Oxidation increases the strength of the methionine-aromatic interaction.

Oxidation of methionine disrupts the structure and function of a range of proteins, but little is understood about the chemistry that underlies these perturbations. Using quantum mechanical calculations, we found that oxidation increased the strength of the methionine-aromatic interaction motif, a driving force for protein folding and protein-protein interaction, by 0.5-1.4 kcal/mol. We found that non-hydrogen-bonded interactions between dimethyl sulfoxide (a methionine analog) and aromatic groups were enriched in both the Protein Data Bank and Cambridge Structural Database. Thermal denaturation and NMR spectroscopy experiments on model peptides demonstrated that oxidation of methionine stabilized the interaction by 0.5-0.6 kcal/mol. We confirmed the biological relevance of these findings through a combination of cell biology, electron paramagnetic resonance spectroscopy and molecular dynamics simulations on (i) calmodulin structure and dynamics, and (ii) lymphotoxin-α binding toTNFR1. Thus, the methionine-aromatic motif was a determinant of protein structural and functional sensitivity to oxidative stress.

Death Receptor 5 Activation Is Energetically Coupled to Opening of the Transmembrane Domain Dimer.

The precise mechanism by which binding of tumor necrosis factor ligands to the extracellular domain of their corresponding receptors transmits signals across the plasma membrane has remained elusive. Recent studies have proposed that activation of several tumor necrosis factor receptors, including Death Receptor 5, involves a scissorlike opening of the disulfide-linked transmembrane (TM) dimer. Using time-resolved fluorescence resonance energy transfer, we provide, to our knowledge, the first direct biophysical evidence that Death Receptor 5 TM-dimers open in response to ligand binding. Then, to probe the importance of the closed-to-open TM domain transition in the overall energetics of receptor activation, we designed point-mutants (alanine to phenylalanine) in the predicted, tightly packed TM domain dimer interface. We hypothesized that the bulky residues should destabilize the closed conformation and eliminate the ∼3 kcal/mol energy barrier to TM domain opening and the âˆ¼2 kcal/mol energy difference between the closed and open states, thus oversensitizing the receptor. To test this, we used all-atom molecular dynamics simulations of the isolated TM domain in explicit lipid bilayers coupled to thermodynamic potential of mean force calculations. We showed that single point mutants at the interface altered the energy landscape as predicted, but were not enough to completely eliminate the barrier to opening. However, the computational model did predict that a double mutation at i, i+4 positions at the center of the TM domain dimer eliminates the barrier and stabilizes the open conformation relative to the closed. We tested these mutants in cells with time-resolved fluorescence resonance energy transfer and death assays, and show remarkable agreement with the calculations. The single mutants had a small effect on TM domain separation and cell death, whereas the double mutant significantly increased the TM domain separation and more than doubled the sensitivity of cells to ligand stimulation.

Piecing it together: Unraveling the elusive structure-function relationship in single-pass membrane receptors.

The challenge of crystallizing single-pass plasma membrane receptors has remained an obstacle to understanding the structural mechanisms that connect extracellular ligand binding to cytosolic activation. For example, the complex interplay between receptor oligomerization and conformational dynamics has been, historically, only inferred from static structures of isolated receptor domains. A fundamental challenge in the field of membrane receptor biology, then, has been to integrate experimentally observable dynamics of full-length receptors (e.g. diffusion and conformational flexibility) into static structural models of the disparate domains. In certain receptor families, e.g. the ErbB receptors, structures have led somewhat linearly to a putative model of activation. In other families, e.g. the tumor necrosis factor (TNF) receptors, structures have produced divergent hypothetical mechanisms of activation and transduction. Here, we discuss in detail these and other related receptors, with the goal of illuminating the current challenges and opportunities in building comprehensive models of single-pass receptor activation. The deepening understanding of these receptors has recently been accelerated by new experimental and computational tools that offer orthogonal perspectives on both structure and dynamics. As such, this review aims to contextualize those technological developments as we highlight the elegant and complex conformational communication between receptor domains. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Open and closed conformations of the isolated transmembrane domain of death receptor 5 support a new model of activation.

It has long been presumed that activation of the apoptosis-initiating Death Receptor 5, as well as other structurally homologous members of the TNF-receptor superfamily, relies on ligand-stabilized trimerization of noninteracting receptor monomers. We and others have proposed an alternate model in which the TNF-receptor dimer-sitting at the vertices of a large supramolecular receptor network of ligand-bound receptor trimers-undergoes a closed-to-open transition, propagated through a scissorslike conformational change in a tightly bundled transmembrane (TM) domain dimer. Here we have combined electron paramagnetic resonance spectroscopy and potential-of-mean force calculations on the isolated TM domain of the long isoform of DR5. The experiments and calculations both independently validate that the opening transition is intrinsic to the physical character of the TM domain dimer, with a significant energy barrier separating the open and closed states.

The methionine-aromatic motif plays a unique role in stabilizing protein structure.

Of the 20 amino acids, the precise function of methionine (Met) remains among the least well understood. To establish a determining characteristic of methionine that fundamentally differentiates it from purely hydrophobic residues, we have used in vitro cellular experiments, molecular simulations, quantum calculations, and a bioinformatics screen of the Protein Data Bank. We show that approximately one-third of all known protein structures contain an energetically stabilizing Met-aromatic motif and, remarkably, that greater than 10,000 structures contain this motif more than 10 times. Critically, we show that as compared with a purely hydrophobic interaction, the Met-aromatic motif yields an additional stabilization of 1-1.5 kcal/mol. To highlight its importance and to dissect the energetic underpinnings of this motif, we have studied two clinically relevant TNF ligand-receptor complexes, namely TRAIL-DR5 and LTα-TNFR1. In both cases, we show that the motif is necessary for high affinity ligand binding as well as function. Additionally, we highlight previously overlooked instances of the motif in several disease-related Met mutations. Our results strongly suggest that the Met-aromatic motif should be exploited in the rational design of therapeutics targeting a range of proteins.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces death receptor 5 networks that are highly organized.

Recent evidence suggests that TNF-related apoptosis-inducing ligand (TRAIL), a death-inducing cytokine with anti-tumor potential, initiates apoptosis by re-organizing TRAIL receptors into large clusters, although the structure of these clusters and the mechanism by which they assemble are unknown. Here, we demonstrate that TRAIL receptor 2 (DR5) forms receptor dimers in a ligand-dependent manner at endogenous receptor levels, and these receptor dimers exist within high molecular weight networks. Using mutational analysis, FRET, fluorescence microscopy, synthetic biochemistry, and molecular modeling, we find that receptor dimerization relies upon covalent and noncovalent interactions between membrane-proximal residues. Additionally, by using FRET, we show that the oligomeric structure of two functional isoforms of DR5 is indistinguishable. The resulting model of DR5 activation should revise the accepted architecture of the functioning units of DR5 and the structurally homologous TNF receptor superfamily members.

TNFR1 signaling is associated with backbone conformational changes of receptor dimers consistent with overactivation in the R92Q TRAPS mutant.

The widely accepted model for tumor necrosis factor 1 (TNFR1) signaling is that ligand binding causes receptor trimerization, which triggers a reorganization of cytosolic domains and thus initiates intracellular signaling. This model of stoichiometrically driven receptor activation does not account for the occurrence of ligand independent signaling in overexpressed systems, nor does it explain the constitutive activity of the R92Q mutant associated with TRAPS. More recently, ligand binding has been shown to result in the formation of high molecular weight, oligomeric networks. Although the dimer, shown to be the preligand structure, is thought to remain present within ligand-receptor networks, it is unknown whether network formation or ligand-induced structural change to the dimer itself is the trigger for TNFR1 signaling. In the present study, we investigate the available crystal structures of TNFR1 to explore backbone dynamics and infer conformational transitions associated with ligand binding. Using normal-mode analysis, we characterize the dynamic coupling between the TNFR1 ligand binding and membrane proximal domains and suggest a mechanism for ligand-induced activation. Furthermore, our data are supported experimentally by FRET showing that the constitutively active R92Q mutant adopts an altered conformation compared to wild-type. Collectively, our results suggest that the signaling competent architecture is the receptor dimer and that ligand binding modifies domain mobilities intrinsic to the receptor structure, allowing it to sample a separate, active conformation mediated by network formation.

Engineered protein-small molecule conjugates empower selective enzyme inhibition.

Potent, specific ligands drive precision medicine and fundamental biology. Proteins, peptides, and small molecules constitute effective ligand classes. Yet greater molecular diversity would aid the pursuit of ligands to elicit precise biological activity against challenging targets. We demonstrate a platform to discover protein-small molecule (PriSM) hybrids to combine unique pharmacophore activities and shapes with constrained, efficiently engineerable proteins. In this platform, a fibronectin protein library is displayed on yeast with a single cysteine coupled to acetazolamide via a maleimide-poly(ethylene glycol) linker. Magnetic and flow cytometric sorts enrich specific binders to carbonic anhydrase isoforms. Isolated PriSMs exhibit potent, specific inhibition of carbonic anhydrase isoforms with efficacy superior to that of acetazolamide or protein alone, including an 80-fold specificity increase and 9-fold potency gain. PriSMs are engineered with multiple linker lengths, protein conjugation sites, and sequences against two different isoforms, which reveal platform flexibility and impacts of molecular designs. PriSMs advance the molecular diversity of efficiently engineerable ligands.

Physical hydrogels photo-cross-linked from self-assembled macromers for potential use in tissue engineering.

To engineer artificial extracellular matrices (ECMs) enabling degradation-independent cell migration that mimicked nonproteolytic cell migration through physically stabilized ECMs in biological systems, polymers having a hydrophilic chain flanked by a terminal self-assembling leucine zipper domain and a terminal photoreactive acrylate group were molecularly engineered to form photo-cross-linkable physical hydrogels. Physical association of the leucine zippers resulted in multifunctional macromers, which were photo-cross-linkable into hydrogels. Gel formation was confirmed by rheological measurements. The physical nature of the hydrogel networks was shown by hydrogel disassembly in denaturing solutions that disrupted the secondary structure of the leucine zippers. Outgrowth of encapsulated fibroblast aggregates was observed in these physical hydrogels but not observed in a control hydrogel in which leucine zippers were covalently linked. The collective properties of these hydrogels, including the physical nature, the photo-cross-linkable characteristic, and the flexibility for systematic engineering of material properties, will provide unique opportunities for tissue engineering.

Minimal Nucleation State of α-Synuclein Is Stabilized by Dynamic Threonine-Water Networks.

The first structures of α-synuclein (αSyn) fibrils have recently been solved. Here, we use a unique combination of molecular dynamics simulation strategies to address the minimal nucleation size of the 11-amino acid NAC protofibril solved by X-ray and to interrogate the dynamic behavior of unexpected crystal waters in the steric zipper. We found that protofibrils of >8 chains are thermodynamically stabilized due to protection of the fibril core from solvent influx and ordering of the end strands by the fibril core. In these stable oligomers, water molecules resolved in the crystal structure freely exchange with bulk solvent but are, on average, stably coordinated along the ß-sheet by inward-facing Thr72 and Thr75. We confirm the persistence of this water coordination via simulations of the full-length Greek-key structure solved by NMR and speculate that these Thr-water networks are important in the context of enhanced fibril nucleation in the familial A53T mutation.

An Innovative High-Throughput Screening Approach for Discovery of Small Molecules That Inhibit TNF Receptors.

Tumor necrosis factor receptor 1 (TNFR1) is a transmembrane receptor that binds tumor necrosis factor or lymphotoxin-alpha and plays a critical role in regulating the inflammatory response. Upregulation of these ligands is associated with inflammatory and autoimmune diseases. Current treatments reduce symptoms by sequestering free ligands, but this can cause adverse side effects by unintentionally inhibiting ligand binding to off-target receptors. Hence, there is a need for new small molecules that specifically target the receptors, rather than the ligands. Here, we developed a TNFR1 FRET biosensor expressed in living cells to screen compounds from the NIH Clinical Collection. We used an innovative high-throughput fluorescence lifetime screening platform that has exquisite spatial and temporal resolution to identify two small-molecule compounds, zafirlukast and triclabendazole, that inhibit the TNFR1-induced IκBα degradation and NF-κB activation. Biochemical and computational docking methods were used to show that zafirlukast disrupts the interactions between TNFR1 pre-ligand assembly domain (PLAD), whereas triclabendazole acts allosterically. Importantly, neither compound inhibits ligand binding, proving for the first time that it is possible to inhibit receptor activation by targeting TNF receptor-receptor interactions. This strategy should be generally applicable to other members of the TNFR superfamily, as well as to oligomeric receptors in general.

Death Receptor 5 Networks Require Membrane Cholesterol for Proper Structure and Function.

Death receptor 5 (DR5) is an apoptosis-inducing member of the tumor necrosis factor receptor superfamily, whose activity has been linked to membrane cholesterol content. Upon ligand binding, DR5 forms large clusters within the plasma membrane that have often been assumed to be manifestations of receptor co-localization in cholesterol-rich membrane domains. However, we have recently shown that DR5 clusters are more than just randomly aggregated receptors. Instead, these are highly structured networks held together by receptor dimers. These dimers are stabilized by specific transmembrane helix-helix interactions, including a disulfide bond in the long isoform of the receptor. The complex relationships among DR5 network formation, transmembrane helix dimerization, membrane cholesterol, and receptor activity has not been established. It is unknown whether the membrane itself plays an active role in driving DR5 transmembrane helix interactions or in the formation of the networks. We show that cholesterol depletion in cells does not inhibit the formation of DR5 networks. However, the networks that form in cholesterol-depleted cells fail to induce caspase cleavage. These results suggest a potential structural difference between active and inactive networks. As evidence, we show that cholesterol is necessary for the covalent dimerization of DR5 transmembrane domains. Molecular simulations and experiments in synthetic vesicles on the DR5 transmembrane dimer suggest that dimerization is facilitated by increased helicity in a thicker bilayer.

Modularly assembled porous cell-laden hydrogels.

Porous cell-laden hydrogels have been modularly assembled to address the challenges in 3-dimensional tissue engineering. Microgels photolithographically fabricated from solutions of poly(ethylene glycol) diacrylate are assembled into large porous constructs in the presence of a polypeptide-based, physically bonded cross-linker. The assembly occurs through a physiologically permissive Michael-type addition reaction between the acrylate groups on the surface of the microgels and the thiol groups on the cross-linker. The constructs assembled from star-shaped microgels exhibit higher porosity, permeability, and pore interconnectivity than those formed from circle- and square-shaped microgels. The correlation between the properties of assembled constructs and the morphological features of microgels suggests the possibility for bottom-up modulation of the construct properties. The high pore interconnectivity revealed on the level of individual microgels suggests that these constructs are suitable for tissue engineering. Cells remain viable inside centimeter-sized constructs when cultured under perfusion. These constructs have the combined advantages of preformed porous scaffolds and in situ forming hydrogels in allowing enhanced mass transfer, uniform cell seeding, and protection of cells from excessive, non-physiological shear stress. Large constructs can be assembled in one step and have no limitations in size. This method will provide opportunities to create large 3-dimensional tissue engineered products.