Human β 2 -glycoprotein I is a heavily glycosylated fivedomain plasma membrane-adhesion protein, which has been implicated in blood coagulation and clearance of apoptotic bodies from the circulation. It is also the key antigen in the autoimmune disease anti-phospholipid syndrome. The crystal structure of β 2 -glycoprotein I isolated from human plasma reveals an elongated fishhook-like arrangement of the globular short consensus repeat domains. Half of the C-terminal fifth domain deviates strongly from the standard fold, as observed in domains one to four. This aberrant half forms a specific phospholipid-binding site. A large patch of 14 positively charged residues provides electrostatic interactions with anionic phospholipid headgroups and an exposed membrane-insertion loop yields specificity for lipid layers. The observed spatial arrangement of the five domains suggests a functional partitioning of protein adhesion and membrane adhesion over the Nand C-terminal domains, respectively, separated by glycosylated bridging domains. Coordinates are in the Protein Data Bank (accession No. 1QUB).

Amyloids are filamentous protein structures approximately 10 nm wide and 0.1-10 mum long that share a structural motif, the cross-beta structure. These fibrils are usually associated with degenerative diseases in mammals. However, recent research has shown that these proteins are also expressed on bacterial and fungal cell surfaces. Microbial amyloids are important in mediating mechanical invasion of abiotic and biotic substrates. In animal hosts, evidence indicates that these protein structures also contribute to colonization by activating host proteases that are involved in haemostasis, inflammation and remodelling of the extracellular matrix. Activation of proteases by amyloids is also implicated in modulating blood coagulation, resulting in potentially life-threatening complications.

When blood is exposed to negatively charged surface materials such as glass, an enzymatic cascade known as the contact system becomes activated. This cascade is initiated by autoactivation of Factor XII and leads to both coagulation (via Factor XI) and an inflammatory response (via the kallikrein-kinin system). However, while Factor XII is important for coagulation in vitro, it is not important for physiological hemostasis, so the physiological role of the contact system remains elusive. Using patient blood samples and isolated proteins, we identified a novel class of Factor XII activators. Factor XII was activated by misfolded protein aggregates that formed by denaturation or by surface adsorption, which specifically led to the activation of the kallikreinkinin system without inducing coagulation. Consistent with this, we found that Factor XII, but not Factor XI, was activated and kallikrein was formed in blood from patients with systemic amyloidosis, a disease marked by the accumulation and deposition of misfolded plasma proteins. These results show that the kallikrein-kinin system can be activated by Factor XII, in a process separate from the coagulation cascade, and point to a protective role for Factor XII following activation by misfolded protein aggregates.

Amyloid fibrils are components of proteinaceous plaques that are associated with conformational diseases such as Alzheimer's disease, transmissible spongiform encephalopathies, and familial amyloidosis. Amyloid polypeptides share a specific quarternary structure element known as cross-␤ structure. Commonly, fibrillar aggregates are modified by advanced glycation end products (AGE). In addition, AGE formation itself induces protein aggregation. Both amyloid proteins and protein-AGE adducts bind multiligand receptors, such as receptor for AGE, CD36, and scavenger receptors A and B type I, and the serine protease tissue-type plasminogen activator (tPA). Based on these observations, we hypothesized that glycation induces refolding of globular proteins, accompanied by formation of cross-␤ structure. Using transmission electron microscopy, we demonstrate here that glycated albumin condensates into fibrous or amorphous aggregates. These aggregates bind to amyloid-specific dyes Congo red and thioflavin T and to tPA. In contrast to globular albumin, glycated albumin contains amino acid residues in ␤-sheet conformation, as measured with circular dichroism spectropolarimetry. Moreover, it displays cross-␤ structure, as determined with x-ray fiber diffraction. We conclude that glycation induces refolding of initially globular albumin into amyloid fibrils comprising cross-␤ structure. This would explain how glycated ligands and amyloid ligands can bind to the same multiligand "cross-␤ structure" receptors and to tPA.

For largely unknown reasons, biopharmaceuticals evoke potentially harmful antibody formation. Such antibodies can inhibit drug efficacy and, when directed against endogenous proteins, cause life-threatening complications. Insight into the mechanisms by which biopharmaceuticals break tolerance and induce an immune response will contribute to finding solutions to prevent this adverse effect. Using a transgenic mouse model, we here demonstrate that protein misfolding, detected with the use of tissue-type plasminogen activator and thioflavin T, markers of amyloid-like properties, results in breaking of tolerance. In wild-type mice, misfolding enhances protein immunogenicity. Several commercially available biopharmaceutical products were found to contain misfolded proteins. In some cases, the level of misfolded protein was found to increase upon storage under conditions prescribed by the manufacturer. Our results indicate that misfolding of therapeutic proteins is an immunogenic signal and a risk factor for immunogenicity. These findings offer novel possibilities to detect immunogenic protein entities with tPA and reduce immunogenicity of biopharmaceuticals.Over the past decades, the use of therapeutic proteins has become common practice in medicine and as their use is very promising, many more biopharmaceuticals are under development (1, 2). Unfortunately, a major drawback of protein therapeutics is the risk of antibody formation (3-7). These immunogenicity problems are of concern regarding therapeutic efficacy and patient safety (5,8). For example, drug-induced neutralizing antibodies to erythropoietin (EPO) 3 result in pure red cell aplasia (9), whereas drug-induced acquired anti-factor VIII (fVIII) antibodies worsen the pathology associated with hemophilia (10). As more and more recombinant therapeutic proteins become licensed for marketing, the incidence of immunogenicity problems is expected to rise.Initially, when mainly proteins from animal origin were used for therapy, it was thought that their foreign (non-self) nature was the main cause of immunogenicity. Unexpectedly, however, both human plasma derived as well as recombinant human protein therapeutics such as EPO (11) and fVIII (12) also elicit immune responses. This suggests that the molecular characteristic evoking antibody responses is at least more complex than being self or non-self to the human immune system. Several additional factors contributing to immunogenicity have been proposed, including contaminants or impurities, protein aggregation (13), chemical degradation and protein modification, such as differences in glycosylation or oxidation (14,15) to explain the induction of antibodies.Protein misfolding is an intrinsic and problematic property of proteins, which underlies a variety of degenerative diseases, such as Alzheimer disease. These diseases are characterized by the occurrence of fibrillar deposits, classically termed amyloid, containing aggregates of misfolded proteins. Whereas the term amyloid is classically used to classify these fi...

The multimeric glycoprotein von Willebrand factor (VWF) mediates platelet adhesion to collagen at sites of vascular damage. The binding site for collagen types I and III is located in the VWF-A3 domain. Recently, we showed that His 1023 , located near the edge between the "front" and "bottom" faces of A3, is critical for collagen binding (Romijn, R. A., Bouma, B., Wuyster, W., Gros, P., Kroon, J., Sixma, J. J., and Huizinga, E. G. (2001) reduced binding affinity about 10-fold. Together, these residues define a flat and rather hydrophobic collagenbinding site located at the front face of the A3 domain. The collagen-binding site of VWF-A3 is distinctly different from that of the homologous integrin ␣ 2 I domain, which has a hydrophilic binding site located at the top face of the domain. Based on the surface characteristics of the collagen-binding site of A3, we propose that it interacts with collagen sequences containing positively charged and hydrophobic residues. Docking of a collagen triple helix on the binding site suggests a range of possible engagements and predicts that at most eight consecutive residues in a collagen triple helix interact with A3.Under conditions of high shear stress, platelet adhesion to collagen at sites of vascular injury is initiated by the interaction of platelet receptor glycoprotein (Gp) 1 Ib-IX-V with collagen-bound von Willebrand factor (VWF) (1). Transient interactions between VWF and GpIb-IX-V mediate platelet rolling, which slows down the platelet and allows other platelet receptors such as integrin ␣ 2 ␤ 1 (2) and GpVI to bind to collagen (2-4). These interactions result in firm adhesion and activation of platelets at the site of vascular injury.VWF is a multimeric glycoprotein consisting of ϳ270-kDa monomers that are linked by disulfide bonds (5). The affinity of VWF for collagen depends on multimer size (6). The binding site for fibrillar collagens type I and III is located in the VWF-A3 domain (7), whereas collagen type VI has been shown to bind to the VWF-A1 domain (8, 9). The latter domain also contains the binding site for GpIb␣ of the GpIb-IX-V complex (10, 11).VWF A-type domains and homologous integrin I domains adopt a so-called dinucleotide-binding fold, or Rossman fold, composed of a central ␤-sheet flanked on both sides by amphipathic ␣-helices (12-15). Binding of the I domains of integrins ␣ 1 ␤ 1 , ␣ 2 ␤ 1 , ␣ 10 ␤ 1 , and ␣ 11 ␤ 1 to collagen involves a divalent cation (16, 17) located in the metal ion-dependent adhesion site (MIDAS) motif, and amino acid residues at the top face of the domain (18,19). Binding of the I domain of integrin ␣ 2 ␤ 1 to collagen induces a major displacement of its carboxyl-terminal ␣-helix that is thought to be critical for integrin signaling (18). The A3 domain of VWF does not contain a functional MIDAS motif, and binding of A3 to collagen is cation-independent (20, 21). The involvement of the top face of A3 in collagen binding has been excluded by mutagenesis studies (13,22). Recently, we showed that His 1023 , located close to th...

The solvent behaviour of¯ash-cooled protein crystals was studied in the range 100±180 K by X-ray diffraction. If the solvent is within large channels it crystallizes at 155 K, as identi®ed by a sharp change in the increase of unit-cell volume upon temperature increase. In contrast, if a similar amount of solvent is con®ned to narrow channels and/or individual cavities it does not crystallize in the studied temperature range. It is concluded that the solvent in large channels behaves similarly to bulk water, whereas when con®ned to narrow channels it is mainly protein-associated. The analogy with the behaviour of pure bulk water provides circumstantial evidence that only solvent in large channels undergoes a glass transition in the 100±180 K temperature range. These studies reveal that¯ash-cooled protein crystals are arrested in a metastable state up to at least 155 K, thus providing an upper temperature limit for their storage and handling. The results are pertinent to the development of rational crystal annealing procedures and to the study of temperature-dependent radiation damage to proteins. Furthermore, they suggest an experimental paradigm for studying the correlation between solvent behaviour, protein dynamics and protein function.

Tissue-type plasminogen activator (tPA) regulates fibrin clot lysis by stimulating the conversion of plasminogen into the active protease plasmin. Fibrin is required for efficient tPA-mediated plasmin generation and thereby stimulates its own proteolysis. Several fibrin regions can bind to tPA, but the structural basis for this interaction is unknown. Amyloid beta (Abeta) is a peptide aggregate that is associated with neurotoxicity in brains afflicted with Alzheimer's disease. Like fibrin, it stimulates tPA-mediated plasmin formation. Intermolecular stacking of peptide backbones in beta sheet conformation underlies cross-beta structure in amyloid peptides. We show here that fibrin-derived peptides adopt cross-beta structure and form amyloid fibers. This correlates with tPA binding and stimulation of tPA-mediated plasminogen activation. Prototype amyloid peptides, including Abeta and islet amyloid polypeptide (IAPP) (associated with pancreatic beta cell toxicity in type II diabetes), have no sequence similarity to the fibrin peptides but also bind to tPA and can substitute for fibrin in plasminogen activation by tPA. Moreover, the induction of cross-beta structure in an otherwise globular protein (endostatin) endows it with tPA-activating potential. Our results classify tPA as a multiligand receptor and show that cross-beta structure is the common denominator in tPA binding ligands.