Structural molecular biology is now becoming part of high school science curriculum thus posing a challenge for teachers who need to convey three-dimensional (3D) structures with conventional text and pictures. In many cases even interactive computer graphics does not go far enough to address these challenges. We have developed a flexible model of the polypeptide backbone using 3D printing technology. With this model we have produced a polypeptide assembly kit to create an idealized model of the Triosephosphate isomerase mutase enzyme (TIM), which forms a structure known as TIM barrel. This kit has been used in a laboratory practical where students perform a step-by-step investigation into the nature of protein folding, starting with the handedness of amino acids to the formation of secondary and tertiary structure. Based on the classroom evidence we collected, we conclude that these models are valuable and inexpensive resource for teaching structural molecular biology.

Protein protein interactions are essential for biological function, but structures of protein protein complexes are difficult to obtain experimentally. To derive the protein complex of the DNA-repair enzyme human uracil-DNA-glycosylase (hUNG) with its protein inhibitor (UGI), we combined rigid-body computational docking with hydrogen/deuterium exchange mass spectrometry (DXMS). Computational docking of the unbound protein structures provides a list of possible three-dimensional models of the complex; DXMS identifies solvent-protected protein residues. DXMS showed that unbound hUNG is compactly folded, but unbound UGI is loosely packed. An increased level of solvent protection of hUNG in the complex was localized to four regions on the same face. The decrease in the number of incorporated deuterons was quantitatively interpreted as the minimum number of main-chain hUNG amides buried in the protein protein interface. The level of deuteration of complexed UGI decreased throughout the protein chain, indicating both tighter packing and direct solvent protection by hUNG. Three UGI regions showing the greatest decreases were best interpreted leniently, requiring just one main-chain amide from each in the interface. Applying the DXMS constraints as filters to a list of docked complexes gave the correct complex as the largest favorable energy cluster. Thus, identification of approximate protein interfaces was sufficient to distinguish the protein complex. Surprisingly, incorporating the DXMS data as added favorable potentials in the docking calculation was less effective in finding the correct complex. The filtering method has greater flexibility, with the capability to test each constraint and enforce simultaneous contact by multiple regions, but with the caveat that the list from the unbiased docking must include correct complexes.

X-rays emerge from a synchrotron to hit a protein-covered substrate. From the film, proteins emerge, which are aggregated into football-like nanoparticles, as described by A. A. Gorodetsky and co-workers on page 8405. The protein and the nanoparticles are present in cephalopod skin; these cells emerge from a purple squid and underpin its camouflage abilities.

This protocol describes the use of the AutoDock suite for computational docking in the study of protein–ligand interactions. A number of methods are described ranging from basic docking of drug molecules to virtual screening using a large ligand library of chemical compounds. Computational docking can be used to predict bound conformations and free energies of binding for small-molecule ligands to macromolecular targets. Docking is widely used for the study of biomolecular interactions and mechanisms, and it is applied to structure-based drug design. The methods are fast enough to allow virtual screening of ligand libraries containing tens of thousands of compounds. This protocol covers the docking and virtual screening methods provided by the AutoDock suite of programs, including a basic docking of a drug molecule with an anticancer target, a virtual screen of this target with a small ligand library, docking with selective receptor flexibility, active site prediction and docking with explicit hydration. The entire protocol will require ∼5 h.

Films from the cephalopod protein reflectin demonstrate multifaceted functionality as infrared camouflage coatings, proton transport media, and substrates for growth of neural stem cells. A detailed study of the in vitro formation, structural characteristics, and stimulus response of such films is presented. The reported observations hold implications for the design and development of advanced cephalopod-inspired functional materials.

Computational docking is a useful tool for predicting macromolecular complexes, which are often difficult to determine experimentally. Here, we present the DOT2 software suite, an updated version of the DOT intermolecular docking program. DOT2 provides straightforward, automated construction of improved biophysical models based on molecular coordinates, offering checkpoints that guide the user to include critical features. DOT has been updated to run more quickly, allow flexibility in grid size and spacing, and generate an infinitive complete list of favorable candidate configurations. Output can be filtered by experimental data and rescored by the sum of electrostatic and atomic desolvation energies. We show that this rescoring method improves the ranking of correct complexes for a wide range of macromolecular interactions and demonstrate that biologically relevant models are essential for biologically relevant results. The flexibility and versatility of DOT2 accommodate realistic models of complex biological systems, improving the likelihood of a successful docking outcome. © 2013 Wiley Periodicals, Inc.

ABSTRACTProtein–DNA interactions are essential for many biological processes. X‐ray crystallography can provide high‐resolution structures, but protein‐DNA complexes are difficult to crystallize and typically contain only small DNA fragments. Thus, there is a need for computational methods that can provide useful predictions to give insights into mechanisms and guide the design of new experiments. We used the program DOT, which performs an exhaustive, rigid‐body search between two macromolecules, to investigate four diverse protein–DNA interactions. Here, we compare our computational results with subsequent experimental data on related systems. In all cases, the experimental data strongly supported our structural hypotheses from the docking calculations: a mechanism for weak, nonsequence‐specific DNA binding by a transcription factor, a large DNA‐binding footprint on the surface of the DNA‐repair enzyme uracil‐DNA glycosylase (UNG), viral and host DNA‐binding sites on the catalytic domain of HIV integrase, and a three‐DNA‐contact model of the linker histone bound to the nucleosome. In the case of UNG, the experimental design was based on the DNA‐binding surface found by docking, rather than the much smaller surface observed in the crystallographic structure. These comparisons demonstrate that the DOT electrostatic energy gives a good representation of the distinctive electrostatic properties of DNA and DNA‐binding proteins. The large, favourably ranked clusters resulting from the dockings identify active sites, map out large DNA‐binding sites, and reveal multiple DNA contacts with a protein. Thus, computational docking can not only help to identify protein–DNA interactions in the absence of a crystal structure, but also expand structural understanding beyond known crystallographic structures. Proteins 2013; 81:2106–2118. © 2013 Wiley Periodicals, Inc.

X-ray crystallography provides excellent structural data on protein-DNA interfaces, but crystallographic complexes typically contain only small fragments of large DNA molecules. We present a new approach that can use longer DNA substrates and reveal new protein-DNA interactions even in extensively studied systems. Our approach combines rigid-body computational docking with hydrogen/deuterium exchange mass spectrometry (DXMS). DXMS identifies solvent-exposed protein surfaces; docking is used to create a 3-dimensional model of the protein-DNA interaction. We investigated the enzyme uracil-DNA glycosylase (UNG), which detects and cleaves uracil from DNA. UNG was incubated with a 30 bp DNA fragment containing a single uracil, giving the complex with the abasic DNA product. Compared with free UNG, the UNG-DNA complex showed increased solvent protection at the UNG active site and at two regions outside the active site: residues 210-220 and 251-264. Computational docking also identified these two DNA-binding surfaces, but neither shows DNA contact in UNG-DNA crystallographic structures. Our results can be explained by separation of the two DNA strands on one side of the active site. These non-sequence-specific DNA-binding surfaces may aid local uracil search, contribute to binding the abasic DNA product and help present the DNA product to APE-1, the next enzyme on the DNA-repair pathway.

The drastic reduction of novel folds in proteins newly determined by x-ray crystallography suggests that large macromolecules are built from domains with already known structures. We have developed a novel integrative protocol that combines experimentally-measured topographic surfaces of single molecules with atomic coordinates of molecular constituents of large proteins or assemblies. Topographic surfaces are obtained using high-resolution atomic force microscopy (AFM) imaging. The present integrative method is based on real-space docking of macromolecular constituents beneath the experimental topographic surface. Assembly of molecular constituents is performed using a combinatorial approach. Only steric clashes between assembled constituents are computed; assemblies having more than a given threshold of bumps are eliminated. The goodness of fit is obtained by a score named E-factor which determines the agreement between the experimental topographic surface with that of the assembled constituents. A proof of concept has been determined on three different systems: Immunoglobulin G, Tobacco mosaic virus, and Aquaporin Z. Results demonstrated that partial topographic surface is adequate for complete macromolecular reconstruction. This protocol may be extremely useful for "difficult proteins" such as membrane proteins, partially unfolded proteins, and hard-to-produce proteins.

The XRCC4-like factor (XLF)-XRCC4 complex is essential for nonhomologous end joining, the major repair pathway for DNA double strand breaks in human cells. Yet, how XLF binds XRCC4 and impacts nonhomologous end joining functions has been enigmatic. Here, we report the XLF-XRCC4 complex crystal structure in combination with biophysical and mutational analyses to define the XLF-XRCC4 interactions. Crystal and solution structures plus mutations characterize alternating XRCC4 and XLF head domain interfaces forming parallel super-helical filaments. XLF Leu-115 ("Leu-lock") inserts into a hydrophobic pocket formed by XRCC4 Met-59, Met-61, Lys-65, Lys-99, Phe-106, and Leu-108 in synergy with pseudo-symmetric β-zipper hydrogen bonds to drive specificity. XLF C terminus and DNA enhance parallel filament formation. Super-helical XLF-XRCC4 filaments form a positively charged channel to bind DNA and align ends for efficient ligation. Collective results reveal how human XLF and XRCC4 interact to bind DNA, suggest consequences of patient mutations, and support a unified molecular mechanism for XLF-XRCC4 stimulation of DNA ligation.

AbstractIron–sulfur clusters reside at the active sites of iron–sulfur proteins and are intimately involved in many critical biological functions. The active sites are usually redox active, and the most common functions include simple electron transfer, proton‐coupled electron transfer, and oxidation or reduction of substrates. Some nonredox functions that involve cluster electronic polarization have also been established. The redox active character of iron–sulfur clusters and iron atom lability are also used for essential biological regulatory functions both in bacteria and eukaryotic cells. The versatility of sulfur ligation and iron–sulfur covalency (partial charge transfer) lie at the heart of iron–sulfur cluster functionality. Iron–sulfur proteins are among the most ancient biological catalysts, and iron–sulfur clusters may have had important prebiotic functions as well. In this review, we summarize the wide variety of iron–sulfur catalytic and regulatory functions. We show how many spectroscopic and physical methods contribute to our current understanding of catalytic and regulatory states and energetic pathways. Quantum mechanical [largely density functional theory (DFT)] methods link electronic structures to energetics and spectroscopic data, and provide a unifying framework for additional progress on many open questions. The active site transition metal complexes also interact with the protein and solvent environment. The main theoretical/computational tools for studying these interactions are Poisson‐Boltzmann electrostatics methods and combined quantum mechanics/molecular mechanics (QM/MM) methods. The principal experimental tools for examining cluster‐protein interactions include comparisons of synthetic complexes with corresponding proteins, the use of alternative substrates or inhibitors for enzymes, redox energetics, reaction kinetics studies, and spectroscopic observations.

The bacterial pathogen Vibrio cholerae uses toxin-coregulated pili (TCP) to colonize the human intestine, causing the severe diarrheal disease cholera. TCP are long, thin, flexible homopolymers of the TcpA subunit that self-associate to hold cells together in microcolonies and serve as the receptor for the cholera toxin phage. To better understand TCP's roles in pathogenesis, we characterized its structure using hydrogen/deuterium exchange mass spectrometry and computational modeling. We show that the pilin subunits are held together by tight packing of the N-terminal alpha helices, but loose packing of the C-terminal globular domains leaves substantial gaps on the filament surface. These gaps expose a glycine-rich, amphipathic segment of the N-terminal alpha-helix, contradicting the consensus view that this region is buried in the filament core. Our results explain extreme filament flexibility, suggest a molecular basis for pilus-pilus interactions, and reveal a previously unrecognized therapeutic target for V. cholerae and other enteric pathogens.

The recent use of Bacillus anthracis as a bioweapon has stimulated the search for novel antitoxins and vaccines that act rapidly and with minimal adverse effects. B. anthracis produces an AB-type toxin composed of the receptor-binding moiety protective antigen (PA) and the enzymatic moieties edema factor and lethal factor. PA is a key target for both antitoxin and vaccine development. We used the icosahedral insect virus Flock House virus as a platform to display 180 copies of the high affinity, PA-binding von Willebrand A domain of the ANTXR2 cellular receptor. The chimeric virus-like particles (VLPs) correctly displayed the receptor von Willebrand A domain on their surface and inhibited lethal toxin action in in vitro and in vivo models of anthrax intoxication. Moreover, VLPs complexed with PA elicited a potent toxin-neutralizing antibody response that protected rats from anthrax lethal toxin challenge after a single immunization without adjuvant. This recombinant VLP platform represents a novel and highly effective, dually-acting reagent for treatment and protection against anthrax.

The crystal structure of the human arginase I−thiosemicarbazide complex reveals an unusual thiocarbonyl μ-sulfide ligand in the binuclear manganese cluster. The CS moiety of thiosemicarbazide bridges Mn2+A and Mn2+B with coordination distances of 2.6 and 2.4 A, respectively. Otherwise, the binding of thiosemicarbazide to human arginase I does not cause any significant structural changes in the active site. The crystal structure of the unliganded enzyme reveals a hydrogen-bonded water molecule that could support proton transfer between a μ-water molecule and H141 to regenerate the nucleophilic μ-hydroxide ion in the final step of catalysis.

Type IV pill (T4P) are long, thin, flexible filaments on bacteria that undergo assembly-disassembly from inner membrane pilin subunits and exhibit astonishing multifunctionality. Neisseria gonorrhoeae (gonococcal or GC) T4P are prototypic virulence factors and immune targets for increasingly antibiotic-resistant human pathogens, yet detailed structures are unavailable for any T4P. Here, we determined a detailed experimental GC-T4P structure by quantitative fitting of a 2.3 angstrom full-length pilin crystal structure into a 12.5 angstrom resolution native GC-T4P reconstruction solved by cryo-electron microscopy (cryo-EM) and iterative helical real space reconstruction. Spiraling three-helix bundles form the filament core, anchor the globular heads, and provide strength and flexibility. Protruding hypervariable loops and posttranslational modifications in the globular head shield conserved functional residues in pronounced grooves, creating a surprisingly corrugated pilus surface. These results clarify T4P multifunctionality and assembly-disassembly while suggesting unified assembly mechanisms for T4P, archaeal flagella, and type II secretion system filaments.

Type IV pili are remarkably strong, flexible filaments with varied roles in bacterial pathogenicity. All Gram-negative bacterial surfaces have type IV pili, which are polymeric assemblies of the protein pilin that evoke the host immune response and are potential drug and vaccine targets. Pilin structures that have been solved using X-ray crystallography and nuclear magnetic resonance, together with models for pilus architectures inferred from electron microscopy, fibre diffraction and computation, have established a molecular basis for assembly and multi-functionality, with implications for therapeutic interventions.