Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P02749 (beta2-glycoprotein I)
 836 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Apolipoprotein H (apoH, protein; APOH, gene) is a 50-kDa glycoprotein that binds to negatively charged substrates, including phospholipids. ApoH is a main target antigen for the binding of antiphospholipid antibodies that are associated with thrombotic events. We have previously characterized the structural organization of the human APOH gene. Because of the significant structural homology between the human and chimpanzee genomes, we have employed oligonucleotides from the human APOH gene sequence to amplify chimpanzee DNA covering the entire transcribed region together with flanking sequence in the 5' region. As in humans, the chimpanzee APOH gene consists of eight exons and seven introns and encodes for a 326-amino-acid protein. The deduced amino acid and nucleotide sequence show 99.4% and 99.6% similarity between human and chimpanzee APOH, respectively. Using isoelectric focusing (IEF) and immunoblotting, we screened 155 chimpanzees (128 unrelated captured parents and 27 captive-born offspring) for the apoH protein polymorphism. The most common IEF pattern in chimpanzees was identical to a previously described APOH*3 allele in humans. In addition, an anodally shifted pattern was observed in chimpanzees with an allele frequency of 0.168, and the corresponding allele was designated as APOH*4. DNA sequencing of APOH*4 carriers revealed a missense mutation in exon 6 (A-->G) at codon 210, which replaces the amino acid lysine by glutamic acid. This mutation does not affect the binding of apoH to cardiolipin as revealed by cardiolipin/enzyme-linked immunosorbent assay (ELISA). We also evaluated the prevalence of anti-apoH antibodies in chimpanzee plasma by using human-apoH-based ELISA and the association of the Lys210Glu mutation with the occurrence of anti-apoH antibodies. The prevalence of anti-apoH antibodies in chimpanzees (64%) was found to be unusually high compared with that found in humans. However, the Lys210Glu mutation showed no association with the occurrence of anti-apoH antibodies. The prevalence of anti-apoH antibodies in chimpanzees may serve as a useful animal model for the human antiphospholipid syndrome, where these antibodies are associated with clinical manifestations.
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PMID:Chimpanzee apolipoprotein H (beta2-glycoprotein I): report on the gene structure, a common polymorphism, and a high prevalence of antiphospholipid antibodies. 1147 37

The phospholipid-binding plasma protein beta2-glycoprotein I (beta2-GPI) is the primary antigen recognized by the circulating autoantibodies in patients with the "anti-phospholipid syndrome" (APS). Although heparin is routinely used in the treatment and prophylaxis of APS patients, the primary heparin-binding site within beta2-GPI has not been identified. More importantly, how heparin exerts its beneficial effects in vivo in APS patients has not been deduced at the molecular level. Using an expression/site-directed mutagenesis approach, we now show that the positively charged site that resides in the first domain of beta2-GPI is not the primary heparin-binding site. Rather it is the second positively charged site located within the fifth domain of the protein that also binds to phospholipids. Lys(284), Lys(286), and Lys(287) in this domain are essential for the interaction of beta2-GPI with heparin. These data indicate that beta2-GPI binds to heparin in a relatively specific manner even though the affinity for the interaction is rather low. Lys(317) resides in the center of the high affinity phospholipid-binding site. Surprisingly, heparin at concentrations that can be achieved in vivo during anticoagulation therapy greatly enhances the plasmin-mediated cleavage of the Lys(317)-Thr(318) site in beta2-GPI. Because the cleaved form cannot bind to phospholipids effectively, the combined actions of heparin and plasmin result in a diminished ability of beta2-GPI to recognize phospholipids. This, in turn, decreases the prothrombotic activity of the endogenous circulating anti-beta2-GPI antibodies in the patients. Thus, heparin exerts its beneficial effects in APS patients by at least two distinct mechanisms.
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PMID:Heparin inhibits the binding of beta 2-glycoprotein I to phospholipids and promotes the plasmin-mediated inactivation of this blood protein. Elucidation of the consequences of the two biological events in patients with the anti-phospholipid syndrome. 1171 50

Complexes formed between beta2GPI (beta2-glycoprotein I), a human plasma protein, and biological membranes are considered to be targets of macrophages and antiphospholipid autoantibodies involved in autoimmune diseases, such as antiphospholipid syndrome or systemic lupus erythematosus. The positively charged lysine-rich fifth domain of beta2GPI facilitates its interaction with phospholipid membranes containing acidic phospholipids, which normally become exposed by apoptotic processes. In the present study, atomic force microscopy was applied to visualize the binding of beta2GPI to a mixed phospholipid model membrane at physiological ionic strength. On supported lipid bilayers the formation of supramolecular assemblies of the protein with a height of approx. 3.3 nm was observed, suggesting a lateral agglomeration of beta2GPI. Detailed analysis of kinetic constants using surface plasmon resonance revealed that the binding can be described by a two-state reaction model, i.e. a very fast interaction step, depending on the content of acidic phospholipids in the bilayer, and a second step with significantly lower k(on) and k(off) values. Taken together, our results suggest a biphasic interaction mechanism: a fast step of beta2GPI binding to negatively charged lipids, mainly based on electrostatic interactions, and a slower phase of agglomeration of the protein on the bilayer surface accompanied by a protein-induced rigidification of the membrane, as revealed by electron paramagnetic resonance.
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PMID:Membrane binding of beta2-glycoprotein I can be described by a two-state reaction model: an atomic force microscopy and surface plasmon resonance study. 1581 6

In the antiphospholipid syndrome (APS), pathogenic antiphospholipid antibodies (aPL) that cause thrombosis or pregnancy morbidity are characterized by binding to anionic phospholipids (PL) and beta2-glycoprotein I (beta(2)GPI). Sequence analysis of human monoclonal aPL has shown that high affinity for these antigens is associated with the presence of three particular amino acids: arginine (Arg), asparagine and lysine in the complementarity determining regions (CDRs) of their heavy and light chains. In vitro expression systems have been used to create variants of the antibodies in which these amino acids have been altered. In general, removal of Arg residues reduces affinity for anionic PL and beta(2)GPI. Arg at different positions in the sequence, however, have different effects on binding affinity and effects on binding are not always mirrored by effects on pathogenicity. This review will focus upon the sequence motifs that have been found to distinguish pathogenic from non-pathogenic aPL, and whether these or other properties may help to identify distinct pathogenic subsets of aPL. In particular, we will focus on our recent work in which we are trying to develop a better understanding of the molecular mechanisms involved in activation of target cells by pathogenic aPL. These studies, together with molecular models of antigen/antibody complexes, help us to understand exactly how pathogenic antibodies interact with antigens. Ultimately, this understanding may aid the design of more powerful diagnostic/prognostic assays and targeted therapeutic agents to block the pathogenic effects of these antibodies.
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PMID:Examining the non-linear relationship between monoclonal antiphospholipid antibody sequence, structure and function. 1882 54

One of the major problems in the study of the dynamics of proteins is the visualization of changing conformations that are important for processes ranging from enzyme catalysis to signaling. A protein exhibiting conformational dynamics is the soluble blood protein beta 2-glycoprotein I (beta2GPI), which exists in two conformations: the closed (circular) form and the open (linear) form. It is hypothesized that an increased proportion of the open conformation leads to the autoimmune disease antiphospholipid syndrome (APS). A characteristic feature of beta2GPI is the high content of lysine residues. However, the potential role of lysine in the conformational dynamics of beta2GPI has been poorly investigated. Here, we report on a strategy to permanently open up the closed protein conformation by chemical acetylation of lysine residues using acetic acid N-hydroxysuccinimide ester (NHS-Ac). Specific and complete acetylation was demonstrated by the quantification of primary amino groups with fluoraldehyde o-phthalaldehyde (OPA) reagent, as well as western blot analysis with an anti-acetylated lysine antibody. Our results demonstrate that acetylated beta2GPI preserves its secondary and tertiary structures, as shown by circular dichroism spectroscopy. We found that after lysine acetylation, the majority of proteins are in the open conformation as revealed by atomic force microscopy high-resolution images. Using this strategy, we proved that the electrostatic interaction of lysine residues plays a major role in stabilizing the beta2GPI closed conformation, as confirmed by lysine charge distribution calculations. We foresee that our approach will be applied to other lysine-rich proteins (e.g. histones) undergoing conformational transitions. For instance, conformational dynamics can be triggered by environmental conditions (e.g. pH, ion concentration, post-translational modifications, and binding of ligands). Therefore, our study may be relevant for investigating the equilibrium of protein conformations causing diseases.
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PMID:Lysine residues control the conformational dynamics of beta 2-glycoprotein I. 3017 30


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