Nuclear localization of Arabidopsis HD-Zip IV transcription factor GLABRA2 is driven by importin α.

GLABRA2 (GL2), a class IV homeodomain leucine-zipper (HD-Zip IV) transcription factor from Arabidopsis, is a developmental regulator of specialized cell types in the epidermis. GL2 contains a monopartite nuclear localization sequence (NLS) that is conserved in most HD-Zip IV members across the plants. We demonstrate that NLS mutations affect nuclear transport and result in a loss-of-function phenotypes. NLS fusions to enhanced yellow fluorescent protein (EYFP) show that it is sufficient for nuclear localization in roots and trichomes. Despite partial overlap of the NLS with the homeodomain, genetic dissection indicates that nuclear localization and DNA binding are separable functions. Affinity purification of GL2 from plants followed by MS-based proteomics identified importin α (IMPα) isoforms as potential GL2 interactors. NLS structural prediction and molecular docking studies with IMPα-3 revealed major interacting residues. Cytosolic yeast two-hybrid assays and co-immunoprecipitation experiments with recombinant proteins verified NLS-dependent interactions between GL2 and several IMPα isoforms. IMPα triple mutants (impα-1,2,3) exhibit abnormal trichome formation and defects in GL2 nuclear localization in trichomes, consistent with tissue-specific and redundant functions of IMPα isoforms. Taken together, our findings provide mechanistic evidence for IMPα-dependent nuclear localization of GL2 in Arabidopsis, a process that is critical for cell type differentiation of the epidermis.

Enhanced Hydrogen Bonding by Urea Functionalization Tunes the Stability and Biological Properties of Peptide Amphiphiles.

Self-assembled nanostructures such as those formed by peptide amphiphiles (PAs) are of great interest in biological and pharmacological applications. Herein, a simple and widely applicable chemical modification, a urea motif, was included in the PA's molecular structure to stabilize the nanostructures by virtue of intermolecular hydrogen bonds. Since the amino acid residue nearest to the lipid tail is the most relevant for stability, we decided to include the urea modification at that position. We prepared four groups of molecules (13 PAs in all), with varying levels of intermolecular cohesion, using amino acids with distinct ß-sheet promoting potential and/or containing hydrophobic tails of distinct lengths. Each subset contained one urea-modified PA and nonmodified PAs, all with the same peptide sequence. The varied responses of these PAs to variations in pH, temperature, counterions, and biologically related proteins were examined using microscopic, X-ray, spectrometric techniques, and molecular simulations. We found that the urea group contributes to the stabilization of the morphology and internal arrangement of the assemblies against environmental stimuli for all peptide sequences. In addition, microbiological and biological studies were performed with the cationic PAs. These assays reveal that the addition of urea linkages affects the PA-cell membrane interaction, showing the potential to increase the selectivity toward bacteria. Our data indicate that the urea motif can be used to tune the stability of a wide range of PA nanostructures, allowing flexibility on the biomaterial's design and opening a myriad of options for clinical therapies.

Thermodynamics of the physisorption of capping agents on silver nanoparticles.

Nanoscale silver particles have growing applications in biomedical and other technologies due to their unique antibacterial, optical, and electrical properties. The preparation of metal nanoparticles requires the action of a capping agent, such as thiol-containing compounds, to provide colloidal stability, prevent agglomeration, stop uncontrolled growth, and attenuate oxidative damage. However, despite the extensive use of these thiol-based capping agents, the structure of the capping agent layers on the metal surface and the thermodynamics of the formation of these layers remains poorly understood. Here, we leverage molecular dynamics simulations and free energy calculation techniques, to study the behavior of citrate and four thiol-containing capping agents commonly used to protect silver nanoparticles from oxidation. We have studied the single-molecule adsorption of these capping agents to the metal-water interface, their coalescence into clusters, and the formation of complete monolayers covering the metal nanoparticle. At sufficiently high concentrations, we find that allylmercaptan, lipoic acid, and mercaptohexanol spontaneously self-assemble into ordered layers with the thiol group in contact with the metal surface. The high density and ordered structure is presumably responsible for their improved protective characteristics relative to the other compounds studied.

Design of Peptides that Fold and Self-Assemble on Graphite.

The graphite-water interface provides a unique environment for polypeptides that generally favors ordered structures more than in solution. Therefore, systems consisting of designed peptides and graphitic carbon might serve as a convenient medium for controlled self-assembly of functional materials. Here, we computationally designed cyclic peptides that spontaneously fold into a ß-sheet-like conformation at the graphite-water interface and self-assemble, and we subsequently observed evidence of such assembly by atomic force microscopy. Using a novel protocol, we screened nearly 2000 sequences, optimizing for formation of a unique folded conformation while discouraging unfolded or misfolded conformations. A head-to-tail cyclic peptide with the sequence GTGSGTGGPGGGCGTGTGSGPG showed the greatest apparent propensity to fold spontaneously, and this optimized sequence was selected for larger scale molecular dynamics simulations, rigorous free-energy calculations, and experimental validation. In simulations ranging from hundreds of nanoseconds to a few microseconds, we observed spontaneous folding of this peptide at the graphite-water interface under many different conditions, including multiple temperatures (295 and 370 K), with different initial orientations relative to the graphite surface, and using different molecular dynamics force fields (CHARMM and Amber). The thermodynamic stability of the folded conformation on graphite over a range of temperatures was verified by replica-exchange simulations and free-energy calculations. On the other hand, in free solution, the folded conformation was found to be unstable, unfolding in tens of picoseconds. Intermolecular hydrogen bonds promoted self-assembly of the folded peptides into linear arrangements where the peptide backbone exhibited a tendency to align along one of the six zigzag directions of the graphite basal plane. For the optimized peptide, atomic force microscopy revealed growth of single-molecule-thick linear patterns of 6-fold symmetry, consistent with the simulations, while no such patterns were observed for a control peptide with the same amino acid composition but a scrambled sequence.

Beta-1,3 Oligoglucans Specifically Bind to Immune Receptor CD28 and May Enhance T Cell Activation.

Beta glucans are known to have immunomodulatory effects that mediated by a variety of mechanisms. In this article, we describe experiments and simulations suggesting that beta-1,3 glucans may promote activation of T cells by a previously unknown mechanism. First, we find that treatment of a T lymphoblast cell line with beta-1,3 oligoglucan significantly increases mRNA levels of T cell activation-associated cytokines, especially in the presence of the agonistic anti-CD3 antibody. This immunostimulatory activity was observed in the absence of dectin-1, a known receptor for beta-1,3 glucans. To clarify the molecular mechanism underlying this activity, we performed a series of molecular dynamics simulations and free-energy calculations to explore the interaction of beta-1,3 oligoglucans with potential immune receptors. While the simulations reveal little association between beta-1,3 oligoglucan and the immune receptor CD3, we find that beta-1,3 oligoglucans bind to CD28 near the region identified as the binding site for its natural ligands CD80 and CD86. Using a rigorous absolute binding free-energy technique, we calculate a dissociation constant in the low millimolar range for binding of 8-mer beta-1,3 oligoglucan to this site on CD28. The simulations show this binding to be specific, as no such association is computed for alpha-1,4 oligoglucan. This study suggests that beta-1,3 glucans bind to CD28 and may stimulate T cell activation collaboratively with T cell receptor activation, thereby stimulating immune function.

Structural Determinants of the Dopamine Transporter Regulation Mediated by G Proteins.

Dopamine clearance in the brain is controlled by the dopamine transporter (DAT), a protein residing in the plasma membrane, which drives reuptake of extracellular dopamine into presynaptic neurons. Studies have revealed that the ßγ subunits of heterotrimeric G proteins modulate DAT function through a physical association with the C-terminal region of the transporter. Regulation of neurotransmitter transporters by Gßγ subunits is unprecedented in the literature; therefore, it is interesting to investigate the structural details of this particular protein-protein interaction. Here, we refined the crystal structure of the Drosophila melanogaster DAT (dDAT), modeling de novo the N- and C-terminal domains; subsequently, we used the full-length dDAT structure to generate a comparative model of human DAT (hDAT). Both proteins were assembled with Gß1γ2 subunits employing protein-protein docking, and subsequent molecular dynamics simulations were run to identify the specific interactions governing the formation of the hDAT:Gßγ and dDAT:Gßγ complexes. A [L/F]R[Q/E]R sequence motif containing the residues R588 in hDAT and R587 in dDAT was found as key to bind the Gßγ subunits through electrostatic interactions with a cluster of negatively charged residues located at the top face of the Gß subunit. Alterations of DAT function have been associated with multiple devastating neuropathological conditions; therefore, this work represents a step toward better understanding DAT regulation by signaling proteins, allowing us to predict therapeutic target regions.

A Structural Model of the Inactivation Gate of Voltage-Activated Potassium Channels.

After opening, the Shaker voltage-gated potassium (KV) channel rapidly inactivates when one of its four N-termini enters and occludes the channel pore. Although it is known that the tip of the N-terminus reaches deep into the central cavity, the conformation adopted by this domain during inactivation and the nature of its interactions with the rest of the channel remain unclear. Here, we use molecular dynamics simulations coupled with electrophysiology experiments to reveal the atomic-scale mechanisms of inactivation. We find that the first six amino acids of the N-terminus spontaneously enter the central cavity in an extended conformation, establishing hydrophobic contacts with residues lining the pore. A second portion of the N-terminus, consisting of a long 24 amino acid α-helix, forms numerous polar contacts with residues in the intracellular entryway of the T1 domain. Double mutant cycle analysis revealed a strong relationship between predicted interatomic distances and empirically observed thermodynamic coupling, establishing a plausible model of the transition of KV channels to the inactivated state.

Potential Information Loss Due to Categorization of Minimum Inhibitory Concentration Frequency Distributions.

A bacterial isolate's susceptibility to antimicrobial is expressed as the lowest drug concentration inhibiting its visible growth, termed minimum inhibitory concentration (MIC). The susceptibilities of isolates from a host population at a particular time vary, with isolates with specific MICs present at different frequencies. Currently, for either clinical or monitoring purposes, an isolate is most often categorized as Susceptible, Intermediate, or Resistant to the antimicrobial by comparing its MIC to a breakpoint value. Such data categorizations are known in statistics to cause information loss compared to analyzing the underlying frequency distributions. The U.S. National Antimicrobial Resistance Monitoring System (NARMS) includes foodborne bacteria at the food animal processing and retail product points. The breakpoints used to interpret the MIC values for foodborne bacteria are those relevant to clinical treatments by the antimicrobials in humans in whom the isolates were to cause infection. However, conceptually different objectives arise when inference is sought concerning changes in susceptibility/resistance across isolates of a bacterial species in host populations among different sampling points or times. For the NARMS 1996-2013 data for animal processing and retail, we determined the fraction of comparisons of susceptibility/resistance to 44 antimicrobial drugs of twelve classes of a bacterial species in a given animal host or product population where there was a significant change in the MIC frequency distributions between consecutive years or the two sampling points, while the categorization-based analyses concluded no change. The categorization-based analyses missed significant changes in 54% of the year-to-year comparisons and in 71% of the slaughter-to-retail within-year comparisons. Hence, analyses using the breakpoint-based categorizations of the MIC data may miss significant developments in the resistance distributions between the sampling points or times. Methods considering the MIC frequency distributions in their entirety may be superior for epidemiological analyses of resistance dynamics in populations.

Inhibition of a Snake Venom Metalloproteinase by the Flavonoid Myricetin.

Most of the snakebite envenomations in Central and South America are caused by species belonging to Bothrops genus. Their venom is composed mainly by zinc-dependent metalloproteinases, responsible of the hemorrhage characteristic of these envenomations. The aim of this study was to determine the inhibitory ability of ten flavonoids on the in-vitro proteolytic activity of Bothrops atrox venom and on the hemorrhagic, edema-forming and myonecrotic activities of Batx-I, the most abundant metalloproteinase isolated from this venom. Myricetin was the most active compound, exhibiting an IC 50 value of 150 µ M and 1021 µ M for the inhibition of proteolytic and hemorrhagic activity, respectively. Independent injection experiments, with a concentration of 1600 µ M of myricetin administered locally, immediately after toxin injection, demonstrated a reduction of 28 ± 6 % in the hemorrhagic lesion. Additionally, myricetin at concentrations 800, 1200 and 1600 µ M promoted a reduction in plasma creatine kinase activity induced by Batx-I of 21 ± 2 % , 60 ± 5 % and 63 ± 2 % , respectively. Molecular dynamics simulations coupled with the adaptive biasing method suggest that myricetin can bind to the metalloproteinase active site via formation of hydrogen bonds between the hydroxyl groups 3', 4' and 5' of the benzyl moiety and amino acid Glu143 of the metalloproteinase. The hydroxyl substitution pattern of myricetin appears to be essential for its inhibitory activity. Based on this evidence, myricetin constitutes a candidate for the development of inhibitors to reduce local tissue damage in snakebite envenomations.

Understanding Noncovalent Interactions of Small Molecules with Carbon Nanotubes.

We combine experimental methods, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations in the quantitative analysis of noncovalent interactions between (6,5)-enriched single-walled carbon nanotubes (SWNTs), as hosts, and a set of pyrene derivatives with different electronic properties and surface areas, as guests. The experiments and calculations were carried out in two solvents with markedly different polarities, namely 1,1',2,2'-tetrachloroethane (TCE) and N,N-dimethylformamide (DMF). Our results show that dispersion forces govern the supramolecular association of small molecules with (6,5)-SWNTs, with negligible contributions from ground-state charge-transfer effects. In the nonpolar solvent (TCE), the binding constants are highly correlated with the contact area between the SWNT and the guests. In the polar solvent (DMF), the binding constants show a complex dependence on the chemical nature of the pyrene substituents, as demonstrated by MD simulations with the explicit inclusion of solvent molecules. The solvation of the small molecules is shown to play a leading role in the binding process. Remarkably, the binding constants obtained from the MD simulations for the five guest molecules correlate with those derived from experiment. Furthermore, the MD simulations also reveal the structure of the adsorbed guest from low to high SWNT surface coverage.

Iodide Binding in Sodium-Coupled Cotransporters.

Several apical iodide translocation pathways have been proposed for iodide efflux out of thyroid follicular cells, including a pathway mediated by the sodium-coupled monocarboxylate transporter 1 (SMCT1), which remains controversial. Herein, we evaluate structural and functional similarities between SMCT1 and the well-studied sodium-iodide symporter (NIS) that mediates the first step of iodide entry into the thyroid. Free-energy calculations using a force field with electronic polarizability verify the presence of a conserved iodide-binding pocket between the TM2, TM3, and TM7 segments in hNIS, where iodide is coordinated by Phe67, Gln72, Cys91, and Gln94. We demonstrate the mutation of residue Gly93 of hNIS to a larger amino acid expels the side chain of a critical tryptophan residue (Trp255) into the interior of the binding pocket, partially occluding the iodide binding site and reducing iodide affinity, which is consistent with previous reports associating mutation of this residue with iodide uptake deficiency and hypothyroidism. Furthermore, we find that the position of Trp255 in this hNIS mutant mirrors that of Trp253 in wild-type hSMCT1, where a threonine (Thr91) occupies the position homologous to that occupied by glycine in wild-type hNIS (Gly93). Correspondingly, mutation of Thr91 to glycine in hSMCT1 makes the pocket structure more like that of wild-type hNIS, increasing its iodide affinity. These results suggest that wild-type hSMCT1 in the inward-facing conformation may bind iodide only very weakly, which may have implications for its ability to transport iodide.

Molecular determinants of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) binding to transient receptor potential V1 (TRPV1) channels.

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) has been recognized as an important activator of certain transient receptor potential (TRP) channels. More specifically, TRPV1 is a pain receptor activated by a wide range of stimuli. However, whether or not PI(4,5)P2 is a TRPV1 agonist remains open to debate. Utilizing a combined approach of mutagenesis and molecular modeling, we identified a PI(4,5)P2 binding site located between the TRP box and the S4-S5 linker. At this site, PI(4,5)P2 interacts with the amino acid residues Arg-575 and Arg-579 in the S4-S5 linker and with Lys-694 in the TRP box. We confirmed that PI(4,5)P2 behaves as a channel agonist and found that Arg-575, Arg-579, and Lys-694 mutations to alanine reduce PI(4,5)P2 binding affinity. Additionally, in silico mutations R575A, R579A, and K694A showed that the reduction in binding affinity results from the delocalization of PI(4,5)P2 in the binding pocket. Molecular dynamics simulations indicate that PI(4,5)P2 binding induces conformational rearrangements of the structure formed by S6 and the TRP domain, which cause an opening of the lower TRPV1 channel gate.

New Insights into Peptide-Silver Nanoparticle Interaction: Deciphering the Role of Cysteine and Lysine in the Peptide Sequence.

We studied the interaction of four new pentapeptides with spherical silver nanoparticles. Our findings indicate that the combination of the thiol in Cys and amines in Lys/Arg residues is critical to providing stable protection for the silver surface. Molecular simulation reveals the atomic scale interactions that underlie the observed stabilizing effect of these peptides, while yielding qualitative agreement with experiment for ranking the affinity of the four pentapeptides for the silver surface.

Simulation-Based Approaches for Determining Membrane Permeability of Small Compounds.

Predicting the rate of nonfacilitated permeation of solutes across lipid bilayers is important to drug design, toxicology, and signaling. These rates can be estimated using molecular dynamics simulations combined with the inhomogeneous solubility-diffusion model, which requires calculation of the potential of mean force and position-dependent diffusivity of the solute along the transmembrane axis. In this paper, we assess the efficiency and accuracy of several methods for the calculation of the permeability of a model DMPC bilayer to urea, benzoic acid, and codeine. We compare umbrella sampling, replica exchange umbrella sampling, adaptive biasing force, and multiple-walker adaptive biasing force for the calculation of the transmembrane PMF. No definitive advantage for any of these methods in their ability to predict the membrane permeability coefficient Pm was found, provided that a sufficiently long equilibration is performed. For diffusivities, a Bayesian inference method was compared to a generalized Langevin method, both being sensitive to chosen parameters and the slow relaxation of membrane defects. Agreement within 1.5 log units of the computed Pm with experiment is found for all permeants and methods. Remaining discrepancies can likely be attributed to limitations of the force field as well as slowly relaxing collective movements within the lipid environment. Numerical calculations based on model profiles show that Pm can be reliably estimated from only a few data points, leading to recommendations for calculating Pm from simulations.

Interfacial Water Is Separated from a Hydrophobic Silica Surface by a Gap of 1.2 nm.

The interaction of liquid water with hydrophobic surfaces is ubiquitous in life and technology. Yet, the molecular structure of interfacial liquid water on these surfaces is not known. By using a 3D atomic force microscope, we characterize with angstrom resolution the structure of interfacial liquid water on hydrophobic and hydrophilic silica surfaces. The combination of 3D AFM images and molecular dynamics simulations reveals that next to a hydrophobic silica surface, there is a 1.2 nm region characterized by a very low density of water. In contrast, the 3D AFM images obtained of a hydrophilic silica surface reveal the presence of hydration layers next to the surface. The gap observed on hydrophobic silica surfaces is filled with two-to-three layers of straight-chain alkanes. We developed a 2D Ising model that explains the formation of a continuous hydrocarbon layer on hydrophobic silica surfaces.

Dangerous liaisons between detergents and membrane proteins. The case of mitochondrial uncoupling protein 2.

The extraction of membrane proteins from their native environment by detergents is central to their biophysical characterization. Recent studies have emphasized that detergents may perturb the structure locally and modify the dynamics of membrane proteins. However, it remains challenging to determine whether these perturbations are negligible or could be responsible for misfolded conformations, altering the protein's function. In this work, we propose an original strategy combining functional studies and molecular simulations to address the physiological relevance of membrane protein structures obtained in the presence of detergents. We apply our strategy to a structure of isoform 2 of an uncoupling protein (UCP2) binding an inhibitor recently obtained in dodecylphosphocholine detergent micelles. Although this structure shares common traits with the ADP/ATP carrier, a member of the same protein family, its functional and biological significance remains to be addressed. In the present investigation, we demonstrate how dodecylphosphocholine severely alters the structure as well as the function of UCPs. The proposed original strategy opens new vistas for probing the physiological relevance of three-dimensional structures of membrane proteins obtained in non-native environments.

Assessing graphene nanopores for sequencing DNA.

Using all-atom molecular dynamics and atomic-resolution Brownian dynamics, we simulate the translocation of single-stranded DNA through graphene nanopores and characterize the ionic current blockades produced by DNA nucleotides. We find that transport of single DNA strands through graphene nanopores may occur in single nucleotide steps. For certain pore geometries, hydrophobic interactions with the graphene membrane lead to a dramatic reduction in the conformational fluctuations of the nucleotides in the nanopores. Furthermore, we show that ionic current blockades produced by different DNA nucleotides are, in general, indicative of the nucleotide type, but very sensitive to the orientation of the nucleotides in the nanopore. Taken together, our simulations suggest that strand sequencing of DNA by measuring the ionic current blockades in graphene nanopores may be possible, given that the conformation of DNA nucleotides in the nanopore can be controlled through precise engineering of the nanopore surface.

Computational design of a cyclic peptide that inhibits the CTLA4 immune checkpoint.

Proteins involved in immune checkpoint pathways, such as CTLA4, PD1, and PD-L1, have become important targets for cancer immunotherapy; however, development of small molecule drugs targeting these pathways has proven difficult due to the nature of their protein-protein interfaces. Here, using a hierarchy of computational techniques, we design a cyclic peptide that binds CTLA4 and follow this with experimental verification of binding and biological activity, using bio-layer interferometry, cell culture, and a mouse tumor model. Beginning from a template excised from the X-ray structure of the CTLA4:B7-2 complex, we generate several peptide sequences using flexible docking and modeling steps. These peptides are cyclized head-to-tail to improve structural and proteolytic stability and screened using molecular dynamics simulation and MM-GBSA calculation. The standard binding free energies for shortlisted peptides are then calculated in explicit-solvent simulation using a rigorous multistep technique. The most promising peptide, cyc(EIDTVLTPTGWVAKRYS), yields the standard free energy -6.6 ± 3.5 kcal mol-1, which corresponds to a dissociation constant of ∼15 µmol L-1. The binding affinity of this peptide for CTLA4 is measured experimentally (31 ± 4 µmol L-1) using bio-layer interferometry. Treatment with this peptide inhibited tumor growth in a co-culture of Lewis lung carcinoma (LLC) cells and antigen primed T cells, as well as in mice with an orthotropic Lewis lung carcinoma allograft model.

Euglena gracilis Extract Protects From Tobacco Smoke Carcinogen-Induced Lung Cancer by Altering Gut Microbiota Metabolome.

Extracts from Euglena gracilis have been shown to prevent cancer growth in mouse models. However, the molecular mechanism of this anti-cancer activity has not been determined nor has the effect of Euglena extracts on tobacco smoke carcinogen-induced carcinogenesis. Here, we investigate the hypothesis that this anti-cancer activity is a result of changes in the intestinal microbiota induced by oral administration of the extract. We found that a Euglena gracilis water extract prevents lung tumorigenesis induced by a tobacco smoke-specific carcinogen (NNK) in mice treated either 2 weeks before or 10 weeks after NNK injection. Both of these treatment regimens are associated with significant increases in 27 microbiota metabolites found in the mouse feces, including large increases in triethanolamine, salicylate, desaminotyrosine, N-acetylserine, glycolate, and aspartate. Increases in the short-chain fatty acids (SCFAs) including acetate, propionate and butyrate are also observed. We also detected a significant attenuation of lung carcinoma cell growth through the induction of cell cycle arrest and apoptosis caused by low levels of SCFAs. This study provides strong evidence of anti-cancer activity in Euglena gracilis extracts against tobacco smoke carcinogen-induced tumorigenesis and demonstrates that this activity is linked to increased production of specific gut microbiota metabolites and the resultant induction of cell cycle arrest and apoptosis of lung carcinoma cells.