Influenza virus infections lead to numerous deaths and millions of hospitalizations each year. One challenge facing anti-influenza drug development is the heterogeneity of the circulating influenza viruses, which comprise several strains with variable susceptibility to antiviral drugs. For example, the wild-type (WT) influenza A viruses, such as the seasonal H1N1, tend to be sensitive to antiviral drugs, amantadine and rimantadine, while the S31N mutant viruses, such as the pandemic 2009 H1N1 (H1N1pdm09) and seasonal H3N2, are resistant to this class of drugs. Thus, drugs targeting both WT and the S31N mutant are highly desired. We report our design of a novel class of dual inhibitors along with their ion channel blockage and antiviral activities. The potency of the most active compound 11 in inhibiting WT and the S31N mutant influenza viruses is comparable with that of amantadine in inhibiting WT influenza virus. Solution NMR studies and molecular dynamics (MD) simulations of drug-M2 interactions supported our design hypothesis: namely, the dual inhibitor binds in the WT M2 channel with an aromatic group facing down toward the C-terminus, while the same drug binds in the S31N M2 channel with its aromatic group facing up toward the N-terminus. The flip-flop mode of drug binding correlates with the structure–activity relationship (SAR) and has paved the way for the next round of rational design of broad-spectrum antiviral drugs.
We have synthesized and characterized a series of compounds containing the 3-azatetracyclo[22.214.171.124,8.01,5]undecane scaffold designed as analogs of amantadine, an inhibitor of the M2 proton channel of influenza A virus. Inhibition of the wild-type (wt) M2 channel and the amantadine-resistant A/M2-S31N and A/M2-V27A mutant ion channels were measured in Xenopus oocytes using two-electrode voltage clamp (TEV) assays. Most of the novel compounds inhibited the wt ion channel in the low micromolar range. Of note, several compounds inhibited the A/M2 V27A mutant ion channel, one of them with submicromolar IC50. None of the compounds was found to inhibit the S31N mutant ion channel. The antiviral activity of three novel dual wt and A/M2-V27A channels inhibitors was confirmed by influenza virus yield assays.
Amantadine; cage compounds; influenza A virus; M2 proton channel; drug design
Foldamers provide an attractive medium to test the mechanisms by which biological macromolecules fold into complex three-dimensional structures, and ultimately to design novel protein-like architectures with properties unprecedented in nature. Here, we describe a large cage-like structure formed from an amphiphilic arylamide foldamer crystallized from aqueous solution. Forty eight copies of the foldamer assemble into a 5 nm cage-like structure, an omnitruncated octahedron filled with well-ordered ice-like water molecules. The assembly is stabilised by a mix of arylamide stacking interaction, hydrogen bonding and hydrophobic forces. The omnitruncated octahedra tessellate to form a cubic crystal. These findings may provide an important step towards the design of nanostructured particles resembling spherical viruses.
de novo design; truncated octahedron; omnitruncated octahedron; foldamer; antimicrobial peptide mimic; supramolecular synthon; organic framework; crystal engineering
The M2 protein of influenza A viruses forms a tetrameric proton channel that is targeted by the amantadine class of antiviral drugs. A S31N mutation in the transmembrane (TM) domain of the protein has caused widespread amantadine resistance in most of the currently circulating flu viruses. Recently, a new family of compounds based on amantadine- and aryl-substituted isoxazole were discovered to potently inhibit the S31N channel activity and reduce replication of S31N-harboring viruses. We now use solid-state NMR spectroscopy to investigate the effects of one of these isoxazole compounds, WJ352, on the conformation of the S31N TM segment and the dynamics of the proton-selective residue, His37. Chemical shift perturbations show that WJ352 changes the conformational equilibrium of multiple TM residues, with the maximal perturbation occurring at the crucial Asn31. 13C-2H distance measurements and 1H-1H NOE cross peaks indicate that the adamantane moiety of the drug is bound in the spacious pore between N31 and G34 while the phenyl tail resides near V27. Thus, the polar amine points to the channel exterior rather than to His37, in contrast to amantadine and rimantadine in the wild-type channel, suggesting that the drug is significantly stabilized by hydrophobic interactions between the adamantane and the TM peptide. 15N and 13C chemical shifts indicate that at low pH, His37 undergoes fast exchange among the τ tautomer, the π tautomer and the cationic state due to proton transfer with water. The exchange rate is higher than the wild-type channel, consistent with the larger single-channel conductance of the mutant. Drug binding at acidic pH largely suppresses this exchange, reverting the histidines to a similar charge distribution as that of the high-pH closed state.
It only takes one mutation: a strategically placed single mutation in a non-enzymatic protein scaffold produced AlleyCat, a small, allosterically regulated catalyst of Kemp elimination. In only 7 rounds of directed evolution enzymatic efficiency of the original 74 amino acid residue catalyst was improved more than 220-fold to achieve kcat value higher than that of catalytic antibodies for the same reaction, still preserving allosteric regulation.
Metalloproteins; Catalysts; Enzyme catalysis
Channel gating and proton conductance of the influenza A virus M2 channel result from complex pH-dependent interactions involving the pore-lining residues His37, Trp41, and Asp44. Protons diffusing from the outside of the virus protonate His37, which opens the Trp41 gate and allows one or more protons to move into the virus interior. The Trp41 gate gives rise to a strong asymmetry in the conductance, favoring rapid proton flux only when the outside is at acid pH. Here, we show that the proton currents recorded for mutants of Asp44, including D44N found in the A/FPV/Rostock/34 strain, lose this asymmetry. Moreover, NMR and MD simulations show that the mutations induce a conformational change similar to that induced by protonation of His37 at low pH, and decrease the structural stability of the hydrophobic seal associated with the Trp41 gate. Thus, Asp44 is able to determine two important properties of the M2 proton channel.
We have designed a highly specific inhibitor of calpain by mimicking a natural protein-protein interaction between calpain and its endogenous inhibitor calpastatin. To enable this goal we established a new method of stabilizing an α-helix in a small peptide by screening twenty-four commercially available crosslinkers for successful cysteine alkylation in a model peptide sequence. The effects of crosslinking on the α-helicity of selected peptides were examined by CD and NMR spectroscopy, and revealed structurally rigid crosslinkers to be the best at stabilizing α-helices. We applied this strategy to the design of inhibitors of calpain that are based on calpastatin, an intrinsically unstable polypeptide that becomes structured upon binding to the enzyme. A two-turn α-helix that binds proximal to the active site cleft was stabilized, resulting in a potent and selective inhibitor for calpain. We further expanded the utility of this inhibitor by developing irreversible calpain family activity-based probes (ABPs), which retained the specificity of the stabilized helical inhibitor. We believe the inhibitor and ABPs and will be useful for future investigation of calpains, while the crosslinking technique will enable exploration of other protein-protein interactions.
De novo proteins provide a unique opportunity for investigating the structure-function relationships of metalloproteins in a minimal, well-defined, and controlled scaffold. Herein, we describe the rational programming of function in a de novo designed di-iron carboxylate protein from the due ferri family. Originally created to catalyze O2-dependent, two-electron oxidation of hydroquinones, the protein was reprogrammed to catalyze the selective N-hydroxylation of arylamines by remodeling the substrate access cavity and introducing a critical third His ligand to the metal binding cavity. Additional second-and third-shell modifications were required to stabilize the His ligand in the core of the protein. These changes resulted in at least a 106 –fold increase in the relative rates of the two reactions. This result highlights the potential for using de novo proteins as scaffolds for future investigations of geometric and electronic factors that influence the catalytic tuning of di-iron active sites.
de novo design; metalloproteins; di-iron proteins; four-helix bundle; oxidase
We describe the use of organosilanes as inhibitors and structural probes of a membrane protein, the M2 proton channel from influenza A virus. Organosilane amine inhibitors were found to be generally as potent as their carbon analogs in targeting WT A/M2, and more potent against the drug resistant A/M2-V27A mutant. In addition, intermolecular NOESY spectra with dimethyl substituted organosilane amine inhibitors clearly located the drug binding site at the N-terminal lumen of the A/M2 channel close to V27.
The soluble monomeric domain of lipoprotein YxeF from the Gram positive bacterium B. subtilis was selected by the Northeast Structural Genomics Consortium (NESG) as a target of a biomedical theme project focusing on the structure determination of the soluble domains of bacterial lipoproteins. The solution NMR structure of YxeF reveals a calycin fold and distant homology with the lipocalin Blc from the Gram-negative bacterium E.coli. In particular, the characteristic β-barrel, which is open to the solvent at one end, is extremely well conserved in YxeF with respect to Blc. The identification of YxeF as the first lipocalin homologue occurring in a Gram-positive bacterium suggests that lipocalins emerged before the evolutionary divergence of Gram positive and Gram negative bacteria. Since YxeF is devoid of the α-helix that packs in all lipocalins with known structure against the β-barrel to form a second hydrophobic core, we propose to introduce a new lipocalin sub-family named ‘slim lipocalins’, with YxeF and the other members of Pfam family PF11631 to which YxeF belongs constituting the first representatives. The results presented here exemplify the impact of structural genomics to enhance our understanding of biology and to generate new biological hypotheses.
Two dimensional vibrational echo spectroscopy has previously been applied to structural determination of small peptides. Here we extend the technique to a more complex, biologically significant system: the homodimeric transmembrane dimer from the α-chain of the integrin αIIbβ3. We prepared micelle suspensions of the pair of 30-residue chains that span the membrane in the native structure, with varying levels of heavy (13C=18O) isotopes substituted in the backbone of the central 10th through 20th positions. The constraints derived from vibrational coupling of the precisely spaced heavy residues led to determination of an optimized structure from a range of model candidates: Glycine residues at the 12th, 15th and 16th positions form a tertiary contact in parallel right handed helix dimers with crossing angles of −58° ± 9° and interhelical distances of 7.7 ± 0.5 Å. The frequency correlation established the dynamical model used in the analysis and it indicated the absence of mobile water associated with labeled residues. Delocalization of vibrational excitations between the helices was also quantitatively established.
The transmembrane domain of the influenza M2 protein (M2TM) forms a tetrameric proton channel important for the virus lifecycle. The proton-channel activity is inhibited by amine-containing adamantyl drugs amantadine and rimantadine, which have been shown to bind specifically to the pore of M2TM near Ser31. However, whether the polar amine points to the N- or C-terminus of the channel has not yet been determined. Elucidating the polar group direction will shed light on the mechanism by which drug binding inhibits this proton channel and will facilitate rational design of new inhibitors. In this study, we determine the polar amine direction using M2TM reconstituted in lipid bilayers as well as DPC micelles. 13C-2H rotational-echo double-resonance NMR experiments of 13C-labeled M2TM and methyl-deuterated rimantadine in lipid bilayers showed that the polar amine pointed to the C-terminus of the channel, with the methyl group close to Gly34. Solution NMR experiments of M2TM in dodecylphosphocholine (DPC) micelles indicate that drug binding causes significant chemical shift perturbations of the protein that are very similar to those seen for M2TM and M2(18–60) bound to lipid bilayers. Specific 2H-labeling of the drugs permitted the assignment of drug-protein cross peaks, which indicate that amantadine and rimantadine bind to the pore in the same fashion as for bilayer-bound M2TM. These results strongly suggest that adamantyl inhibition of M2TM is achieved not only by direct physical occlusion of the pore but also by perturbing the equilibrium constant of the proton-sensing residue His37. The reproduction of the pharmacologically relevant specific pore-binding site in DPC micelles, which was not observed with a different detergent, DHPC, underscores the significant influence of the detergent environment on the functional structure of membrane proteins.
We describe a computational protocol, called DDMI, for redesigning scaffold proteins to bind to a specified region on a target protein. The DDMI protocol is implemented within the Rosetta molecular modeling program and uses rigid-body docking, sequence design, and gradient-based minimization of backbone and side chain torsion angles to design low energy interfaces between the scaffold and target protein. Iterative rounds of sequence design and conformational optimization were needed to produce models that have calculated binding energies that are similar to binding energies calculated for native complexes. We also show that additional conformation sampling with molecular dynamics can be iterated with sequence design to further lower the computed energy of the designed complexes. To experimentally test the DDMI protocol we redesigned the human hyperplastic discs protein to bind to the kinase domain of p21-activated kinase 1 (PAK1). Six designs were experimentally characterized. Two of the designs aggregated and were not characterized further. Of the remaining four designs, three bound to the PAK1 with affinities tighter than 350 μM. The tightest binding design, named Spider Roll, bound with an affinity of 100 μM. NMR –based structure prediction of Spider Roll based on backbone and 13Cβ chemical shifts using the program CS-ROSETTA indicated that the architecture of human hyperplastic discs protein is preserved. Mutagenesis studies confirmed that Spider Roll binds the target patch on PAK1. Additionally, Spider Roll binds to full length PAK1 in its activated state, but does not bind PAK1 when it forms an auto-inhibited conformation that blocks the Spider Roll target site. Subsequent NMR characterization of the binding of Spider Roll to PAK1 revealed a comparably small binding `on-rate' constant (<< 105 M−1 s−1). The ability to rationally design the site of novel protein-protein interactions is an important step towards creating new proteins that are useful as therapeutics or molecular probes.
Computational protein design; protein-protein interactions; protein docking; Rosetta molecular modeling program; NMR; CS-ROSETTA
analytical methods; clean absorption mode; GFT projection NMR; NMR spectroscopy; resolution enhancement
Clean absorption mode NMR data acquisition is presented based on mirrored time domain sampling and widely used time-proportional phase incrementation (TPPI) for quadrature detection. The resulting NMR spectra are devoid of dispersive frequency domain peak components. Those peak components exacerbate peak identification and shift peak maxima, and thus impede automated spectral analysis. The new approach is also of unique value for obtaining clean absorption mode reduced-dimensionality projection NMR spectra, which can rapidly provide high-dimensional spectral information for high-throughput NMR structure determination.
Clean Absorption Mode NMR; TPPI; GFT projection NMR; RD projection NMR
Proteins that discriminate between cisplatin–DNA adducts and oxaliplatin–DNA adducts are thought to be responsible for the differences in tumor range, toxicity, and mutagenicity of these two important chemotherapeutic agents. However, the structural basis for differential protein recognition of these adducts has not been determined and could be important for the design of more effective platinum anticancer agents. We have determined high-resolution NMR structures for cisplatin–GG and undamaged DNA dodecamers in the AGGC sequence context and have compared these structures with the oxaliplatin–GG structure in the same sequence context determined previously in our laboratory. This structural study allows the first direct comparison of cisplatin–GG DNA and oxaliplatin–GG DNA solution structures referenced to undamaged DNA in the same sequence context. Non-hydrogen atom rmsds of 0.81 and 1.21 were determined for the 15 lowest-energy structures for cisplatin–GG DNA and undamaged DNA, respectively, indicating good structural convergence. The theoretical NOESY spectra obtained by back-calculation from the final average structures showed excellent agreement with the experimental data, indicating that the final structures are consistent with the NMR data. Several significant conformational differences were observed between the cisplatin–GG adduct and the oxaliplatin–GG adduct, including buckle at the 5′ G6•C19 base pair, opening at the 3′ G7•C18 base pair, twist at the A5G6•T20C19 base pair step, slide, twist, and roll at the G6G7•C19C18 base pair step, slide at the G7C8•C18G17 base pair step, G6G7 dihedral angle, and overall bend angle. We hypothesize that these conformational differences may be related to the ability of various DNA repair proteins, DNA binding proteins, and DNA polymerases to discriminate between cisplatin–GG and oxaliplatin–GG adducts.