Multiple domains contribute to heparin/heparan sulfate binding by human HIP/L29

Human heparin/heparan sulfate interacting protein/L29 (HIP/L29) is thought to be involved in the promotion of cell adhesion, the promotion of cell growth in the cancerous state, and the modulation of blood coagulation. These activities are consistent with the proposed function of HIP/L29 as a heparin/heparan sulfate (Hp/HS) binding growth factor that has a preference for anticoagulantly active Hp/HS. Previous studies showed that a peptide derived from the C terminus of human HIP/L29 (HIP peptide-1) can selectively bind anticoagulant Hp and support cell adhesion. However, a murine ortholog does not have an identical HIP peptide-1 sequence, yet still retains the ability to bind Hp, suggesting that there may be additional Hp/HS binding sites outside of the HIP peptide-1 domain. To test this hypothesis, a systematic study of the domains within human and murine HIP/L29 responsible for Hp/HS binding activity was undertaken. Using deletion mutants, proteolytic fragments, and protease protection of HIP/L29 by Hp, we demonstrate that multiple binding domains contribute to the overall Hp/HS binding activity of HIP/L29 proteins. Furthermore, a conformational change is induced in human HIP/L29 upon Hp binding as detected by circular dichroism spectroscopy. These studies demonstrate the multiplicity of Hp/HS binding sequences within human and murine HIP/L29.     

Protein-protein interactions:possible location of hemoglobin-haptoglobin contacts

We have studied the binding of α and β chain tryptic peptides to Hp linked to agarose. The results obtained provide evidence for a specific binding of peptides to Hp. Addition of Hb, which binds irreversibly Hp resulted in a specific displacement of the peptides bound to Hp. The analysis of the peptides displaced and their location in the Hb molecule suggested that Hb interacts with Hp through surfaces by which (I) αβ dimers and (II) α and β monomers face each other in the Hb tetramer.     

Steady-state and time-resolved spectroscopic studies on the hematoporphyrin-lipoprotein complex

The interaction of hematoporphyrin (Hp) with the isolated rabbit lipoprotein fractions very low density lipoproteins, low-density lipoproteins, and high-density lipoproteins has been studied by steady-state and time-resolved spectroscopy. The porphyrin appears to be bound to both the apoprotein and the lipid phase. The two populations of lipoprotein-bound Hp molecules can be distinguished on the basis of the fluorescence excitation spectrum, decay constants of the lowest excited singlet and triplet states, and accessibility to oxygen. Upon Hp binding, the intrinsic fluorescence emission of apolipoproteins is quenched at least in part via singlet-singlet energy transfer from tryptophyl residues to the porphyrin moiety. The binding of Hp with the protein matrix can be adequately described on the basis of Scatchard analysis, whereas the interaction of Hp with the lipid core can be described as the partitioning of the dye between a hydrophobic and an aqueous phase. The Hp binding capacity of lipoproteins is maximal for very low density lipoproteins.     

The binding of haptoglobin to apolipoprotein AI: influence of hemoglobin and concanavalin A

Haptoglobin (Hp) can be purified by affinity chromatography using hemoglobin (Hb)-linked Sepharose. Elution with 8 M urea is generally performed, resulting in heavy contamination of the Hp preparation by apolipoprotein AI (ApoAI), and partial loss of Hb binding activity. Hp, separated from ApoAI, was recovered by elution with glycine-HCl at pH 3. Complexes of the isolated protein with Hb or ApoAI were detected by enzyme-linked immunosorbent assay (ELISA). Competition between the two ligands in their interaction with Hp was observed. Concanavalin A (ConA), which binds the Hp carbohydrate chains, did not influence Hp binding to ApoAI. These results suggest that changes in the plasma levels of ApoAI or Hb affect the Hp role in regulating the reverse transport of cholesterol or preventing Hb-dependent oxidative damage.     

Regulation of CD163 associated casein kinase II activity is haptoglobin genotype dependent

Free hemoglobin is now recognized as a major mediator of a variety of vascular diseases. The abundant serum protein haptoglobin irreversibly binds to hemoglobin and promotes the uptake of hemoglobin via the macrophage CD163 receptor. The haptoglobin gene is polymorphic in man with two common alleles denoted 1 and 2. The haptoglobin genotype specifies the nature of the response of the macrophage to free hemoglobin. Hp 1-Hb complexes stimulate an anti-inflammatory macrophage phenotype while Hp 2-Hb complexes do not. We have previously demonstrated that Hp 1-Hb induced anti-inflammatory cytokine production is critically dependent on casein kinase II. In this study we set out to determine whether the amount or the activity of casein kinase II associated with CD163 was altered by the binding of Hp 1-1-Hb to CD163. Our results indicate that casein kinase II activity is increased by the binding of Hp 1-1-Hb to CD163.     

TNF-induced haptoglobin release from human neutrophils: pivotal role of the TNF p55 receptor

Haptoglobin (Hp), TNF-alpha, and neutrophils are parts of a highly interactive ensemble participating in inflammatory processes. Hp is taken up by neutrophils, stored within a cytoplasmic granular compartment, and is secreted during phagocytosis by those cells. In the present study, the effects of TNF-alpha on the release of Hp from human neutrophils were investigated. Incubation of neutrophils with TNF-alpha induced the release of Hp from cells in a time- and concentration-dependent manner as revealed by Western blot analysis and immunofluorescence. The release of Hp induced by TNF-alpha was not due to nonspecific lysis of the cells. TNF-alpha is a highly pleiotropic cytokine that mediates its effects by binding to two distinct receptors (p55 and p75). Administration of TNF-alpha mutants binding specifically either to the p55 or to the p75 TNF receptors showed that there is a preference of TNF-alpha for the p55 receptor in the mediation of Hp release by neutrophils. A stimulated release of Hp was also induced by the chemotactic tripeptide fMLP. The TNF-alpha-induced release of Hp from neutrophils was inhibited by erbstatin, a tyrosine kinase inhibitor. These findings suggest that TNF-alpha may promptly increase the level of Hp at sites of infection or injury, leading to the modulation of the acute inflammatory response.     

Haptoglobin polymorphism--not only a genetic marker]

The biological activities of the haptoglobin polymorphism are controlled by continuous DNA sequences coding for the HP alpha and Hp beta polypeptide chains and forming with a linked Hp related gene the haptoglobin gene complex on chromosome 16. Probably, this DNA domain originates from the gene family of the serine proteases after having lost the informations for the proteolytic functions. Instead of this, the haptoglobins have acquired other qualities, among them the hemoglobin binding capacity, inserted into the Hp beta chain. The Hp polymorphism is constituted by the evolutionary progressive DNA sequences for the Hp alpha chains, which probably have activation functions. The haptoglobins display immunoregulative abilities, which can be immunosuppressive by inhibition of the lymphocyte reactivity or immunoinductive by influencing the IgM biosynthesis, adapted to the functional requirements. In this field, Hp 2-2 has a stronger effect than the two other Hp types. Moreover, the haptoglobins inhibit the prostaglandin synthesis and protect against harmful oxidation processes. These qualities are based on the hemoglobin binding ability and can be realized by Hp 1-1 with the comparatively highest efficacy. Further on, the haptoglobins are protease inhibitors. Finally, Hp 2-2 is associated with higher albumin and ceruloplasmin serum levels than Hp 2-1 and Hp 1-1. Evidently, the haptoglobins are inserted into a widely ramified network of biological functions. The selective advantages and disadvantages of the Hp polymorphism are noticeable under pathological conditions in case of malignant tumors, inflammations, autoimmune diseases, allergic illness, affective psychoses and affective lability favouring addiction.     

Haptoglobin Preferentially Binds β but Not α Subunits Cross-Linked Hemoglobin Tetramers with Minimal Effects on Ligand and Redox Reactions

Human hemoglobin (Hb) and haptoglobin (Hp) exhibit an extremely high affinity for each other, and the dissociation of Hb tetramers into dimers is generally believed to be a prerequisite for complex formation. We have investigated Hp interactions with native Hb, αα, and ββ cross-linked Hb (ααXLHb and ββXLHb, respectively), and rapid kinetics of Hb ligand binding as well as the redox reactivity in the presence of and absence of Hp. The quaternary conformation of ββ subunit cross-linking results in a higher binding affinity than that of αα subunit cross-linked Hb. However, ββ cross-linked Hb exhibits a four fold slower association rate constant than the reaction rate of unmodified Hb with Hp. The Hp contact regions in the Hb dimer interfaces appear to be more readily exposed in ββXLHb than ααXLHb. In addition, apart from the functional changes caused by chemical modifications, Hp binding does not induce appreciable effects on the ligand binding and redox reactions of ββXLHb. Our findings may therefore be relevant to the design of safer Hb-based oxygen therapeutics by utilizing this preferential binding of ββXLHb to Hp. This may ultimately provide a safe oxidative inactivation and clearance pathway for chemically modified Hbs in circulation.     

Dual inhibition of cyclooxygenase and lipoxygenase by human haptoglobin: its polymorphism and relation to hemoglobin binding

Haptoglobin (Hp) binds hemoglobin (Hb) specifically and stoichiometrically. Since Hb stimulates prostaglandin (PG biosynthesis), we investigated if Hp effects arachidonic acid (AA) metabolism. The results showed that Hp (50-250 microg protein) inhibited the biosynthesis of PGs via cyclooxygenase (COX) and 12-HETE via lipoxygenase pathway in human platelets. Additional evidence was obtained by the loss of Hp inhibitory activity upon removal of Hp by affinity chromatography on hemoglobin sepharose and by inhibition of AA or bradykinin-induced bronchoconstriction in the guinea pig. Hb reduced the inhibitory effect of Hp in a concentration-related manner such that all its inhibitory activity was lost when completely bound by Hb. Of the three Hp phenotypes, Hp 1-1 showed maximum binding capacity to Hb indicating its greater protective role. These findings implicate Hp in the regulation of COX and lipoxygenase pathways and show Hp involvement in the body's endogenous defense system against inflammation. This indicates that mammals have dual defense system, i.e., a specific immune system and non-specific Hp defense system.     

Binding of bovine serum albumin to heparin determined by turbidimetric titration and frontal analysis continuous capillary electrophoresis

The association of proteins with glycosaminoglycans is a subject of growing interest, but few techniques exist for elucidating this interaction quantitatively. Here we demonstrate the application of capillary electrophoresis to the system of serum albumin (SA) and heparin (Hp). These two species form soluble complexes, the interaction increasing with reduction in pH and/or ionic strength (I). The acid-base property of Hp was characterized by potentiometric titration of ion-exchanged Hp. Conditions for complex formation with SA were qualitatively determined by turbidimetry, which revealed points of incipient binding (pH(c)) and phase separation (pH(phi)), both of which depend on I. At pH > pH(phi), i.e., prior to phase separation, frontal analysis continuous capillary electrophoresis was used to measure the concentration of free protein and to determine the protein-HP binding isotherm. The binding isotherms were well fit by the McGhee-von Hippel model to yield quantitative binding information in the form of intrinsic binding constants (K(obs)) and binding site size (n). The strong increase in K(obs) with decrease of pH or I could be explained on the basis of electrostatic interactions, considering the effects of protein charge heterogeneity. The value of n, independent of pH, was rationalized on the basis of size considerations. The implications of these findings for clinical applications of Hp and for its physiological behavior are discussed.     

Evidence of tandem repeat and extra thiol-groups resulted in the polymeric formation of bovine haptoglobin: a unique structure of Hp 2-2 phenotype

Human plasma Hp is classified as 1-1, 2-1, and 2-2. They are inherited from two alleles Hp 1 and Hp 2, but there is only Hp 1 in almost all the animal species. Hp 2-2 molecule is extremely large and heterogeneous associated with the development of inflammatory-related diseases. In this study, we expressed entire bovine Hp in E. coli as a alphabeta linear form. Interestingly, the antibodies prepared against this form could recognize the subunit of native Hp. In stead of a complicated column method, the antibody was able to isolate bovine Hp via immunoaffinity and gel-filtration columns. The isolated Hp is polymeric containing two major molecular forms (660 and 730 kDa). Their size and hemoglobin binding complex are significantly larger than that of human Hp 2-2. The amino-acid sequence deducted from the nucleotide sequence is similar to human Hp 2 containing a tandem repeat over the alpha chain. Thus, the Hp 2 allele is not unique in human. We also found that there is one additional -SH group (Cys-97) in bovine alpha chain with a total of 8 -SH groups, which may be responsible for the overall polymeric structure that is markedly different from human Hp 2-2. The significance of the finding and its relationship to structural evolution are also discussed.     

The mitochondrial p55 accessory subunit of human DNA polymerase gamma enhances DNA binding, promotes processive DNA synthesis, and confers N-ethylmaleimide resistance

Human DNA polymerase gamma is composed of a 140-kDa catalytic subunit and a smaller accessory protein variously reported to be 43-54 kDa. Immunoblot analysis of the purified, heterodimeric native human polymerase gamma complex identified the accessory subunit as 55 kDa. We isolated the full-length cDNA encoding a 55-kDa polypeptide, expressed the cDNA in Escherichia coli and purified the 55-kDa protein to homogeneity. Recombinant Hp55 forms a high affinity, salt-stable complex with Hp140 during protein affinity chromatography. Immunoprecipitation, gel filtration, and sedimentation analyses revealed a 190-kDa complex indicative of a native heterodimer. Reconstitution of Hp140.Hp55 raises the salt optimum of Hp140, stimulates the polymerase and exonuclease activities, and increases the processivity of the enzyme by several 100-fold. Similar to Hp140, isolated Hp55 binds DNA with moderate strength and was a specificity for double-stranded primer-template DNA. However, Hp140.Hp55 has a surprisingly high affinity for DNA, and kinetic analyses indicate Hp55 enhances the affinity of Hp140 for primer termini by 2 orders of magnitude. Thus the enhanced DNA binding caused by Hp55 is the basis for the salt tolerance and high processivity characteristic of DNA polymerase gamma. Observation of native DNA polymerase gamma both as an Hp140 monomer and as a heterodimer with Hp55 supports the notion that the two forms act in mitochondrial DNA repair and replication. Additionally, association of Hp55 with Hp140 protects the polymerase from inhibition by N-ethylmaleimide.     

CD163 Binding to Haptoglobin-Hemoglobin Complexes Involves a Dual-point Electrostatic Receptor-Ligand Pairing

Formation of the haptoglobin (Hp)-hemoglobin (Hb) complex in human plasma leads to a high affinity recognition by the endocytic macrophage receptor CD163. A fast segregation of Hp-Hb from CD163 occurs at endosomal conditions (pH <6.5). The ligand binding site of CD163 has previously been shown to involve the scavenger receptor cysteine-rich (SRCR) domain 3. This domain and the adjacent SRCR domain 2 of CD163 contain a consensus motif for a calcium-coordinated acidic amino acid triad cluster as originally identified in the SRCR domain of the scavenger receptor MARCO. Here we show that site-directed mutagenesis in each of these acidic triads of SRCR domains 2 and 3 abrogates the high affinity binding of recombinant CD163 to Hp-Hb. In the ligand, Hp Arg-252 and Lys-262, both present in a previously identified CD163 binding loop of Hp, were revealed as essential residues for the high affinity receptor binding. These findings are in accordance with pairing of the calcium-coordinated acidic clusters in SRCR domains 2 and 3 with the two basic Arg/Lys residues in the Hp loop. Such a two-point electrostatic pairing is mechanistically similar to the pH-sensitive pairings disclosed in crystal structures of ligands in complex with tandem LDL receptor repeats or tandem CUB domains in other endocytic receptors.     

2型猪链球菌表面蛋白 Hp0272的原核表达及其结合人血清 IgA 的活性分析

目的：构建带GST标签的2型猪链球菌功能未知表面蛋白Hp0272的原核表达质粒并在大肠杆菌中表达,获得纯度较高的GST-Hp0272重组融合蛋白,鉴定其与人血清IgA（higA）的结合活性。方法：采用分子生物学方法构建pGEX-4T-1-0272重组表达载体,将其转入大肠杆菌BL21（DE3）菌株,IPTG诱导表达后,通过SDS-PAGE分析蛋白表达情况;用GSTrap柱亲和纯化目的蛋白并经Western印迹验证,采用配体印迹和ELISA方法鉴定GST-Hp0272与hlgA的结合活性。结果：构建了GST融合蛋白原核表达质粒,并获得GST-Hp0272融合蛋白,鉴定到Hp0272特异性地与hisA结合,且结合区域位于Hp0272的N端（41-318an）。结论：获得了GST-Hp0272融合蛋白,并鉴定到其能特异性结合hlgA,为进一步了解Hp0272在猪链球菌致病过程中的作用提供了基础。