Prion Protein-Antibody Complexes Characterized by Chromatography-Coupled Small-Angle X-Ray Scattering.

Aberrant self-assembly, induced by structural misfolding of the prion proteins, leads to a number of neurodegenerative disorders. In particular, misfolding of the mostly α-helical cellular prion protein (PrP(C)) into a ß-sheet-rich disease-causing isoform (PrP(Sc)) is the key molecular event in the formation of PrP(Sc) aggregates. The molecular mechanisms underlying the PrP(C)-to-PrP(Sc) conversion and subsequent aggregation remain to be elucidated. However, in persistently prion-infected cell-culture models, it was shown that treatment with monoclonal antibodies against defined regions of the prion protein (PrP) led to the clearing of PrP(Sc) in cultured cells. To gain more insight into this process, we characterized PrP-antibody complexes in solution using a fast protein liquid chromatography coupled with small-angle x-ray scattering (FPLC-SAXS) procedure. High-quality SAXS data were collected for full-length recombinant mouse PrP [denoted recPrP(23-230)] and N-terminally truncated recPrP(89-230), as well as their complexes with each of two Fab fragments (HuM-P and HuM-R1), which recognize N- and C-terminal epitopes of PrP, respectively. In-line measurements by fast protein liquid chromatography coupled with SAXS minimized data artifacts caused by a non-monodispersed sample, allowing structural analysis of PrP alone and in complex with Fab antibodies. The resulting structural models suggest two mechanisms for how these Fabs may prevent the conversion of PrP(C) into PrP(Sc).

Integrative structure-function mapping of the nucleoporin Nup133 suggests a conserved mechanism for membrane anchoring of the nuclear pore complex.

The nuclear pore complex (NPC) is the sole passageway for the transport of macromolecules across the nuclear envelope. Nup133, a major component in the essential Y-shaped Nup84 complex, is a large scaffold protein of the NPC's outer ring structure. Here, we describe an integrative modeling approach that produces atomic models for multiple states of Saccharomyces cerevisiae (Sc) Nup133, based on the crystal structures of the sequence segments and their homologs, including the related Vanderwaltozyma polyspora (Vp) Nup133 residues 55 to 502 (VpNup133(55-502)) determined in this study, small angle X-ray scattering profiles for 18 constructs of ScNup133 and one construct of VpNup133, and 23 negative-stain electron microscopy class averages of ScNup133(2-1157). Using our integrative approach, we then computed a multi-state structural model of the full-length ScNup133 and validated it with mutational studies and 45 chemical cross-links determined via mass spectrometry. Finally, the model of ScNup133 allowed us to annotate a potential ArfGAP1 lipid packing sensor (ALPS) motif in Sc and VpNup133 and discuss its potential significance in the context of the whole NPC; we suggest that ALPS motifs are scattered throughout the NPC's scaffold in all eukaryotes and play a major role in the assembly and membrane anchoring of the NPC in the nuclear envelope. Our results are consistent with a common evolutionary origin of Nup133 with membrane coating complexes (the protocoatomer hypothesis); the presence of the ALPS motifs in coatomer-like nucleoporins suggests an ancestral mechanism for membrane recognition present in early membrane coating complexes.

The structure of yeast glutaminyl-tRNA synthetase and modeling of its interaction with tRNA.

Eukaryotic glutaminyl-tRNA synthetase (GlnRS) contains an appended N-terminal domain (NTD) whose precise function is unknown. Although GlnRS structures from two prokaryotic species are known, no eukaryotic GlnRS structure has been reported. Here we present the first crystallographic structure of yeast GlnRS, finding that the structure of the C-terminal domain is highly similar to Escherichia coli GlnRS but that 214 residues, including the NTD, are crystallographically disordered. We present a model of the full-length enzyme in solution, using the structures of the C-terminal domain, and the isolated NTD, with small-angle X-ray scattering data of the full-length molecule. We proceed to model the enzyme bound to tRNA, using the crystallographic structures of GatCAB and GlnRS-tRNA complex from bacteria. We contrast the tRNA-bound model with the tRNA-free solution state and perform molecular dynamics on the full-length GlnRS-tRNA complex, which suggests that tRNA binding involves the motion of a conserved hinge in the NTD.

Trapping and structure determination of an intermediate in the allosteric transition of aspartate transcarbamoylase.

X-ray crystallography and small-angle X-ray scattering (SAXS) in solution have been used to show that a mutant aspartate transcarbamoylase exists in an intermediate quaternary structure between the canonical T and R structures. Additionally, the SAXS data indicate a pH-dependent structural alteration consistent with either a pH-induced conformational change or a pH-induced alteration in the T to R equilibrium. These data indicate that this mutant is not a model for the R state, as has been proposed, but rather represents the enzyme trapped along the path of the allosteric transition between the T and R states.

Atomic structure of the nuclear pore complex targeting domain of a Nup116 homologue from the yeast, Candida glabrata.

The nuclear pore complex (NPC), embedded in the nuclear envelope, is a large, dynamic molecular assembly that facilitates exchange of macromolecules between the nucleus and the cytoplasm. The yeast NPC is an eightfold symmetric annular structure composed of ~456 polypeptide chains contributed by ~30 distinct proteins termed nucleoporins. Nup116, identified only in fungi, plays a central role in both protein import and mRNA export through the NPC. Nup116 is a modular protein with N-terminal "FG" repeats containing a Gle2p-binding sequence motif and a NPC targeting domain at its C-terminus. We report the crystal structure of the NPC targeting domain of Candida glabrata Nup116, consisting of residues 882-1034 [CgNup116(882-1034)], at 1.94 Å resolution. The X-ray structure of CgNup116(882-1034) is consistent with the molecular envelope determined in solution by small-angle X-ray scattering. Structural similarities of CgNup116(882-1034) with homologous domains from Saccharomyces cerevisiae Nup116, S. cerevisiae Nup145N, and human Nup98 are discussed.

An integrated high-throughput data acquisition system for biological solution X-ray scattering studies.

A fully automated high-throughput solution X-ray scattering data collection system has been developed for protein structure studies at beamline 4-2 of the Stanford Synchrotron Radiation Lightsource. It is composed of a thin-wall quartz capillary cell, a syringe needle assembly on an XYZ positioning arm for sample delivery, a water-cooled sample rack and a computer-controlled fluid dispenser. It is controlled by a specifically developed software component built into the standard beamline control program Blu-Ice/DCS. The integrated system is intuitive and very simple to use, and enables experimenters to customize data collection strategy in a timely fashion in concert with an automated data processing program. The system also allows spectrophotometric determination of protein concentration for each sample aliquot in the beam via an in situ UV absorption spectrometer. A single set of solution scattering measurements requires a 20-30 µl sample aliquot and takes typically 3.5 min, including an extensive capillary cleaning cycle. Over 98.5% of measurements are valid and free from artefacts commonly caused by air-bubble contamination. The sample changer, which is compact and light, facilitates effortless switching with other sample-handling devices required for other types of non-crystalline X-ray scattering experiments.

Ni(II) coordination to mixed sites modulates DNA binding of HpNikR via a long-range effect.

Helicobacter pylori NikR (HpNikR) is a nickel-dependent transcription factor that regulates multiple genes in the H. pylori pathogen. There are conflicting data regarding the locations of the Ni(II) sites and the role of Ni(II) coordination in DNA recognition. Herein, we report crystal structures of (i) the metal-binding domain (MBD) of HpNikR (3.08 Å) and (ii) a mutant, H74A (2.04 Å), designed to disrupt native Ni(II) coordination. In the MBD structure, four nickel ions are coordinated to two different types of nickel sites (4-coordinate, square planar, and 5/6-coordinate, square pyramidal/octahedral). In the H74A structure, all four nickel ions are coordinated to 4-coordinate square-planar sites. DNA-binding studies reveal tighter binding for target DNA sequences for holo-HpNikR compared with the affinities of Ni(II) reconstituted apo-HpNikR and H74A for these same DNA targets, supporting a role for Ni(II) coordination to 5/6 sites in DNA recognition. Small-angle X-ray scattering studies of holo-HpNikR and H74A reveal a high degree of conformational flexibility centered at the DNA-binding domains of H74A, which is consistent with disorder observed in the crystal structure of the protein. A model of DNA recognition by HpNikR is proposed in which Ni(II) coordination to specific sites in the MBD have a long-range effect on the flexibility of the DNA-binding domains and, consequently, the DNA recognition properties.

Characterization of lipid matrices for membrane protein crystallization by high-throughput small angle X-ray scattering.

The lipidic cubic phase (LCP) has repeatedly proven to serve as a successful membrane-mimetic matrix for a variety of difficult-to-crystallize membrane proteins. While monoolein has been the predominant lipid of choice, there is a growing need for the characterization and use of other LCP host lipids, allowing exploration of a range of structural parameters such as bilayer thickness and curvature for optimal insertion, stability and crystallogenesis of membrane proteins. Here, we describe the development of a high-throughput (HT) pipeline to employ small angle X-ray scattering (SAXS) - the most direct technique to identify lipid mesophases and measure their structural parameters - to interrogate rapidly a large number of lipid samples under a variety of conditions, similar to those encountered during crystallization. Leveraging the identical setup format for LCP crystallization trials, this method allows the quickly assessment of lipid matrices for their utility in membrane protein crystallization, and could inform the tailoring of lipid and precipitant conditions to overcome specific crystallization challenges. As proof of concept, we present HT LCP-SAXS analysis of lipid samples made of monoolein with and without cholesterol, and of monovaccenin, equilibrated with solutions used for crystallization trials and LCP fluorescence recovery after photobleaching (FRAP) experiments.

Small angle X-ray scattering as a complementary tool for high-throughput structural studies.

Structural crystallography and nuclear magnetic resonance (NMR) spectroscopy are the predominant techniques for understanding the biological world on a molecular level. Crystallography is constrained by the ability to form a crystal that diffracts well and NMR is constrained to smaller proteins. Although powerful techniques, they leave many soluble, purified structurally uncharacterized protein samples. Small angle X-ray scattering (SAXS) is a solution technique that provides data on the size and multiple conformations of a sample, and can be used to reconstruct a low-resolution molecular envelope of a macromolecule. In this study, SAXS has been used in a high-throughput manner on a subset of 28 proteins, where structural information is available from crystallographic and/or NMR techniques. These crystallographic and NMR structures were used to validate the accuracy of molecular envelopes reconstructed from SAXS data on a statistical level, to compare and highlight complementary structural information that SAXS provides, and to leverage biological information derived by crystallographers and spectroscopists from their structures. All the ab initio molecular envelopes calculated from the SAXS data agree well with the available structural information. SAXS is a powerful albeit low-resolution technique that can provide additional structural information in a high-throughput and complementary manner to improve the functional interpretation of high-resolution structures.

Balanced electrostatic and structural forces guide the large conformational change associated with maturation of T = 4 virus.

Nudaurelia capensis omega virus has a well-characterized T = 4 capsid that undergoes a pH-dependent large conformational changes (LCC) and associated auto-catalytic cleavage of the subunit. We examined previously the particle size at different pH values and showed that maturation occurred at pH 5.5. We now characterized the LCC with time-resolved small-angle x-ray scattering and showed that there were three kinetic stages initiated with an incremental drop in pH: 1), a rapid (<10 ms) collapse to an incrementally smaller particle; 2), a continuous size reduction over the next 5 s; and 3), a smaller final transition occurring in 2-3 min. Equilibrium measurements similar to those reported previously, but now more precise, showed that the particle dimension between pH 5.5 and 5 requires the autocatalytic cleavage to achieve its final compact size. A balance of electrostatic and structural forces shapes the energy landscape of the LCC with the latter requiring annealing of portions of the subunit. Equilibrium experiments showed that many intermediate states could be populated with a homogeneous ensemble of particles by carefully controlling the pH. A titration curve for the LCC was generated that showed that the virtual pK(a) (i.e., the composite of all titratable residues that contribute to the LCC) is 5.8.

The origin of the electrostatic perturbation in acetoacetate decarboxylase.

Acetoacetate decarboxylase (AADase) has long been cited as the prototypical example of the marked shifts in the pK(a) values of ionizable groups that can occur in an enzyme active site. In 1966, it was hypothesized that in AADase the origin of the large pK(a) perturbation (-4.5 log units) observed in the nucleophilic Lys 115 results from the proximity of Lys 116, marking the first proposal of microenvironment effects in enzymology. The electrostatic perturbation hypothesis has been demonstrated in a number of enzymes, but never for the enzyme that inspired its conception, owing to the lack of a three-dimensional structure. Here we present the X-ray crystal structures of AADase and of the enamine adduct with the substrate analogue 2,4-pentanedione. Surprisingly, the shift of the pK(a) of Lys 115 is not due to the proximity of Lys 116, the side chain of which is oriented away from the active site. Instead, Lys 116 participates in the structural anchoring of Lys 115 in a long, hydrophobic funnel provided by the novel fold of the enzyme. Thus, AADase perturbs the pK(a) of the nucleophile by means of a desolvation effect by placement of the side chain into the protein core while enforcing the proximity of polar residues, which facilitate decarboxylation through electrostatic and steric effects.

Structures of the signal recognition particle receptor from the archaeon Pyrococcus furiosus: implications for the targeting step at the membrane.

In all organisms, a ribonucleoprotein called the signal recognition particle (SRP) and its receptor (SR) target nascent proteins from the ribosome to the translocon for secretion or membrane insertion. We present the first X-ray structures of an archeal FtsY, the receptor from the hyper-thermophile Pyrococcus furiosus (Pfu), in its free and GDP*magnesium-bound forms. The highly charged N-terminal domain of Pfu-FtsY is distinguished by a long N-terminal helix. The basic charges on the surface of this helix are likely to regulate interactions at the membrane. A peripheral GDP bound near a regulatory motif could indicate a site of interaction between the receptor and ribosomal or SRP RNAs. Small angle X-ray scattering and analytical ultracentrifugation indicate that the crystal structure of Pfu-FtsY correlates well with the average conformation in solution. Based on previous structures of two sub-complexes, we propose a model of the core of archeal and eukaryotic SRP*SR targeting complexes.

Virus capsid expansion driven by the capture of mobile surface loops.

The capsids of tailed-DNA bacteriophages first assemble as procapsids, which mature by converting into a new form that is strong enough to contain a densely packed viral chromosome. We demonstrate that the intersubunit crosslinking that occurs during maturation of HK97 capsids actually promotes the structural transformation. Small-angle X-ray scattering and crosslinking assays reveal that a shift in the crosslink pattern accompanies conversion of a semimature particle, Expansion Intermediate-I/II, to a more mature state, Balloon. This transition occurs in a switch-like fashion. We find that crosslink formation shifts the global conformational balance to favor the balloon state. A pseudoatomic model of EI-I/II derived from cryo-EM provides insight into the relationship between crosslink formation and conformational switching.

Time evolution of the quaternary structure of Escherichia coli aspartate transcarbamoylase upon reaction with the natural substrates and a slow, tight-binding inhibitor.

Here, we present a study of the conformational changes of the quaternary structure of Escherichia coli aspartate transcarbamoylase, as monitored by time-resolved small-angle X-ray scattering, upon combining with substrates, substrate analogs, and nucleotide effectors at temperatures between 5 and 22 degrees C, obviating the need for ethylene glycol. Time-resolved small-angle X-ray scattering time courses tracking the T-->R structural change after mixing with substrates or substrate analogs appeared to be a single phase under some conditions and biphasic under other conditions, which we ascribe to multiple ligation states producing a time course composed of multiple rates. Increasing the concentration of substrates up to a certain point increased the T-->R transition rate, with no further increase in rate beyond that point. Most strikingly, after addition of N-phosphonacetyl-l-aspartate to the enzyme, the transition rate was more than 1 order of magnitude slower than with the natural substrates. These results on the homotropic mechanism are consistent with a concerted transition between structural and functional states of either low affinity, low activity or high affinity, high activity for aspartate. Addition of ATP along with the substrates increased the rate of the transition from the T to the R state and also decreased the duration of the R-state steady-state phase. Addition of CTP or the combination of CTP/UTP to the substrates significantly decreased the rate of the T-->R transition and caused a shift in the enzyme population towards the T state even at saturating substrate concentrations. These results on the heterotropic mechanism suggest a destabilization of the T state by ATP and a destabilization of the R state by CTP and CTP/UTP, consistent with the T and R state crystallographic structures of aspartate transcarbamoylase in the presence of the heterotropic effectors.

A covalent linker allows for membrane targeting of an oxylipin biosynthetic complex.

A naturally occurring bifunctional protein from Plexaura homomalla links sequential catalytic activities in an oxylipin biosynthetic pathway. The C-terminal lipoxygenase (LOX) portion of the molecule catalyzes the transformation of arachidonic acid (AA) to the corresponding 8 R-hydroperoxide, and the N-terminal allene oxide synthase (AOS) domain promotes the conversion of the hydroperoxide intermediate to the product allene oxide (AO). Small-angle X-ray scattering data indicate that in the absence of a covalent linkage the two catalytic domains that transform AA to AO associate to form a complex that recapitulates the structure of the bifunctional protein. The SAXS data also support a model for LOX and AOS domain orientation in the fusion protein inferred from a low-resolution crystal structure. However, results of membrane binding experiments indicate that covalent linkage of the domains is required for Ca (2+)-dependent membrane targeting of the sequential activities, despite the noncovalent domain association. Furthermore, membrane targeting is accompanied by a conformational change as monitored by specific proteolysis of the linker that joins the AOS and LOX domains. Our data are consistent with a model in which Ca (2+)-dependent membrane binding relieves the noncovalent interactions between the AOS and LOX domains and suggests that the C2-like domain of LOX mediates both protein-protein and protein-membrane interactions.

Integration of small-angle X-ray scattering data into structural modeling of proteins and their assemblies.

A major challenge in structural biology is to determine the configuration of domains and proteins in multidomain proteins and assemblies, respectively. All available data should be considered to maximize the accuracy and precision of these models. Small-angle X-ray scattering (SAXS) efficiently provides low-resolution experimental data about the shapes of proteins and their assemblies. Thus, we integrated SAXS profiles into our software for modeling proteins and their assemblies by satisfaction of spatial restraints. Specifically, we modeled the quaternary structures of multidomain proteins with structurally defined rigid domains as well as quaternary structures of binary complexes of structurally defined rigid proteins. In addition to SAXS profiles and the component structures, we used stereochemical restraints and an atomic distance-dependent statistical potential. The scoring function is optimized by a biased Monte Carlo protocol, including quasi-Newton and simulated annealing schemes. The final prediction corresponds to the best scoring solution in the largest cluster of many independently calculated solutions. To quantify how well the quaternary structures are determined based on their SAXS profiles, we used a benchmark of 12 simulated examples as well as an experimental SAXS profile of the homotetramer D-xylose isomerase. Optimization of the SAXS-dependent scoring function generally results in accurate models if sufficiently precise approximations for the constituent rigid bodies are available; otherwise, the best scoring models can have significant errors. Thus, SAXS profiles can play a useful role in the structural characterization of proteins and assemblies if they are combined with additional data and used judiciously. Our integration of a SAXS profile into modeling by satisfaction of spatial restraints will facilitate further integration of different kinds of data for structure determination of proteins and their assemblies.

Detection of intermediates and kinetic control during assembly of bacteriophage P22 procapsid.

Bacteriophage P22 serves as a model for the assembly and maturation of other icosahedral double-stranded DNA viruses. P22 coat and scaffolding proteins assemble in vitro into an icosahedral procapsid, which then expands during DNA packaging (maturation). Efficient in vitro assembly makes this system suitable for design and production of monodisperse spherical nanoparticles (diameter approximately 50 nm). In this work, we explore the possibility of controlling the outcome of assembly by scaffolding protein engineering. The scaffolding protein exists in monomer-dimer-tetramer equilibrium. We address the role of monomers and dimers in assembly by using three different scaffolding proteins with altered monomer-dimer equilibrium (weak dimer, covalent dimer, monomer). The progress and outcome of assembly was monitored by time-resolved X-ray scattering, which allowed us to distinguish between closed shells and incomplete assembly intermediates. Binding of scaffolding monomer activates the coat protein for assembly. Excess dimeric scaffolding protein resulted in rapid nucleation and kinetic trapping yielding incomplete shells. Addition of monomeric wild-type scaffold with excess coat protein completed these metastable shells. Thus, the monomeric scaffolding protein plays an essential role in the elongation phase by activating the coat and effectively lowering its critical concentration for assembly.

Use of L-asparagine and N-phosphonacetyl-L-asparagine to investigate the linkage of catalysis and homotropic cooperativity in E. coli aspartate transcarbomoylase.

The mechanism of domain closure and the allosteric transition of Escherichia coli aspartate transcarbamoylase (ATCase) are investigated using L-Asn, in the presence of carbamoyl phosphate (CP), and N-phosphonacetyl-L-asparagine (PASN). ATCase was found to catalyze the carbamoylation of L-Asn with a K(m) of 122 mM and a maximal velocity 10-fold lower than observed with the natural substrate, L-Asp. As opposed to L-Asp, no cooperativity was observed with respect to L-Asn. Time-resolved small-angle X-ray scattering (SAXS) and fluorescence experiments revealed that the combination of CP and L-Asn did not convert the enzyme from the T to the R state. PASN was found to be a potent inhibitor of ATCase exhibiting a K(D) of 8.8 microM. SAXS experiments showed that PASN was able to convert the entire population of molecules to the R state. Analysis of the crystal structure of the enzyme in the presence of PASN revealed that the binding of PASN was similar to that of the R-state complex of ATCase with N-phosphonaceyl-L-aspartate, another potent inhibitor of the enzyme. The linking of CP and L-Asn into one molecule, PASN, correctly orients the asparagine moiety in the active site to induce domain closure and the allosteric transition. This entropic effect allows for the high affinity binding of PASN. However, the binding of L-Asn, in the presence of a saturating concentration of CP, does not induce the closure of the two domains of the catalytic chain, nor does the enzyme undergo the transition to the high-activity high- affinity R structure. These results imply that Arg229, which interacts with the beta-carboxylate of L-Asp, plays a critical role in the orientation of L-Asp in the active site and demonstrates the requirement of the beta-carboxylate of L-Asp in the mechanism of domain closure and the allosteric transition in E. coli ATCase.