocket dynamics; upon photoexcitation with a visible laser pulse, CO is released and the infrared-active stretch frequency of the CO molecule changes, as it is very sensitive to interactions with the surrounding protein. The infrared absorption difference spectra indicate that the escape of photolyzed CO to solvent is preceded by transient docking within the protein in a manner similar to globins. A small-scale spectral change of the CO molecule on a picosecond time scale is likely due to changes in heme-protein conformation associated with cooling. A larger scale spectral evolution on a nanosecond time scale indicates a structural change in the protein, possibly related to changes in the beta-strands associated with the transition from CO-bound to deoxy in BjFixLH (Key, J.; Srajer, V.; Pahl, R.; Moffat, K. Biochemistry 2007, 46, 4706).

Photosensing in chemotrophic, non-phototrophic bacteria: let there be light sensing too.

Putative light-sensing proteins are ubiquitously encoded in the genomes of chemotrophic, non-photosynthetic bacteria. Surprisingly, these are not limited to UV-receptors: the metagenome of the chemotrophic prokaryotes encodes representatives of all known major families of photoreceptors. Insight into the mechanism of light-mediated signaling is relatively advanced, but most light-induced physiological and behavioral responses in chemotrophic bacteria are not well understood. In the current era of 'omics' studies, this knowledge gap could be closed rapidly. Here we review the state of the art in this field. Because light signals can be manipulated accurately, these photoreceptors might help provide a systems-level understanding of the cytology of bacteria.

Computational Repacking of HIF-2α Cavity Replaces Water-Based Stabilized Core.

Hypoxia-inducible factors (HIFs) are heterodimeric transcription factors central to hypoxia response and cancer development. Within the HIF-2 complex, one domain (HIF-2α PAS-B) contains a large (290 Å3) buried cavity filled with water molecules within its hydrophobic core. Such cavities are uncommon except in the case of ligand-binding proteins, leading to the hypothesis that HIF-2α can be regulated by small molecules. The development of artificial HIF-2α inhibitors validates this hypothesis but raises questions about the impact of this cavity on HIF-2α PAS-B structure and function. To answer these points, we used computational methods to construct a repacked protein containing a smaller cavity within the native fold. Experimental validation of a five-mutation variant confirms achieving these objectives and stabilizing the folded structure. Complementary functional data establish that ligands cannot bind this variant although heterodimerization remains unchanged. Altogether, our strategy innovatively addresses the roles of solvated cavities in maintaining protein stability and function.

Data publication with the structural biology data grid supports live analysis.

Access to experimental X-ray diffraction image data is fundamental for validation and reproduction of macromolecular models and indispensable for development of structural biology processing methods. Here, we established a diffraction data publication and dissemination system, Structural Biology Data Grid (SBDG; data.sbgrid.org), to preserve primary experimental data sets that support scientific publications. Data sets are accessible to researchers through a community driven data grid, which facilitates global data access. Our analysis of a pilot collection of crystallographic data sets demonstrates that the information archived by SBDG is sufficient to reprocess data to statistics that meet or exceed the quality of the original published structures. SBDG has extended its services to the entire community and is used to develop support for other types of biomedical data sets. It is anticipated that access to the experimental data sets will enhance the paradigm shift in the community towards a much more dynamic body of continuously improving data analysis.

Purification and initial characterization of a putative blue light-regulated phosphodiesterase from Escherichia coli.

The Escherichia coli protein YcgF contains a photosensory flavin adenine dinucleotide (FAD)-binding BLUF domain covalently linked to an EAL domain, which is predicted to have cyclic-di-guanosine monophosphate (GMP) phosphodiesterase activity. We have cloned, overexpressed and purified this protein, which we refer to as blue light-regulated phosphodiesterase (Blrp) for its putative activity. Blrp undergoes a reversible photocycle after exposure to light in which the spectrum of its photostationary state and kinetics of recovery of the dark state are similar to those of the isolated BLUF domain of the AppA protein. Unlike the AppA BLUF domain, the chromophore environment in the context of full-length Blrp is asymmetric, and the protein does not undergo any detectable global changes on exposure to blue light. When overexpressed in E. coli, Blrp copurifies with certain proteins which suggests that it plays a protective role in response to oxidative stress. Predicted proteins from Klebsiella pneumoniae and from a bacterium in the Sargasso Sea are similar to E. coli Blrp in both their BLUF and EAL domains, which suggests that blue light sensing in these bacteria may follow similar pathways.

Collaboration gets the most out of software.

By centralizing many of the tasks associated with the upkeep of scientific software, SBGrid allows researchers to spend more of their time on research.

Regulating the ARNT/TACC3 axis: multiple approaches to manipulating protein/protein interactions with small molecules.

For several well-documented reasons, it has been challenging to develop artificial small molecule inhibitors of protein/protein complexes. Such reagents are of particular interest for transcription factor complexes given links between their misregulation and disease. Here we report parallel approaches to identify regulators of a hypoxia signaling transcription factor complex, involving the ARNT subunit of the HIF (Hypoxia Inducible Factor) activator and the TACC3 (Transforming Acidic Coiled Coil Containing Protein 3) coactivator. In one route, we used in vitro NMR and biochemical screening to identify small molecules that selectively bind within the ARNT PAS (Per-ARNT-Sim) domain that recruits TACC3, identifying KG-548 as an ARNT/TACC3 disruptor. A parallel, cell-based screening approach previously implicated the small molecule KHS101 as an inhibitor of TACC3 signaling. Here, we show that KHS101 works indirectly on HIF complex formation by destabilizing both TACC3 and the HIF component HIF-1α. Overall, our data identify small molecule regulators for this important complex and highlight the utility of pursuing parallel strategies to develop protein/protein inhibitors.

Development of inhibitors of the PAS-B domain of the HIF-2α transcription factor.

Hypoxia inducible factors (HIFs) are heterodimeric transcription factors induced in a variety of pathophysiological settings, including cancer. We describe the first detailed structure-activity relationship study of small molecules designed to inhibit HIF-2α-ARNT heterodimerization by binding an internal cavity of the HIF-2α PAS-B domain. Through a series of biophysical characterizations of inhibitor-protein interactions (NMR and X-ray crystallography), we have established the structural requirements for artificial inhibitors of the HIF-2α-ARNT PAS-B interaction. These results may serve as a foundation for discovering therapeutic agents that function by a novel mode of action.

Structure of the redox sensor domain of Azotobacter vinelandii NifL at atomic resolution: signaling, dimerization, and mechanism.

NifL is a multidomain sensor protein responsible for the transcriptional regulation of genes involved in response to changes in cellular redox state and ADP concentration. Cellular redox is monitored by the N-terminal PAS domain of NifL which contains an FAD cofactor. Flavin-based PAS domains of this type have also been referred to as LOV domains. To explore the mechanism of signal recognition and transduction in NifL, we determined the crystal structure of the FAD-bound PAS domain of NifL from Azotobacter vinelandii to 1.04 A resolution. The structure reveals a novel cavity within the PAS domain which contains two water molecules directly coordinated to the FAD. This cavity is connected to solvent by multiple access channels which may facilitate the oxidation of the FAD by molecular oxygen and the release of hydrogen peroxide. The structure contains a dimer of the NifL PAS domain that is structurally very similar to those described in other crystal structures of PAS domains and identifies a conserved dimerization motif. An N-terminal amphipathic helix constitutes part of the dimerization interface, and similar N-terminal helices are identified in other PAS domain proteins. The structure suggests a model for redox-mediated signaling in which a conformational change is initiated by redox-dependent changes in protonation at the N5 atom of FAD that lead to reorganization of hydrogen bonds within the flavin binding pocket. A structural signal is subsequently transmitted to the beta-sheet interface between the monomers of the PAS domain.

Time-resolved crystallographic studies of the heme domain of the oxygen sensor FixL: structural dynamics of ligand rebinding and their relation to signal transduction.

The FixL protein of Bradyrhizobium japonicum is a dimeric oxygen sensor responsible for initiating regulation of transcription of genes encoding proteins involved in nitrogen fixation and oxidative stress. It consists of an N-terminal heme-bound PAS domain, denoted bjFixLH, and a C-terminal histidine kinase domain whose enzymatic activity depends on the ligation state of the heme. To investigate the molecular basis for this dependence and the dynamics associated with conversion between ligated and unligated states, we have conducted time-resolved Laue diffraction studies of CO recombination in bjFixLH. Time-dependent difference Fourier maps from 1 micros to 10 ms after photolysis of the heme-CO bond show movement of the side chain of Leu236 and the H and I beta-strands into the ligand binding pocket formerly occupied by CO. Long-range conformational changes are evident in the protein, driven by relaxation of steric interactions between the bound ligand and amino acid side chains and/or changes in heme stereochemistry. These structural changes fully reverse as CO rebinds to the heme. Spectroscopic measurements of CO recombination kinetics in bjFixLH crystals relate the behavior of crystalline bjFixLH to solution and provide a framework for our time-resolved crystallographic experiments. Analysis of the time-dependent difference Fourier maps by singular value decomposition reveals that only one significant singular value accounts for the data. Thus only two structural states are present, the photolyzed and the CO-bound states. The first left singular vector represents the difference in density between these two states and shows features common to difference maps calculated from the static CO and deoxy states. The first right singular vector represents the time course of this difference density and agrees well with the CO recombination kinetics measured spectroscopically. We refine the structure of the photolyzed state present in the early-microsecond time range and find that it does not differ significantly in conformation from static, deoxy bjFixLH. Thus, structural relaxation from CO-bound to deoxy bjFixLH is complete in less than 1 micros.

Hydrogen-bond switching through a radical pair mechanism in a flavin-binding photoreceptor.

BLUF (blue light sensing using FAD) domains constitute a recently discovered class of photoreceptor proteins found in bacteria and eukaryotic algae, where they control a range of physiological responses including photosynthesis gene expression, photophobia, and negative phototaxis. Other than in well known photoreceptors such as the rhodopsins and phytochromes, BLUF domains are sensitive to light through an oxidized flavin rather than an isomerizable cofactor. To understand the physicochemical basis of BLUF domain photoactivation, we have applied femtosecond transient absorption spectroscopy to the Slr1694 BLUF domain of Synechocystis PCC6803. We show that photoactivation of BLUF domains proceeds by means of a radical-pair mechanism, driven by electron and proton transfer from the protein to the flavin, resulting in the transient formation of anionic and neutral flavin radical species that finally result in the long-lived signaling state on a 100-ps timescale. A pronounced deuteration effect is observed on the lifetimes of the intermediate radical species, indicating that proton movements underlie their molecular transformations. We propose a photoactivation mechanism that involves a successive rupture of hydrogen bonds between a conserved tyrosine and glutamine by light-induced electron transfer from tyrosine to flavin and between the glutamine and flavin by subsequent protonation at flavin N5. These events allow a reorientation of the conserved glutamine, resulting in a switching of the hydrogen-bond network connecting the chromophore to the protein, followed by radical-pair recombination, which locks the glutamine in place. It is suggested that the redox potential of flavin generally defines the light sensitivity of flavin-binding photoreceptors.

Crystal structures of deoxy and CO-bound bjFixLH reveal details of ligand recognition and signaling.

Rhizobia directly regulate the expression of genes required for symbiotic nitrogen fixation in response to oxygen concentration via the sensor protein FixL. The N-terminal PAS domain of FixL contains a histidine-coordinated heme and regulates the activity of its effector domain, a C-terminal histidine kinase, in response to binding of oxygen and other ligands at the heme. To further investigate ligand-induced inhibition of FixL, we have determined the crystal structures of the heme domain in both the deoxy state and bound to carbon monoxide, a weak inhibitor of FixL kinase activity. Structures collected at room temperature are presented in each state from two crystallographic space groups at 1.8 and 2 A resolution. These structures reveal displacement of the residues of the H(beta) and I(beta) strands by Leu236 upon CO binding, and this structural change propagates more than 15 A to a region of the structure implicated in signal transduction in PAS proteins. Displacement of residues Ile215, Ile216, and Gly217 in the FG loop is also evident, accompanied by the movement of heme propionate 6 upon change in iron ligation. CO binding increases the temperature factors in the FG loop of the protein and disorders the side chain of Arg206, a conserved residue involved in the FG loop switch mechanism. We relate these results to structural changes in other PAS sensor domains and their involvement in catalytic control.

The applicability of a computer model for predicting head injury incurred during actual motor vehicle collisions.

BACKGROUND: Head injury is a significant cause of both morbidity and mortality. Motor vehicle collisions (MVCs) are the most common source of head injury in the United States. No studies have conclusively determined the applicability of computer models for accurate prediction of head injuries sustained in actual MVCs. This study sought to determine the applicability of such models for predicting head injuries sustained by MVC occupants. METHODS: The Crash Injury Research and Engineering Network (CIREN) database was queried for restrained drivers who sustained a head injury. These collisions were modeled using occupant dynamic modeling (MADYMO) software, and head injury scores were generated. The computer-generated head injury scores then were evaluated with respect to the actual head injuries sustained by the occupants to determine the applicability of MADYMO computer modeling for predicting head injury. RESULTS: Five occupants meeting the selection criteria for the study were selected from the CIREN database. The head injury scores generated by MADYMO were lower than expected given the actual injuries sustained. In only one case did the computer analysis predict a head injury of a severity similar to that actually sustained by the occupant. CONCLUSION: Although computer modeling accurately simulates experimental crash tests, it may not be applicable for predicting head injury in actual MVCs. Many complicating factors surrounding actual MVCs make accurate computer modeling difficult. Future modeling efforts should consider variables such as age of the occupant and should account for a wider variety of crash scenarios.

Control mechanisms in the regulation of telomerase reverse transcriptase expression in differentiating human teratocarcinoma cells.

Telomerase is active in about 90% of cancers and contributes to the immortality of cancer cells by maintaining the lengths of the ends of chromosomes. Undifferentiated embryonic human teratocarcinoma (HT) cells were found to express high levels of hTERT, the catalytic subunit of telomerase, and the hTERT promoter was unmethylated in these cells. Retinoic acid (RA)-induced differentiation led to hTERT gene silencing and increased methylation of the hTERT promoter. Treatment with trichostatin A, a histone deacetylase inhibitor, resulted in hTERT reactivation only in very early differentiating HT cells. After methylation patterns had been established within the hTERT promoter region in late differentiating cells, 5-azacytidine, a common demethylating agent, activated the hTERT gene but trichostatin A had no effect on hTERT transcription. These studies suggest that histone deacetylation is involved in early hTERT gene down-regulation and that DNA methylation may maintain silencing of the hTERT gene in these cells.