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Protein-folding intermediates have been implicated in amyloid fibril formation involved in neurodegenerative disorders. However, the structural mechanisms by which intermediates initiate fibrillar aggregation have remained largely elusive. To gain insight, we used relaxation dispersion nuclear magnetic resonance spectroscopy to determine the structure of a low-populated, on-pathway folding intermediate of the A39V/N53P/V55L (A, Ala; V, Val; N, Asn; P, Pro; L, Leu) Fyn SH3 domain. The carboxyl terminus remains disordered in this intermediate, thereby exposing the aggregation-prone amino-terminal â strand. Accordingly, mutants lacking the carboxyl terminus and thus mimicking the intermediate fail to safeguard the folding route and spontaneously form fibrillar aggregates. The structure provides a detailed characterization of the non-native interactions stabilizing an aggregation-prone intermediate under native conditions and insight into how such an intermediate can derail folding and initiate fibrillation.

We present the software Peak INTegration (PINT), designed to perform integration of peaks in NMR spectra. The program is very simple to run, yet powerful enough to handle complicated spectra. Peaks are integrated by fitting predefined line shapes to experimental data and the fitting can be customized to deal with, for instance, heavily overlapped peaks. The results can be inspected visually, which facilitates systematic optimization of the line shape fitting. Finally, integrated peak volumes can be used to extract parameters such as relaxation rates and information about low populated states. The utility of PINT is demonstrated by applications to the 59 residue SH3 domain of the yeast protein Abp1p and the 289 residue kinase domain of murine EphB2.

Suppose that R is an associative unital ring and that $E=(E^0,E^1,r,s)$ is a directed graph. Using results from graded ring theory, we show that the associated Leavitt path algebra $L_R(E)$ is simple if and only if R is simple, $E^0$ has no nontrivial hereditary and saturated subset, and every cycle in E has an exit. We also give a complete description of the centre of a simple Leavitt path algebra.

Peak overlap in crowded regions of two-dimensional spectra prevents characterization of dynamics for many sites of interest in globular and intrinsically disordered proteins. We present new three-dimensional pulse sequences for measurement of Carr-Purcell-Meiboom-Gill relaxation dispersions at backbone nitrogen and carbonyl positions. To alleviate increase in the measurement time associated with the additional spectral dimension, we use non-uniform sampling in combination with two distinct methods of spectrum reconstruction: compressed sensing and co-processing with multi-dimensional decomposition. The new methodology was validated using disordered protein CD79A from B-cell receptor and an SH3 domain from Abp1p in exchange between its free form and bound to a peptide from the protein Ark1p. We show that, while providing much better resolution, the 3D NUS experiments give the similar accuracy and precision of the dynamic parameters to ones obtained using traditional 2D experiments. Furthermore, we show that jackknife resampling of the spectra yields robust estimates of peak intensities errors, eliminating the need for recording duplicate data points.

Tripartite motif-containing (TRIM) proteins are defined by the sequential arrangement of RING, B-box and coiled-coil domains (RBCC), where the B-box domain is a unique feature of the TRIM protein family. TRIM21 is an E3 ubiquitin-protein ligase implicated in innate immune signaling by acting as an autoantigen and by modifying interferon regulatory factors. Here we report the three-dimensional solution structure of the TRIM21 B-box2 domain by nuclear magnetic resonance (NMR) spectroscopy. The structure of the B-box2 domain, comprising TRIM21 residues 86-130, consists of a short α-helical segment with an N-terminal short β-strand and two anti-parallel β-strands jointly found the core, and adopts a RING-like fold. This ββαβ core largely defines the overall fold of the TRIM21 B-box2 and the coordination of one Zn2+ ion stabilizes the tertiary structure of the protein. Using NMR titration experiments, we have identified an exposed interaction surface, a novel interaction patch where the B-box2 is likely to bind the N-terminal RING domain. Our structure together with comparisons with other TRIM B-box domains jointly reveal how its different surfaces are employed for various modular interactions, and provides extended understanding of how this domain relates to flanking domains in TRIM proteins.

Structural- and functional heterogeneity, as well as allosteric regulation, in homo-monomeric enzymes is a highly active area of research. One such enzyme is human nuclear-associated deoxyuridine 5’-triphosphate nucleotidohydrolase (dUTPase), which has emerged as an interesting drug target in combination therapy with traditional nucleotide analogue treatment of cancer. We report, for the first time, a full structural dynamics study of human dUTPase by NMR. dUTPase has been investigated in terms of structural dynamics in its apo form, in complex with the modified substrate resistant to hydrolysis, 2’-deoxyuridine 5’-α,β-imido-triphosphate (dUpNHpp), as well as the product, 2’-deoxy-uridine-monophosphate (dUMP). The apo form of the enzyme displayed slow dynamics in the milli- to microsecond regime in relaxation dispersion experiments, which was further slowed down to observable heterogeneity upon substrate-analogue binding. The results suggest that the non-hydrolysable substrate-analogue traps the enzyme in the conformational isomerization step that has been previously suggested to be part of the enzyme catalysis kinetics cycle. The observed heterogeneity fits well with the pattern expected to emerge from the suggested kinetic model, and no evidence for homotropic allosterism was found. The heatmaps of the slow dynamics, chemical shift perturbation upon substrate binding and conserved regions of the enzyme sequence all displayed a similar pattern, which suggests that the structural dynamics is finely tuned and important for the biological function of the enzyme for binding, conformational shift, catalysis and substrate release.

The utility of nuclear magnetic resonance (NMR) spectroscopy as a tool for the study of biomolecular structure and dynamics has benefited from the development of facile labeling methods that incorporate NMR active probes at key positions in the molecule. Here we describe a protocol for the labeling of proteins that facilitates their study using a technique that is sensitive to millisecond conformational exchange processes. The samples necessary for an analysis of exchange dynamics are discussed, using the Abp1p SH3 domain from Saccharomyces cerevisiae as an example. For this system, the time frame for production of each sample, including in vitro refolding, is about 80 h. The samples so produced facilitate the measurement of accurate chemical shifts of low populated, invisible conformers that are part of the exchange pathway. The accuracy of the methodology has been established experimentally and the chemical shifts that are obtained provide important restraints in structure calculations of the excited state.

A selective isotope labeling scheme based on the utilization of [2- 13 C]-glycerol as the carbon source during protein overexpression has been evaluated for the measurement of excited state 13 Cα chemical shifts using Carr–Purcell–Meiboom–Gill (CPMG) relaxation dispersion (RD) experiments. As expected, the fractional incorporation of label at the Cα positions is increased two-fold relative to labeling schemes based on [2- 13 C]-glucose, effectively doubling the sensitivity of NMR experiments. Applications to a binding reaction involving an SH3 domain from the protein Abp1p and a peptide from the protein Ark1p establish that accurate excited state 13 Cα chemical shifts can be obtained from RD experiments, with errors on the order of 0.06 ppm for exchange rates ranging from 100 to 1000 s −1 , despite the small fraction of 13 Cα– 13 Cβ spin-pairs that are present for many residue types. The labeling approach described here should thus be attractive for studies of exchanging systems using 13 Cα spin probes.

A labeling scheme is introduced that facilitates the measurement of accurate ¹³Cβ chemical shifts of invisible, excited states of proteins by relaxation dispersion NMR spectroscopy. The approach makes use of protein over-expression in a strain of E. coli in which the TCA cycle enzyme succinate dehydrogenase is knocked out, leading to the production of samples with high levels of ¹³C enrichment (30-40%) at Cβ side-chain carbon positions for 15 of the amino acids with little ¹³C label at positions one bond removed ([almost equal to]5%). A pair of samples are produced using [1-¹³C]-glucose/NaH¹²CO₃ or [2-¹³C]-glucose as carbon sources with isolated and enriched (>30%) ¹³Cβ positions for 11 and 4 residues, respectively. The efficacy of the labeling procedure is established by NMR spectroscopy. The utility of such samples for measurement of ¹³Cβ chemical shifts of invisible, excited states in exchange with visible, ground conformations is confirmed by relaxation dispersion studies of a protein-ligand binding exchange reaction in which the extracted chemical shift differences from dispersion profiles compare favorably with those obtained directly from measurements on ligand free and fully bound protein samples.

Inhibition of the ternary protein complex of the synaptic scaffolding protein postsynaptic density protein-95 (PSD-95), neuronal nitric oxide synthase (nNOS), and the N-methyl-D-aspartate (NMDA) receptor is a potential strategy for treating ischemic brain damage, but high-affinity inhibitors are lacking. Here we report the design and synthesis of a novel dimeric inhibitor, Tat-NPEG4(IETDV)2 (Tat-N-dimer), which binds the tandem PDZ1-2 domain of PSD-95 with an unprecedented high affinity of 4.6 nM, and displays extensive protease-resistance as evaluated in vitro by stability-measurements in human blood plasma. X-ray crystallography, NMR, and small-angle X-ray scattering (SAXS) deduced a true bivalent interaction between dimeric inhibitor and PDZ1-2, and also provided a dynamic model of the conformational changes of PDZ1-2 induced by the dimeric inhibitor. A single intravenous injection of Tat-N-dimer (3 nmol/g) to mice subjected to focal cerebral ischemia reduces infarct volume with 40% and restores motor functions. Thus, Tat-N-dimer is a highly efficacious neuroprotective agent with therapeutic potential in stroke.