Christina Redfield

A camelid antibody fragment inhibits the formation of amyloid fibrils by human lysozyme.

Amyloid diseases are characterized by an aberrant assembly of a specific protein or protein fragment into fibrils and plaques that are deposited in various organs and tissues, often with serious pathological consequences. Non-neuropathic systemic amyloidosis is associated with single point mutations in the gene coding for human lysozyme. Here we report that a single-domain fragment of a camelid antibody raised against wild-type human lysozyme inhibits the in vitro aggregation of its amyloidogenic variant, D67H. Our structural studies reveal that the epitope includes neither the site of mutation nor most residues in the region of the protein structure that is destabilized by the mutation. Instead, the binding of the antibody fragment achieves its effect by restoring the structural cooperativity characteristic of the wild-type protein. This appears to occur at least in part through the transmission of long-range conformational effects to the interface between the two structural domains of the protein. Thus, reducing the ability of an amyloidogenic protein to form partly unfolded species can be an effective method of preventing its aggregation, suggesting approaches to the rational design of therapeutic agents directed against protein deposition diseases.

Local cooperativity in the unfolding of an amyloidogenic variant of human lysozyme.

Hydrogen exchange experiments monitored by NMR and mass spectrometry reveal that the amyloidogenic D67H mutation in human lysozyme significantly reduces the stability of the β-domain and the adjacent C-helix in the native structure. In addition, mass spectrometric data reveal that transient unfolding of these regions occurs with a high degree of cooperativity. This behavior results in the occasional population of a partially structured intermediate in which the three α-helices that form the core of the α-domain still have native-like structure, whereas the β-domain and C-helix are simultaneously substantially unfolded. This finding suggests that the extensive intermolecular interactions that will be possible in such a species are likely to initiate the aggregation events that ultimately lead to the formation of the well-defined fibrillar structures observed in the tissues of patients carrying this mutation in the lysozyme gene.

A conserved face of the Jagged/Serrate DSL domain is involved in Notch trans -activation and cis -inhibition

The Notch receptor and its ligands are key components in a core metazoan signaling pathway that regulates the spatial patterning, timing and outcome of many cell-fate decisions. Ligands contain a disulfide-rich Delta/Serrate/LAG-2 (DSL) domain required for Notch trans-activation or cis-inhibition. Here we report the X-ray structure of a receptor binding region of a Notch ligand, the DSL-EGF3 domains of human Jagged-1 (J-1DSL-EGF3). The structure reveals a highly conserved face of the DSL domain, and we show, by functional analysis of Drosophila melanogster ligand mutants, that this surface is required for both cis- and trans-regulatory interactions with Notch. We also identify, using NMR, a surface of Notch-1 involved in J-1DSL-EGF3 binding. Our data imply that cis- and trans-regulation may occur through the formation of structurally distinct complexes that, unexpectedly, involve the same surfaces on both ligand and receptor.

A refined solution structure of hen lysozyme determined using residual dipolar coupling data.

A high resolution NMR structure of hen lysozyme has been determined using 209 residual 1H–15N dipolar coupling restraints from measurements made in two different dilute liquid crystalline phases (bicelles) in conjunction with a data set of 1632 NOE distance restraints, 110 torsion angle restraints, and 60 hydrogen bond restraints. The ensemble of 50 low‐energy calculated structures has an average backbone RMSD of 0.50±0.13Å to the mean structure and of 1.49±0.10Å to the crystal structure of hen lysozyme. To assess the importance of the dipolar coupling data in the structure determination, the final structures are compared with an ensemble calculated using an identical protocol but excluding the dipolar coupling restraints. The comparison shows that structures calculated with the dipolar coupling data are more similar to the crystal structure than those calculated without, and have better stereochemical quality. The structures also show improved quality factors when compared with additional dipolar coupling data that were not included in the structure calculations, with orientation‐dependent 15N chemical shift changes measured in the bicelle solutions, and with T1/T2 values obtained from 15N relaxation measurements. Analysis of the ensemble of NMR structures and comparisons with crystal structures, 15N relaxation data, and molecular dynamics simulations of hen lysozyme provides a detailed description of the solution structure of this protein and insights into its dynamical behavior.

α-Lactalbumin forms a compact molten globule in the absence of disulfide bonds

Human α-lactalbumin (α-LA) is a four disulfide-bonded protein that adopts partially structured conformations under a variety of mildly denaturing conditions. At low pH, the protein is denatured but compact, with a high degree of secondary structure and a native-like fold. This is commonly referred to as a molten globule. A variant of α-LA, in which all eight cysteines have been mutated to alanine (all-Ala α-LA), has been studied using NMR spectroscopy. At low pH all-Ala α-LA is nearly as compact as wild type α-LA. Urea-induced unfolding experiments reveal that the residues that remain compact in the absence of disulfide bonds are those that are most resistant to unfolding in the wild-type α-LA molten globule. This is particularly remarkable because this stable core is formed by segments of the polypeptide chain from both the N- and C-termini. These results show that the overall architecture of the protein fold of α-LA is determined by the polypeptide sequence itself, and not as the result of cross-linking by disulfide bonds, and provide insight into the way in which the sequence codes for the fold.

The Interaction of the Molecular Chaperone α-Crystallin with Unfolding α-Lactalbumin: A Structural and Kinetic Spectroscopic Study

The unfolding of the apo and holo forms of bovine α-lactalbumin (α-LA) upon reduction by dithiothreitol (DTT) in the presence of the small heat-shock protein α-crystallin, a molecular chaperone, has been monitored by visible and UV absorption spectroscopy, mass spectrometry and 1H NMR spectroscopy. From these data, a description and a time-course of the events that result from the unfolding of both forms of the protein, and the state of the protein that interacts with α-crystallin, have been obtained. α-LA contains four disulphide bonds and binds a calcium ion. In apo α-LA, the disulphide bonds are reduced completely over a period of ∼1500 seconds. Fully reduced α-LA adopts a partly folded, molten globule conformation that aggregates and, ultimately, precipitates. In the presence of an equivalent mass of α-crystallin, this precipitation can be prevented via complexation with the chaperone. α-Crystallin does not interfere with the kinetics of the reduction of disulphide bonds in apo α-LA but does stabilise the molten globule state. In holo α-LA, the disulphide bonds are less accessible to DTT, because of the stabilisation of the protein by the bound calcium ion, and reduction occurs much more slowly. A two-disulphide intermediate aggregates and precipitates rapidly. Its precipitation can be prevented only in the presence of a 12-fold mass excess of α-crystallin. It is concluded that kinetic factors are important in determining the efficiency of the chaperone action of α-crystallin. It interacts efficiently with slowly aggregating, highly disordered intermediate (molten globule) states of α-LA. Real-time NMR spectroscopy shows that the kinetics of the refolding of apo α-LA following dilution from denaturant are not affected by the presence of α-crystallin. Thus, α-crystallin is not a chaperone that is involved in protein folding per se. Rather, its role is to stabilise compromised, partly folded, molten globule states of proteins that are destined for precipitation.

Measurement of the individual pKa values of acidic residues of hen and turkey lysozymes by two-dimensional 1H NMR.

The pH dependence of the two-dimensional 1H nuclear magnetic resonance spectra of hen and turkey egg-white lysozymes has been recorded over the pH range 1-7. By monitoring the chemical shifts of the resonances of the various protons of ionizable residues, individual pKa values for the acidic residues have been determined for both proteins. The pKa values are displaced, with the exception of those of the residues in the active site cleft, by an average of 1 unit to low pH compared to model compounds.

A nuclear magnetic resonance study of the hydrogen-exchange behaviour of lysozyme in crystals and solution

Amide hydrogen/deuterium exchange behaviour has been studied for all of the peptide amides of hen lysozyme by means of two-dimensional n.m.r. spectroscopy. The amides have been grouped into four categories on the basis of their rates of exchange in solution at pH 4.2 and 7.5. The distribution of the amides into the different categories has been examined in the light of the crystallographic structural information, considering the type of secondary structure, the nature of hydrogen bonding and the distance from the protein surface. None of these features was found to determine uniquely the pattern of hydrogen exchange rates within the protein. The exchange behaviour of the individual amides could, however, in general be rationalized by a combination of these features. Hydrogen exchange was also monitored in both tetragonal and triclinic crystals of lysozyme, by allowing exchange to take place in the crystals prior to dissolution and recording of n.m.r. spectra under conditions where further exchange was minimized. This enabled direct comparison to be made of the exchange behaviour in the crystals and solution. A reduction in exchange rate was observed in the crystalline state relative to solution for a substantial number of amides and distinct differences between exchange in the different crystals could be observed. These differences between the solution and the different crystal states do not, however, correlate in a simple manner with proximity to intermolecular contacts in the crystals. However, the existence of these contacts, which are on the surface of the protein molecule, have a profound effect on the exchange of amides in the interior of the protein. The results indicate that the spectrum of fluctuations giving rise to hydrogen exchange may be significantly altered by the intermolecular interactions present within the crystalline state.

Engineering a Camelid Antibody Fragment That Binds to the Active Site of Human Lysozyme and Inhibits Its Conversion into Amyloid Fibrils

A single-domain fragment, cAb-HuL22, of a camelid heavy-chain antibody specific for the active site of human lysozyme has been generated, and its effects on the properties of the I56T and D67H amyloidogenic variants of human lysozyme, which are associated with a form of systemic amyloidosis, have been investigated by a wide range of biophysical techniques. Pulse-labeling hydrogen-deuterium exchange experiments monitored by mass spectrometry reveal that binding of the antibody fragment strongly inhibits the locally cooperative unfolding of the I56T and D67H variants and restores their global cooperativity to that characteristic of the wild-type protein. The antibody fragment was, however, not stable enough under the conditions used to explore its ability to perturb the aggregation behavior of the lysozyme amyloidogenic variants. We therefore engineered a more stable version of cAb-HuL22 by adding a disulfide bridge between the two beta-sheets in the hydrophobic core of the protein. The binding of this engineered antibody fragment to the amyloidogenic variants of lysozyme inhibited their aggregation into fibrils. These findings support the premise that the reduction in global cooperativity caused by the pathogenic mutations in the lysozyme gene is the determining feature underlying their amyloidogenicity. These observations indicate further that molecular targeting of enzyme active sites, and of protein binding sites in general, is an effective strategy for inhibiting or preventing the aberrant self-assembly process that is often a consequence of protein mutation and the origin of pathogenicity. Moreover, this work further demonstrates the unique properties of camelid single-domain antibody fragments as structural probes for studying the mechanism of aggregation and as potential inhibitors of fibril formation.

Selective small molecule inhibitors of the potential breast cancer marker, human arylamine N-acetyltransferase 1, and its murine homologue, mouse arylamine N-acetyltransferase 2.

The identification, synthesis and evaluation of a series of rhodanine and thiazolidin-2,4-dione derivatives as selective inhibitors of human arylamine N-acetyltransferase 1 and mouse arylamine N-acetyltransferase 2 is described. The most potent inhibitors identified have submicromolar activity and inhibit both the recombinant proteins and human NAT1 in ZR-75 cell lysates in a competitive manner. (1)H NMR studies on purified mouse Nat2 demonstrate that the inhibitors bind within the putative active site of the enzyme.