A1 Domain Research Articles

Identification and Immunological Characterization of the Ligand Domain of Plasmodium vivax Reticulocyte Binding Protein 1a.

Erythrocyte invasion by malaria parasites is essential for blood-stage development. Consequently, parasite proteins critically involved in erythrocyte invasion, such as the Plasmodium vivax reticulocyte binding proteins (RBPs) that mediate preferential invasion of reticulocytes, are considered potential vaccine targets. Thus, targeting the RBPs could prevent blood-stage infection and disease. The RBPs are large, and little is known about their functional domains and whether individuals naturally exposed to P. vivax acquire binding-inhibitory antibodies to these critical binding regions. This study aims to functionally and immunologically characterize Plasmodium vivax RBP1a. Recombinant proteins of overlapping fragments of RBP1a were used to determine binding specificity to erythrocytes and immunogenicity in laboratory animals. The naturally acquired antibody response to these proteins was evaluated using serum samples from individuals in regions of endemicity. The N-terminal extracellular region, RBP1157-650 (RBP1:F8), was determined to bind both reticulocytes and normocytes, with a preference for immature reticulocytes. Antibodies elicited against rRBP1:F8 blocked binding between RBP1:F8 and erythrocytes. Naturally acquired anti-RBP1 binding-inhibitory antibodies were detected in serum specimens from P. vivax-exposed individuals from Papua New Guinea and Brazil. Recombinant RBP1:F8 binds human erythrocytes, elicits artificially induced functional blocking antibodies, and is a target of naturally acquired binding-inhibitory antibodies.

Abstract 039: A Small Fragment Derived from von Willebrand Factor Improves Survival in a Mouse Model of Endotoxemia-Induced Disseminated Intravascular Coagulation

Introduction: Disseminated intravascular coagulation (DIC) is a life-threatening condition that results in wide-spread clot formation and organ failure. Sepsis-induced DIC is associated with a high mortality rate, prompting the need for an effective therapeutic intervention. Our lab has identified the A2 domain of the von Willebrand factor (VWF) protein as a potent treatment for sepsis-induced DIC. Historically, VWF has only been described as a contributor of clot formation; however, our lab has demonstrated that purified recombinant full length A2 domain of the VWF protein can dampen fibrin-rich microthrombi formation in mice with endotoxin. The mode of action appears to be via the interaction with the cell-surface expressed vimentin on endothelium, inhibiting VWF strings formation and thus, platelet adhesion. Goal: Since the VWF protein is naturally cleaved in circulation by the enzyme ADAMTS-13 via the A2 domain, one can argue whether the beneficial effect observed with full len gth A2 domain is terminated by the cleavage of ADAMTS-13. We produced recombinant proteins encompassing the sequences of the two proteolytic products of the A2 domain; the A2Nt and A2Ct proteins. I will identify which fragment retains the mode of action of the full length A2 domain in a mouse model of endotoxin-induced DIC. Methods: The recombinant A2 and vimentin proteins were produced using the bacterial expression system. Protein: protein interactions were determined by optical interferometry. The endotoxin-induced DIC model was achieved by injecting C57B/6 mice with lipopolysaccharide (LPS). Treatment with A2 or its fragments was started approximately 2 hours after LPS insult. Results: The A2N t protein has a higher affinity for vimentin than the A2Ct. Treatment of A2Nt in a LPS-induced DIC mouse model was effective in maintaining survival in males, a comparable finding that mimics treatment of mice with full-length recombinant A2. Interestingly, A2Nt showed less survival than full length A2 in females. These findings highlight the potential differences in therapy responses between genders, an occurrence that some studies have described to also occur in humans.

Prediction of spacer-\u03b16 complex: a novel insight into binding of ADAMTS13 with A2 domain of von Willebrand factor under forces

Force-regulated cleavage of A2 domain of von Willebrand factor (vWF) by ADAMTS13 is a key event in preventing thrombotic thrombocytopenic purpura (TTP). Recognition and cleavage depend on cooperative and modular contacts between several ADAMTS13 subdomains and discrete segments of vWF A2 domain. Spacer domain of ADAMTS13 contains an important exosite interacting with α6 helix of unfold A2 domain, but it remains unclear whether stretching of α6 regulates binding to spacer. To understand the molecular mechanism underlying the interactions between spacer and α6 under stretching, we successfully predicted spacer-α6 complex by a novel computer strategy combined the steered molecular dynamics (SMD) and flexible docking techniques. This strategy included three steps: (1) constant-velocity SMD simulation of α6; (2) zero-velocity SMD simulations of α6, and (3) flexible dockings of α6 to spacer. In our spacer-α6 complex model, 13 key residues, six in α6 and seven in spacer, were identified. Our data demonstrated a biphasic extension-regulated binding of α6 to spacer. The binding strength of the complex increased with α6 extension until it reaches its optimum of 0.25 nm, and then decreased as α6 extension further increased, meaning that spacer is in favor to binding with a partially extended α6, which may contribute to the optimal contact and proteolysis. Changes of interface area and intermolecular salt bridge may serve as the molecular basis for this characteristic. These findings provide a novel insight into mechano-chemical regulation on interaction between ADAMTS13 and vWF A2 domain under forces.

Contribution of Factor VIII A2 Domain Residues 400-409 to a Factor X-Interactive Site in the Factor Xase Complex.

The link between factor (F)VIII and FX is essential for optimum activity of the tenase complex. The interactive site(s) in FVIII for FX remains to be completely clarified, however. We investigated the FVIII A2 domain-FX association that was speculated from inhibitory mechanism(s) by an anti-A2 autoantibody. SDS-PAGE demonstrated that the purified inhibitor IgG recognizing residues 373-562 blocked FXa cleavage at Arg372 in FVIII, and surface-plasmon resonance (SPR)-based assays showed that intact A2 subunit directly bound to FX (Kd; 63 nM). The FVIII structure model indicated possible FX-binding site(s) in residues 400-429 in A2. One peptide corresponding to residues 400-409 competitively inhibited both the A2-FX binding and FVIIIa/FIXa-dependent FXa generation. Covalent cross-linking was observed between this peptide and FX following reaction with EDC (1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide) using SDS-PAGE. K408 and S409 were not evident in N-terminal sequence analysis of the cross-linked product, suggesting that two residues participated in cross-link formation. SPR-based assays using recombinant FVIII mutants with one or both residues substituted to alanine demonstrated that K408A and K408A/S409A had approximately fourfold high Kd values of wild-type (WT-)FVIII. FXa cleavages at Arg372 in both mutants were significantly delayed, suggesting a contribution of K408 for FXa cleavage at Arg372. Furthermore, FXa generation assays with these mutants demonstrated that the Km values were 1.4- to 1.7-fold greater, and overall catalytic efficiency (kcat/Km) was 0.49- to 0.89-fold lower than with WT-FVIII, suggesting a significant contribution of K408 for FVIII-FX interaction in tenase assembly. We concluded that the K408 in the A2 domain provided an interactive-site for FX.

Describing the molecular mechanism by which the A2 protein improves survival in mice with endotoxemia.

Von Willebrand factor (VWF) is an essential protein that mediates the platelet adhesion to the exposed subendothelium surface and significantly contributes to arterial thrombosis and stroke. Secreted VWF from the stimulated endothelium as long strings remain anchored to the surface of the endothelial cells and interact with circulating platelets, leading to vessel occlusion. We have shown that the isolated A2 domain of VWF (A2 protein) contains bind activity for the A1 domain, vimentin, and fibrin(ogen). In fact, as a result of those interactions, the A2 protein impairs platelet adhesion to VWF and fibrin(ogen), formation of VWF strings on the endothelium, and fibrin polymerization. The A2 protein was effective in preventing fibrin‐rich microvascular thrombosis and improving survival (100%) of mice with endotoxemia (LPS)‐induced disseminated intravascular coagulation (DIC). Thus, a goal was to identify the binding sites of A1 domain, vimentin, and fibrin in the A2 protein and dissect the mode of action by which the A2 protein recuperates the sick animals. To determine the amino acid residues that are important in the recognition for vimentin, A1 and fibrin, I used the crystal structure of the A2 domain as a guide for point mutations. Two mutations, E1523A and E1567A were introduced in the A2 protein. The purified mutants were tested for their binding capacity for the ligands using ELISA. Moreover, the A2 mutants were tested in mice with endotoxemia (LPS). We are now demonstrating that E1523A mutation had a reduced binding affinity for Vimentin and A1 domain. Strikingly, E1567A and E1523A mutations significantly abolished the binding to fibrin(ogen). Importantly, both mutations failed to replicate the beneficial effect of the wild type A2 protein in improving survival of mice with LPS‐induced DIC. Together, these findings suggest that the A2 protein impairs microvascular thrombosis in mice with endotoxemia mainly via its interaction with fibrin.Support or Funding InformationThis research was supported by funding provided by the following NIH Diversity Supplement: R01GM112806‐0351.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

"Cooperative collapse" of the denatured state revealed through Clausius-Clapeyron analysis of protein denaturation phase diagrams.

Protein phase diagrams have a unique potential to identify the presence of additional thermodynamic states even when non-2-state character is not readily apparent from the experimental observables used to follow protein unfolding transitions. Two-state analysis of the von Willebrand factor A3 domain has previously revealed a discrepancy in the calorimetric enthalpy obtained from thermal unfolding transitions as compared with Gibbs-Helmholtz analysis of free energies obtained from the Linear Extrapolation Method (Tischer and Auton, Prot Sci 2013; 22(9):1147-60). We resolve this thermodynamic conundrum using a Clausius-Clapeyron analysis of the urea-temperature phase diagram that defines how and the urea m-value interconvert through the slope of cm versus T, . This relationship permits the calculation of at low temperature from m-values obtained through iso-thermal urea denaturation and high temperature m-values from obtained through iso-urea thermal denaturation. Application of this equation uncovers sigmoid transitions in both cooperativity parameters as temperature is increased. Such residual thermal cooperativity of and the m-value confirms the presence of an additional state which is verified to result from a cooperative phase transition between urea-expanded and thermally-compact denatured states. Comparison of the equilibria between expanded and compact denatured ensembles of disulfide-intact and carboxyamidated A3 domains reveals that introducing a single disulfide crosslink does not affect the presence of the additional denatured state. It does, however, make a small thermodynamically favorable free energy (∼-13 ± 1 kJ/mol) contribution to the cooperative denatured state collapse transition as temperature is raised and urea concentration is lowered. The thermodynamics of this "cooperative collapse" of the denatured state retain significant compensations between the enthalpy and entropy contributions to the overall free energy.

Nuclear Lamina Stress Measured with FRET Based Stress Sensor

Cellular stress transmitted to the nuclear lamina can modulate gene expression and affect DNA repair. While static cell-type dependent nuclear stiffness differences have been measured, the dynamic changes in nuclear stress that occur during the cell cycle, cell adhesion and differentiation have been more difficult to determine. To measure dynamic changes in nuclear stress we created chimeric Lamin A and B expression vectors with the fluorescent cpstFRET stress sensing probe inserted into the linker region between helical rod domains 1B and 2A. C2C12 myoblast cells expressing approximately a 1:1 ratio of exogenous:endogenous lamin A progress through the cell cycle and differentiate into multinucleated myotubes. Lamin stress increased depending on cell stiffness as shown when expressed in cells ranging from soft HEK cells to stiffer C2C12 myoblasts and differentiated myotubes. Lamin showed higher variability in stress compared to other cytoskeletal proteins, but this variation was reduced in differentiated myotubes. Monitoring nuclear stress during the cell cycle shows that lamin stress transiently decreases during G1-phase followed by stress recovery during S-phase suggesting the cell cycle contributes to stress variations. Lamin stress increases rapidly when trypsinized cells are deposited onto glass coated with different extracellular matrix proteins, but the kinetics were sensitive to the substrate chemistry. The stress on lamin increased when isolated nuclei were swollen by cation depletion and were consistent with probe sensitivity in the pN range. Aspiration of isolated nuclei showed inconsistent stress changes, possibly due to lamina rupture and plastic properties from lamin interactions with the nuclear membrane and the DNA. The magnitude of these stress changes was significantly muted when the cpstFRET cassette was linked to the C-terminus. This new probe will prove useful for correlating lamina stress changes associated cell division, differentiation and pathology.

Combined effects of two mutations in von Willebrand disease 2M phenotype

Unlabelled Box BackgroundType 2M von Willebrand disease (VWD2M) is usually characterized by VWF:RCo/VWF:Ag<0.6 and normal multimeric profile; desmopressin (DDAVP) challenge test commonly shows poor response of VWF:RCo. ObjectiveWe describe the bleeding tendency and the laboratory phenotype in a patient carrying two heterozygous mutations affecting VWF‐A1 domain and VWF‐A2 domain. Subjects/methodsA 12‐year‐old patient (O blood group) with severe hemorrhagic tendency was phenotypically and genotypically analyzed; his parents were also studied. ResultsThe proband showed decrease FVIII:C, VWF:RCo/VWF:Ag, and VWF:CB6/VWF:Ag ratios, but normal platelet count, VWF:CB1/VWF:Ag ratio, VWFpp and multimeric pattern, suggesting a VWD2M phenotype. The DDAVP challenge test, compared to controls (VWD2M patients with mutations in VWF‐A1 domain), showed lower increase of FVIII:C and VWF:Ag than in heterozygous, but very similar to homozygous control. Two mutations were found in heterozygous and trans presentation: p.Pro1648fs*45 and a novel missense mutation, p.Arg1426Cys. The mother was p.Arg1426Cys heterozygous carrier, with few clinical symptoms. The father was asymptomatic, with no mutations. The p.Pro1648fs*45 was considered an apparent de novo mutation; proband's AS‐PCR revealed mosaicism in the paternal allele. According to the predicted models, p.Arg1426Cys would not be affecting the binding of GPIbα to A1 domain, whereas p.Pro1648fs*45 seems to modify the folding of A2 domain, and in this way, it would affect the binding to GPIbα and type VI collagen. We believe that the combination of these two heterozygous mutations, in a child with O blood group, could result in a defective phenotype enhancer.

The effect of mismatch strain induced by the a1–a2 domain pattern on dislocation generation of epitaxial ferroelectric films

This study theoretically models both mismatch strain and lattice dislocation generation induced by the a1–a2 domain pattern in epitaxial ferroelectric films. For the ferroelectric film/substrate structure, the precise complex potential functions for the Somigliana screw dislocation quadrupole that represents the lattice mismatch strain are presented, the free boundary condition of the film is approximately processed by an equivalent method, and the effect of mismatch strain on lattice dislocation generation is discussed. The results show that: the higher mismatch degree and domain wall energy evidently lead to interfacial dislocation multiplication; the density of the dislocations decreases with decreasing film thickness; the generated dislocations are mainly concentrated at the sites far away from domain walls.

High resolution crystal structures of Clostridium botulinum neurotoxin A3 and A4 binding domains.

Clostridium botulinum neurotoxins (BoNTs) cause the life-threatening condition, botulism. However, while they have the potential to cause serious harm, they are increasingly being utilised for therapeutic applications. BoNTs comprise of seven distinct serotypes termed BoNT/A through BoNT/G, with the most widely characterised being sub-serotype BoNT/A1. Each BoNT consists of three structurally distinct domains, a binding domain (HC), a translocation domain (HN), and a proteolytic domain (LC). The HC domain is responsible for the highly specific targeting of the neurotoxin to neuronal cell membranes. Here, we present two high-resolution structures of the binding domain of subtype BoNT/A3 (HC/A3) and BoNT/A4 (HC/A4) at 1.6 Å and 1.34 Å resolution, respectively. The structures of both proteins share a high degree of similarity to other known BoNT HC domains whilst containing some subtle differences, and are of benefit to research into therapeutic neurotoxins with novel characteristics.

Interaction of von Willebrand factor domains with collagen investigated by single molecule force spectroscopy.

von Willebrand factor (VWF) is a huge multimeric protein that plays a key role in primary hemostasis. Sites for collagen binding, an initial event of hemostasis, are located in the VWF-domains A1 and A3. In this study, we investigated single molecule interactions between collagen surfaces and wild type VWF A1A2A3 domain constructs, as well as clinically relevant VWF A3 domain point mutations, such as p.Ser1731Thr, p.Gln1734His, and p.His1786Arg. For this, we utilized atomic force microscopy based single molecular force spectroscopy. The p.Ser1731Thr mutant had no impact on the VWF-collagen type III and VI interactions, while the p.Gln1734His and p.His1786Arg mutants showed a slight increase in bond stability to collagen type III. This effect probably arises from additional hydrogen bonds that come along with the introduction of these mutations. Using the same mutants, but collagen type VI as a binding partner, resulted in a significant increase in bond stability. VWF domain A1 was reported to be essential for the interaction with collagen type VI and thus our findings strengthen the hypothesis that the VWF A1 domain can compensate for mutations in the VWF A3 domain. Additionally, our data suggest that the mutations could even stabilize the interaction between VWF and collagen without shear. VWF-collagen interactions seem to be an important system in which defective interactions between one VWF domain and one type of collagen can be compensated by alternative binding events.

Gene Therapy with BMN 270 Results in Therapeutic Levels of FVIII in Mice and Primates and Normalization of Bleeding in Hemophilic Mice.

Hemophilia A is an X-linked bleeding disorder caused by mutations in the gene encoding the factor VIII (FVIII) coagulation protein. Bleeding episodes in patients are reduced by prophylactic therapy or treated acutely using recombinant or plasma-derived FVIII. We have made an adeno-associated virus 5 vector containing a B domain-deleted (BDD) FVIII gene (BMN 270) with a liver-specific promoter. BMN 270 injected into hemophilic mice resulted in a dose-dependent expression of BDD FVIII protein and a corresponding correction of bleeding time and blood loss. At the highest dose tested, complete correction was achieved. Similar corrections in bleeding were observed at approximately the same plasma levels of FVIII protein produced either endogenously by BMN 270 or following exogenous administration of recombinant BDD FVIII. No evidence of liver dysfunction or hepatocyte endoplasmic reticulum stress was observed. Comparable doses in primates produced similar levels of circulating FVIII. These preclinical data support evaluation of BMN 270 in hemophilia A patients.

Identification of extant vertebrate Myxine glutinosa VWF: evolutionary conservation of primary hemostasis

AbstractHemostasis in vertebrates involves both a cellular and a protein component. Previous studies in jawless vertebrates (cyclostomes) suggest that the protein response, which involves thrombin-catalyzed conversion of a soluble plasma protein, fibrinogen, into a polymeric fibrin clot, is conserved in all vertebrates. However, similar data are lacking for the cellular response, which in gnathostomes is regulated by von Willebrand factor (VWF), a glycoprotein that mediates the adhesion of platelets to the subendothelial matrix of injured blood vessels. To gain evolutionary insights into the cellular phase of coagulation, we asked whether a functional vwf gene is present in the Atlantic hagfish, Myxine glutinosa. We found a single vwf transcript that encodes a simpler protein compared with higher vertebrates, the most striking difference being the absence of an A3 domain, which otherwise binds collagen under high-flow conditions. Immunohistochemical analyses of hagfish tissues and blood revealed Vwf expression in endothelial cells and thrombocytes. Electron microscopic studies of hagfish tissues demonstrated the presence of Weibel-Palade bodies in the endothelium. Hagfish Vwf formed high-molecular-weight multimers in hagfish plasma and in stably transfected CHO cells. In functional assays, botrocetin promoted VWF-dependent thrombocyte aggregation. A search for vwf sequences in the genome of sea squirts, the closest invertebrate relatives of hagfish, failed to reveal evidence of an intact vwf gene. Together, our findings suggest that VWF evolved in the ancestral vertebrate following the divergence of the urochordates some 500 million years ago and that it acquired increasing complexity though sequential insertion of functional modules.

Results of the Randomized, Double-Blind, Placebo-Controlled, Phase 3 Hercules Study of Caplacizumab in Patients with Acquired Thrombotic Thrombocytopenic Purpura

Introduction: Acquired or immune-mediated thrombotic thrombocytopenic purpura (aTTP) is a life-threatening thrombotic microangiopathy characterized by severe thrombocytopenia, microangiopathic hemolytic anemia, and organ ischemia. Inhibitory autoantibodies cause a severe deficiency of the von Willebrand factor (vWF) cleaving enzyme ADAMTS13, leading to intravascular vWF-platelet aggregation and microvascular thrombosis. The mainstays of treatment are plasma exchange (PE) and immunosuppression. Caplacizumab, a bivalent Nanobody, targets the A1 domain of vWF, inhibiting the interaction between ultra-large vWF and platelets. Methods: Patients with an acute episode of aTTP who had received one PE treatment were randomized 1:1 to placebo or 10 mg caplacizumab, in addition to daily PE and corticosteroids. A single IV dose of study drug was given before the first on-study PE and a SC dose was given daily during the PE period and 30 days thereafter. If at the end of this period there was evidence of ongoing disease, such as suppressed ADAMTS13 activity, investigators were encouraged to extend the blinded treatment for a maximum of 4 weeks together with optimization of immunosuppression. All patients entered a 28-day treatment-free follow up period after the last dose of study drug (Figure 1). Primary endpoint was time to platelet count response, defined as platelet count ≥ 150×109/L with stop of daily PE within 5 days. There were 4 key secondary endpoints, hierarchically ranked. The 1st was a composite of aTTP-related death, aTTP recurrence, or major thromboembolic event during the study drug treatment period. A blinded, independent committee adjudicated aTTP-related deaths and major thromboembolic events. The 2nd looked at recurrences during the entire study period, including the follow up period. The 3d evaluated refractoriness to therapy, defined as absence of platelet count doubling after 4 days of treatment and LDH still above normal. The 4th was the time to normalization of 3 organ damage markers: LDH, cardiac troponin I and serum creatinine. Results: 145 patients were randomized, 73 to placebo and 72 to caplacizumab. Demographics and baseline disease characteristics were balanced between groups, except for a higher proportion of initial episodes in the caplacizumab arm. Compared to patients treated with placebo, those on caplacizumab were >50% more likely to achieve a platelet response at any given time point (platelet count normalization rate 1.55, 95% CI 1.10 - 2.20, p Conclusions: Results of the Phase 3 HERCULES study confirm that treatment with caplacizumab reduces the time to platelet count response, resulting in faster resolution of aTTP. Treatment with caplacizumab also resulted in a highly clinically meaningful reduction in aTTP-related death, recurrence of aTTP, or a major thromboembolic event during study drug treatment. The relapses after stop of study drug in patients with ADAMTS13 activity Disclosures Scully:Ablynx: Honoraria, Research Funding; Shire: Honoraria, Research Funding; Novartis: Honoraria; Alexion: Honoraria. Cataland:Ablynx NV: Membership on an entity9s Board of Directors or advisory committees. Peyvandi:Ablynx, Roche: Membership on an entity9s Board of Directors or advisory committees; Ablynx, Bayer, Grifols, Novo Nordisk, Sobi: Speakers Bureau;Freeline, Kedrion, LFB, Octapharma: Consultancy. Coppo:Ablynx: Membership on an entity9s Board of Directors or advisory committees; Alexion: Membership on an entity9s Board of Directors or advisory committees; Octapharma: Membership on an entity9s Board of Directors or advisory committees; Shire: Membership on an entity9s Board of Directors or advisory committees. Knobl:Novo Nordisk: Consultancy, Research Funding; Shire: Consultancy, Research Funding. Kremer Hovinga:Baxalta/Shire: Other: unrestricted grant hereditary TTP registry; Ablynx NV: Membership on an entity9s Board of Directors or advisory committees. Metjian:Ablynx NV, Shire, Omeros: Research Funding; Shire: Membership on an entity9s Board of Directors or advisory committees. de la Rubia:Amgen: Other: Honoraria; Celgene: Other: Honoraria; Janssen: Other: Honoraria. Pavenski:Alexion Pharmaceuticals: Honoraria; Novartis: Honoraria; Ablynx: Other: participation in industry sponsored RCT; CSL Behring: Research Funding. Callewaert:Ablynx NV: Employment. Biswas:Ablynx NV: Employment. De Winter:Ablynx NV: Employment. Zeldin:Ablynx NV: Employment.

Production of recombinant human tektin 1, 2, and 4 and in vitro assembly of human tektin 1.

Proteins predicted to be composed of large stretches of coiled-coil structure have often proven difficult to crystallize for structural determination. We have successfully applied EPR spectroscopic techniques to the study of the structure and assembly of full-length human vimentin assembled into native 11 nm filaments, in physiologic solution, circumventing the limitations of crystallizing shorter peptide sequences. Tektins are a small family of highly alpha helical filamentous proteins found in the doublet microtubules of cilia and related structures. Tektins exhibit several similarities to intermediate filaments (IFs): moderate molecular weight, highly alpha helical, hypothesized to be coiled-coil, and homo- and heteromeric assembly into long smooth filaments. In this report, we show the application of IF research methodologies to the study of tektin structure and assembly. To begin in vitro studies, expression constructs for human tektins 1, 2, and 4 were synthesized. Recombinant tektins were produced in E. coli and purified by chromatography. Preparations of tektin 1 successfully formed filaments. The recombinant human tektin 1 was used to produce antibodies which recognized an antigen in mouse testes, most likely present in sperm flagella. Finally, we report the creation of seven mutants to analyze predictions of coiled-coil structure in the rod 1A domain of tektin 1. Although this region is predicted to be coiled-coil, our EPR analysis does not reflect the parallel, in register, coiled-coil structure as demonstrated in vimentin and kinesin. These results document that tektin can be successfully expressed and assembled in vitro, and that SDSL EPR techniques can be used for structural analysis.

A common mechanism by which type 2A von Willebrand disease mutations enhance ADAMTS13 proteolysis revealed with a von Willebrand factor A2 domain FRET construct.

Rheological forces in the blood trigger the unfolding of von Willebrand factor (VWF) and its A2 domain, exposing the scissile bond for proteolysis by ADAMTS13. Under quiescent conditions, the scissile bond is hidden by the folded structure due to the stabilisation provided by the structural specialisations of the VWF A2 domain, a vicinal disulphide bond, a calcium binding site and a N1574-glycan.The reduced circulating high MW multimers of VWF in patients with type 2A von Willebrand disease (VWD) may be associated with mutations within the VWF A2 domain and this is attributed to enhanced ADAMTS13 proteolysis. We investigated 11 VWF A2 domain variants identified in patients with type 2A VWD. In recombinant full-length VWF, enhanced ADAMTS13 proteolysis was detected for all of the expressed variants in the presence of urea-induced denaturation. A subset of the FLVWF variants displayed enhanced proteolysis in the absence of urea. The mechanism of enhancement was investigated using a novel VWF A2 domain FRET construct. In the absence of induced unfolding, 7/8 of the expressed mutants exhibited a disrupted domain fold, causing spatial separation of the N- and C- termini. Three of the type 2A mutants were not secreted when studied within the VWF A2 domain FRET construct. Urea denaturation revealed for all 8 secreted mutants reduced unfolding cooperativity and stability of the VWF A2 domain. As folding stability was progressively disrupted, proteolysis by ADAMTS13 increased. Due to the range of folding stabilities and wide distribution of VWF A2 domain mutations studied, we conclude that these mutations disrupt regulated folding of the VWF A2 domain. They enhance unfolding by inducing separation of N- and C-termini, thereby promoting a more open conformation that reveals its binding sites for ADAMTS13 and the scissile bond.

Specific electrostatic interactions between charged amino acid residues regulate binding of von Willebrand factor to blood platelets

The plasma protein von Willebrand factor (VWF) is essential for hemostasis initiation at sites of vascular injury. The platelet-binding A1 domain of VWF is connected to the VWF N-terminally located D'D3 domain through a relatively unstructured amino acid sequence, called here the N-terminal linker. This region has previously been shown to inhibit the binding of VWF to the platelet surface receptor glycoprotein Ibα (GpIbα). However, the molecular mechanism underlying the inhibitory function of the N-terminal linker has not been elucidated. Here, we show that an aspartate at position 1261 is the most critical residue of the N-terminal linker for inhibiting binding of the VWF A1 domain to GpIbα on platelets in blood flow. Through a combination of molecular dynamics simulations, mutagenesis, and A1-GpIbα binding experiments, we identified a network of salt bridges between Asp1261 and the rest of A1 that lock the N-terminal linker in place such that it reduces binding to GpIbα. Mutations aimed at disrupting any of these salt bridges activated binding unless the mutated residue also formed a salt bridge with GpIbα, in which case the mutations inhibited the binding. These results show that interactions between charged amino acid residues are important both to directly stabilize the A1-GpIbα complex and to indirectly destabilize the complex through the N-terminal linker.

Comparative profiling of HLA-DR and HLA-DQ associated factor VIII peptides presented by monocyte-derived dendritic cells.

The development of anti-factor VIII antibodies is a major complication of the treatment of patients with hemophilia A. Generation of high affinity anti-factor VIII antibodies is dependent on help provided by CD4+ T cells that recognize factor VIII-derived peptides presented on class II major histocompatibility complex on the surface of antigen-presenting cells. In order to identify the immune-dominant epitopes that can be presented to CD4+ T cells, we previously developed a mass spectrometry-based method to identify factor VIII-derived peptides that are presented on human leukocyte antigen (HLA)-DR. In the present work, we compared the repertoire of FVIII-derived peptide presented on HLA-DR and HLA-DQ. Monocyte-derived dendritic cells from nine HLA-typed healthy donors were pulsed with recombinant factor VIII. HLA-DR and HLA-DQ molecules were purified using monoclonal antibodies. Our data show that HLA-DQ and HLA-DR present a similar repertoire of factor VIII-derived peptides. However, the number of peptides associated with HLA-DQ was lower than that with HLA-DR. We also identified a peptide, within the acidic a3 domains of factor VIII, which is presented with higher frequency on HLA-DQ. Interestingly, this peptide was found to have a higher predicted affinity for HLA-DQ than for HLA-DR. Taken together, our data suggest that HLA-DQ participates in the presentation of factor VIII peptides, thereby contributing to the development of inhibitory antibodies in a proportion of patients with severe hemophilia A.

SH3PXD2A (SH3 and PX domains 2A)

SH3PXD2A (SH3 and PX domains 2A)

A discontinuous autoinhibitory module masks the A1 domain of von Willebrand factor

Essentials The mechanism for the auto-inhibition of von Willebrand factor (VWF) remains unclear. Hydrogen exchange of two VWF A1 fragments with disparate activities was measured and compared. Discontinuous residues flanking A1 form a structural module that blocks A1 binding to the platelet. Our results suggest a potentially unified model of VWF activation. Click to hear an ISTH Academy presentation on the domain architecture of VWF and activation by elongational flow by Dr Springer SUMMARY: Background How von Willebrand factor (VWF) senses and responds to shear flow remains unclear. In the absence of shear flow, VWF or its fragments can be induced to bind spontaneously to platelet GPIbα. Objectives To elucidate the auto-inhibition mechanism of VWF. Methods Hydrogen-deuterium exchange (HDX) of two recombinant VWF fragments expressed from baby hamster kidney cells were measured and compared. Results The shortA1 protein contains VWF residues 1261-1472 and binds GPIbα with a significantly higher affinity than the longA1 protein that contains VWF residues 1238-1472. Both proteins contain the VWF A1 domain (residues 1272-1458). Many residues in longA1, particularly those in the N- and C-terminal sequences flanking the A1 domain, and in helix α1, loops α1β2 and β3α2, demonstrated markedly reduced HDX compared with their counterparts in shortA1. The HDX-protected region in longA1 overlaps with the GPIbα-binding interface and is clustered with type 2B von Willebrand disease (VWD) mutations. Additional comparison with the HDX of denatured longA1 and ristocetin-bound longA1 indicates the N- and C-terminal sequences flanking the A1 domain form cooperatively an integrated autoinhibitory module (AIM) that interacts with the HDX-protected region. Binding of ristocetin to the C-terminal part of the AIM desorbs the AIM from A1 and enables longA1 binding to GPIbα. Conclusion The discontinuous AIM binds the A1 domain and prevents it from binding to GPIbα, which has significant implications for the pathogenesis of type 2B VWD and the shear-induced activation of VWF activity.