Design of buried charged networks in artificial proteins.

Soluble proteins are universally packed with a hydrophobic core and a polar surface that drive the protein folding process. Yet charged networks within the central protein core are often indispensable for the biological function. Here, we show that natural buried ion-pairs are stabilised by amphiphilic residues that electrostatically shield the charged motif from its surroundings to gain structural stability. To explore this effect, we build artificial proteins with buried ion-pairs by combining directed computational design and biophysical experiments. Our findings illustrate how perturbation in charged networks can introduce structural rearrangements to compensate for desolvation effects. We validate the physical principles by resolving high-resolution atomic structures of the artificial proteins that are resistant towards unfolding at extreme temperatures and harsh chemical conditions. Our findings provide a molecular understanding of functional charged networks and how point mutations may alter the protein's conformational landscape.

Quantifying the insertion of membrane proteins into lipid bilayer nanodiscs using a fusion protein strategy.

A membrane protein's oligomeric state modulates its functionality in various cellular processes. Since membrane proteins have to be solubilized in an appropriate membrane mimetic, the use of classical biophysical methods to analyze protein oligomers is challenging. We here present a method to determine the number of membrane proteins inserted into lipid nanodiscs. It is based on the ability to selectively quantify the amount of a small and robust fusion protein that can be proteolytically cleaved off from a membrane protein after incorporation into lipid nanodiscs. A detailed knowledge of the number of membrane proteins per nanodisc at defined assembly conditions is essential to estimate the tendency for oligomerization, but also for guiding sample optimization for structural investigations that require the presence of a homogenous oligomeric state. We show that this method can efficiently be used to determine the number of VDAC1 channels in nanodiscs at various assembly conditions, as confirmed by negative stain EM. The presented method is suitable in particular for membrane proteins that cannot be probed easily by other methods such as single span transmembrane helices. This assay can be applied to any membrane protein that can be incorporated into a nanodisc without the requirement for special instrumentation and will thus be widely applicable and complementary to other methods that quantify membrane protein insertion in lipid nanodiscs.

The N-Terminal Segment of the Voltage-Dependent Anion Channel: A Possible Membrane-Bound Intermediate in Pore Unbinding.

The voltage-dependent anion channel (VDAC) resides in the outer mitochondrial membrane and can adopt a closed or open configuration, most likely depending on whether the N-terminal segment (NTS) occupies the pore or protrudes into the cytoplasm. In this study, we calculate the free energy of releasing the NTS from the pore using molecular dynamics simulation. This is complicated by the flexible nature of the NTS, in particular its disordered structure in aqueous solution compared to the pore lumen. We carried out potential of mean force calculations using enhanced sampling or conformational restraints to address the conformational sampling problem. For the binding to the VDAC pore, two systems were considered, featuring either the native VDAC system or a modified system where the NTS is detached from the pore, that is, noncovalently bound in the pore lumen. The calculated free energies required to translocate the NTS from the pore into the solvent moiety are 83.8 or 74.3 kJ mol-1, respectively. The dissociation pathway in VDAC presents two in-pore minima, separated by a low free energy barrier and a membrane-bound intermediate state. Since we observe small changes in pore shape along the NTS dissociation pathway, we suggest that rigidification of the VDAC pore might impair NTS dissociation. The stability of the membrane-bound state of the VDAC NTS is confirmed by independent molecular dynamics simulations showing spontaneous membrane binding of a NTS-derived peptide as well as nuclear magnetic resonance experiments where chemical shift perturbations of the NTS-derived peptide evidence binding to phospholipid nanodiscs.

Nanodiscs for INPHARMA NMR Characterization of GPCRs: Ligand Binding to the Human A2A Adenosine Receptor.

G-protein-coupled-receptors (GPCRs) are of fundamental importance for signal transduction through cell membranes. This makes them important drug targets, but structure-based drug design (SBDD) is still hampered by the limitations for structure determination of unmodified GPCRs. We show that the interligand NOEs for pharmacophore mapping (INPHARMA) method can provide valuable information on ligand poses inside the binding site of the unmodified human A2A adenosine receptor reconstituted in nanodiscs. By comparing experimental INPHARMA spectra with back-calculated spectra based on ligand poses obtained from molecular dynamics simulations, a complex structure for A2A R with the low-affinity ligand 3-pyrrolidin-1-ylquinoxalin-2-amine was determined based on the X-ray structure of ligand ZM-241,358 in complex with a modified A2A R.

Automated assignment of NMR chemical shifts based on a known structure and 4D spectra.

Apart from their central role during 3D structure determination of proteins the backbone chemical shift assignment is the basis for a number of applications, like chemical shift perturbation mapping and studies on the dynamics of proteins. This assignment is not a trivial task even if a 3D protein structure is known and needs almost as much effort as the assignment for structure prediction if performed manually. We present here a new algorithm based solely on 4D [(1)H,(15)N]-HSQC-NOESY-[(1)H,(15)N]-HSQC spectra which is able to assign a large percentage of chemical shifts (73-82 %) unambiguously, demonstrated with proteins up to a size of 250 residues. For the remaining residues, a small number of possible assignments is filtered out. This is done by comparing distances in the 3D structure to restraints obtained from the peak volumes in the 4D spectrum. Using dead-end elimination, assignments are removed in which at least one of the restraints is violated. Including additional information from chemical shift predictions, a complete unambiguous assignment was obtained for Ubiquitin and 95 % of the residues were correctly assigned in the 251 residue-long N-terminal domain of enzyme I. The program including source code is available at https://github.com/thomasexner/4Dassign .

On-the-Fly Integration of Data from a Spin-Diffusion-Based NMR Experiment into Protein-Ligand Docking.

INPHARMA (interligand nuclear Overhauser enhancement for pharmacophore mapping) determines the relative orientation of two competitive ligands in the protein binding pocket. It is based on the observation of interligand transferred NOEs mediated by spin diffusion through protons of the protein and is, therefore, sensitive to the specific interactions of each of the two ligands with the protein. We show how this information can be directly included into a protein-ligand docking program to guide the prediction of the complex structures. Agreement between the experimental and back-calculated spectra based on the full relaxation matrix approach is translated into a score contribution that is combined with the scoring function ChemPLP of our docking tool PLANTS. This combined score is then used to predict the poses of five weakly bound cAMP-dependent protein kinase (PKA) ligands. After optimizing the setup, which finally also included trNOE data and optimized protonation states, very good success rates were obtained for all combinations of three ligands. For one additional ligand, no conclusive results could be obtained due to the ambiguous electron density of the ligand in the X-ray structure, which does not disprove alternative ligand poses. The failures of the remaining ligand are caused by suboptimal locations of specific protein side chains. Therefore, side-chain flexibility should be included in an improved INPHARMA-PLANTS version. This will reduce the strong dependence on the used protein input structure leading to improved scores overall, not only for this last ligand.

The solution structure of the amino-terminal domain of human DNA polymerase epsilon subunit B is homologous to C-domains of AAA+ proteins.

DNA polymerases alpha, delta and epsilon are large multisubunit complexes that replicate the bulk of the DNA in the eukaryotic cell. In addition to the homologous catalytic subunits, these enzymes possess structurally related B subunits, characterized by a carboxyterminal calcineurin-like and an aminoproximal oligonucleotide/oligosaccharide binding-fold domain. The B subunits also share homology with the exonuclease subunit of archaeal DNA polymerases D. Here, we describe a novel domain specific to the N-terminus of the B subunit of eukaryotic DNA polymerases epsilon. The N-terminal domain of human DNA polymerases epsilon (Dpoe2NT) expressed in Escherichia coli was characterized. Circular dichroism studies demonstrated that Dpoe2NT forms a stable, predominantly alpha-helical structure. The solution structure of Dpoe2NT revealed a domain that consists of a left-handed superhelical bundle. Four helices are arranged in two hairpins and the connecting loops contain short beta-strand segments that form a short parallel sheet. DALI searches demonstrated a striking structural similarity of the Dpoe2NT with the alpha-helical subdomains of ATPase associated with various cellular activity (AAA+) proteins (the C-domain). Like C-domains, Dpoe2NT is rich in charged amino acids. The biased distribution of the charged residues is reflected by a polarization and a considerable dipole moment across the Dpoe2NT. Dpoe2NT represents the first C-domain fold not associated with an AAA+ protein.

Peptide binding and NMR analysis of the interaction between SAP97 PDZ2 and GluR-A: potential involvement of a disulfide bond.

Synaptic delivery of GluR-A (GluR1) subunit-containing glutamate receptors depends on a C-terminal type I PDZ binding motif in GluR-A. Synapse-associated protein 97 (SAP97) is the only PDZ domain protein known to associate with GluR-A. We have used NMR spectroscopy and a biotinylated peptide binding assay to characterize the interaction between synthetic GluR-A C-terminal peptides and the PDZ2 domain of SAP97 (SAP97(PDZ2)), previously determined to be the dominant factor responsible for the interaction. The binding mode appeared to be strongly influenced by redox conditions. Chemical shift changes observed in NMR spectra indicate that under reducing conditions, the last four residues of GluR-A peptides bind to PDZ2 in a fashion typical of class I PDZ interactions. The binding is weak and relatively nonselective as it occurs similarly with a PDZ2 domain derived from PSD-95, a related protein not believed to directly interact with GluR-A. In the absence of reducing agents, conserved cysteine residues in SAP97(PDZ2) and the GluR-A C-terminus gave rise to an anomalous behavior in a microplate assay with a biotinylated GluR-A 18-mer peptide. A covalent disulfide-linked complex between SAP97(PDZ2) and the GluR-A peptide was seen in the binding assay and in the NMR experiments performed under oxidizing conditions. The results are consistent with a two-step binding mechanism consisting of an initial PDZ interaction followed by stabilization of the complex by a disulfide bond. The possible physiological relevance of redox regulation of SAP97-GluR-A interaction remains to be established.

A set of HA-detected experiments for measuring scalar and residual dipolar couplings.

A new set of HCACO based three-dimensional NMR experiments for measuring residual dipolar couplings in proteins is presented. Using spin-state selection and editing in three dimensions, the experiments allow accurate measurement of intraresidual (1)D(C'H(alpha)), (1)D(C'C(alpha)) and (2)D(C'H(alpha)) scalar and residual dipolar couplings of (15) N/(13)C labeled proteins in D(2)O and dilute liquid crystals with minimal spectral crowding. The presented experiments are especially suitable for small or medium sized proline-rich proteins, or proteins that require high pH solvent conditions, making (1)H(N) detected experiments unattractive. In addition, the tetrahedral coordination of C(alpha) is superior to the planar peptide bond for determination of local alignments in partially structured polypeptides. For the efficient use of spectrometer time and to avoid complications arising from the varying magnitude of the alignment tensor during relatively long experiments, the (1)D(C'H(alpha)) and (2)D(C'H(alpha)) couplings can also be measured simultaneously in an E.COSY like manner with high accuracy. The pulse sequences are balanced for cross-correlation effects and minimized for relaxation losses. The pulse sequences are tested with a sample of (15)N/(13)C human ubiquitin. We find internuclear vector directions determined from the dipolar couplings to have an excellent correlation with those of ubiquitin's refined solution structure.

On the interpretation of residual dipolar couplings as reporters of molecular dynamics.

The analysis of residual dipolar couplings from an ensemble of conformations to extract molecular dynamics is intricate. The very mechanism that is necessary to perturb overall molecular tumbling to generate nonvanishing residual dipolar couplings gives rise to convoluted data. The measured values are essentially weighted averages over conformations. However, the weights are not simply the populations of conformations. Consequently, the observed order parameter is not exactly the true measure of motion. In the case of paramagnetic alignment, the apparent order parameter is expected to depend on the number of torsions that separate the locus of interest from the paramagnetic site. In the case of alignment due to steric obstruction, the uneven selection of conformations by their differing Saupe order matrices leads to a bias in the residual dipolar couplings-probed molecular dynamics.

Alignment of chain-like molecules.

The steric obstruction model, that describes the enhanced alignment of folded proteins by anisotropic medium, is extended to account for the residual dipolar couplings of chain-like polypeptides. The average alignment of each chain segment is calculated from an ensemble of conformations represented by a spatial probability distribution. The segmental alignment depends on chain length, flexibility and segment's position in the chain. Residual dipolar couplings in turn depend on internuclear vector directions within each fragment. The results of calculations and simulations explain salient features of the experimental data. With this insight residual dipolar couplings can be interpreted to assess the degree of denaturation, local structures and spatial organization of weakly structured proteins.

On the origin of residual dipolar couplings from denatured proteins.

Effects of steric obstruction on random flight chains are examined. Spatial probability distributions are elaborated to calculate residual dipolar couplings and residual chemical shift anisotropy, parameters that are acquired by NMR spectroscopy from solutes dissolved in dilute liquid crystals. Calculations yield chain length and residue position-dependent values in good agreement with simulations to provide understanding of recently acquired data from denatured proteins.