Calcium-binding properties of bovine factor X lacking the gamma-carboxyglutamic acid-containing region

Role of the alpha-helix 163-170 in factor Xa catalytic activity

Factor Xa (FXa) is a key protease of the coagulation pathway whose activity is known to be in part modulated by binding to factor Va (FVa) and sodium ions. Previous investigations have established that solvent-exposed, charged residues of the FXa alpha-helix 163-170 (h163-170), Arg(165) and Lys(169), participate in its binding to FVa. In the present study we aimed to investigate the role of the other residues of h163-170 in the catalytic functions of the enzyme. FX derivatives were constructed in which point mutations were made or parts of h163-170 were substituted with the corresponding region of either FVIIa or FIXa. Purified FXa derivatives were compared with wild-type FXa. Kinetic studies in the absence of FVa revealed that, compared with wild-type FXa, key functional parameters (catalytic activity toward prothrombin and tripeptidyl substrates and non-enzymatic interaction of a probe with the S1 site) were diminished by mutations in the NH(2)-terminal portion of h163-170. The defective amidolytic activity of these FXa derivatives appears to result from their impaired interaction with Na(+) because using a higher Na(+) concentration partially restored normal catalytic parameters. Furthermore, kinetic measurements with tripeptidyl substrates or prothrombin indicated that assembly of these FXa derivatives with an excess of FVa in the prothrombinase complex improves their low catalytic efficiency. These data indicate that residues in the NH(2)-terminal portion of the FVa-binding h163-170 are energetically linked to the S1 site and Na(+)-binding site of the protease and that residues Val(163) and Ser(167) play a key role in this interaction.

Prothrombinase assembly and S1 site occupation restore the catalytic activity of FXa impaired by mutation at the sodium-binding site

Two loop segments (183-189 and 221-225) in the protease domain of factor Xa contribute to the formation of a Na(+)-binding site. Studies with factor Xa indicate that binding of a single Na(+) ion to this site influences its activity by altering the S1 specificity site, and substitution of Tyr(225) with Pro diminishes sensitivity to Na(+). Using full-length factor Xa(Y225P), the allosteric relationship between the Na(+) site and other structural determinants in factor Xa and prothrombinase was investigated. Direct binding and kinetic measurements with probes that target the S1 specificity pocket indicate that assembly of the mutant in prothrombinase corrected the impaired binding of these probes observed with free factor Xa(Y225P). This appears to result from the apparent allosteric linkage between the factor Va, S1, and Na(+)-binding sites, since binding of the cofactor to membrane-bound factor Xa(Y225P) enhances binding at the S1 site and vice versa. Additional studies revealed that the internal salt bridge (Ile(16)-Asp(194)) of factor Xa(Y225P) is partially destabilized, a process that is reversible upon occupation of the S1 site. The data establish that alterations at the factor Xa Na(+)-binding site shift the zymogen-protease equilibrium to a more zymogen-like state, and as a consequence binding of S1-directed probes and factor Va are adversely affected. Therefore, the zymogen-like characteristics of factor Xa(Y225P) have allowed for the apparent allosteric linkage between the S1, factor Va, and Na(+) sites to become evident and has provided insight into the structural transitions which accompany the conversion of factor X to factor Xa.

Mechanisms by which soluble endothelial cell protein C receptor modulates protein C and activated protein C function

The endothelial cell protein C receptor (EPCR) functions as an important regulator of the protein C anticoagulant pathway by binding protein C and enhancing activation by the thrombin-thrombomodulin complex. EPCR binds to both protein C and activated protein C (APC) with high affinity. A soluble form of EPCR (sEPCR) circulates in plasma and inhibits APC anticoagulant activity. In this study, we investigate the mechanisms by which sEPCR modulates APC function. Soluble EPCR inhibited the inactivation of factor Va by APC only in the presence of phospholipid vesicles. By using flow cytometric analysis in the presence of 3 mM CaCl(2) and 0. 6 mM MgCl(2), sEPCR inhibited the binding of protein C and APC to phospholipid vesicles (K(i) = 40 +/- 7 and 33 +/- 4 nM, respectively). Without MgCl(2), the K(i) values increased approximately 4-fold. Double label flow cytometric analysis using fluorescein-APC and Texas Red-sEPCR indicated that the APC.sEPCR complex does not interact with phospholipid vesicles. By using surface plasmon resonance, we found that sEPCR also inhibited binding of protein C to phospholipid in a dose-dependent fashion (K(i) = 32 nM). To explore the possibility that sEPCR evokes structural changes in APC, fluorescence spectroscopy studies were performed to monitor sEPCR/Fl-APC interactions. sEPCR binds saturably to Fl-APC (K(d) = 27 +/- 13 nM) with a maximum decrease in Fl-APC fluorescence of 10.8 +/- 0.6%. sEPCR also stimulated the amidolytic activity of APC toward synthetic substrates. We conclude that sEPCR binding to APC blocks phospholipid interaction and alters the active site of APC.

The interaction of thrombomodulin with Ca2+

Thrombomodulin (TM) is a cofactor for protein C activation by thrombin and each residue of a consensus Ca2+ site in the sixth epidermal growth factor domain (EGF6) is essential for this cofactor activity [Nagashima, M., Lundh, E., Leonard, J.C., Morser, J. & Parkinson, J.F. (1993) J. Biol. Chem. 268, 2888-2892]. Three soluble analogs of the extracellular domain of TM, solulin (Glu4-Pro490), TME1-6 (Cys227-Cys462) and TMEi4-6 (Val345-Cys462) were prepared for equilibrium dialysis experiments by exhaustive dialysis against Ca2+-depleted buffer. However, all three analogs still contained one tightly bound Ca2+ (Kd approximately 2 microm), which could only be removed by EDTA. Epitope mapping with Ca2+-dependent monoclonal antibodies to EGF6 provided further localization of this tight Ca2+ site. Equilibrium dialysis of the soluble TM analogs in [45Ca2+] between 10 and 200 microm revealed a second Ca2+ site (Kd = 30 +/- 10 microm) in both solulin and TME1-6, but not in TMEi4-6. Ca2+ binding to this second site was unaffected by bound thrombin and we attribute it to the consensus Ca2+ site in EGF3. A 75-fold decrease in the binding affinity of thrombin to TM was observed with immobilized solulin treated with EDTA to remove the high affinity Ca2+ by measuring kassoc and kdiss rates in a BIAcoretrade mark instrument. Ca2+-dependent conformational transitions detected by CD spectroscopy in the far UV indicate a more ordered structure upon Ca2+ binding. Bound Ca2+ stabilized soluble TM against protease digestion at a trypsin-like protease-sensitive site between Arg456 and His457 in EGF6 compared with protease treatment in EDTA. Finally, TM containing EGF domains 4-6, but lacking the interdomain loop between EGF3 and 4 (TME4-6), has an identical Ca2+ dependence for the activation of protein C as found for TMEi4-6, indicating this interdomain loop is not involved in Ca2+ binding.

Identification of chicken and C. elegans fibulin-1 homologs and characterization of the C. elegans fibulin-1 gene

Fibulin-1, a member of the emerging family of fibulin proteins, is a component of elastic extracellular matrix fibers, basement membranes and blood. Homologs of fibulin-1 have been described in man, mouse and zebrafish. In this study, we describe the isolation and sequencing of chicken fibulin-1C and D cDNA variants. We also describe identification of a C. elegans cDNA encoding fibulin-1D and cosmids containing the C. elegans fibulin-1 gene. Using the cDNA, RT-PCR and computer-based analysis of genomic sequences, the exon/intron organization of the C. elegans fibulin-1 gene was determined. The C. elegans fibulin-1 gene is located on chromosome IV, is approximately 6 kb in length, contains 16 exons and encodes fibulin-1C and D variants. Comparative analysis of the deduced amino acid sequences of nematode and chicken fibulin-1 variants with other known vertebrate fibulin-1 polypeptides showed that the number and organization of structural modules are identical. The results of this study indicate that the structure of the fibulin-1 protein has remained highly conserved over a large period of evolution, suggestive of functional conservation.

The interaction between the endothelial cell protein C receptor and protein C is dictated by the gamma-carboxyglutamic acid domain of protein C

The endothelial cell protein C receptor (EPCR) binds to both protein C and activated protein C (APC) with similar affinity. Removal of the Gla domain of protein C results in the loss of most of the binding affinity. This observation is compatible with at least two models: 1) the Gla domain of protein C interacts with phospholipid on cell surfaces to stabilize interaction with EPCR or 2) the Gla domain of protein C makes specific protein-protein interactions with EPCR. The latter model predicts that chimeric proteins containing the protein C Gla domain should interact with EPCR. To test this, we constructed a prothrombin chimera in which the Gla domain and aromatic stack of prothrombin were replaced with the corresponding region of protein C. The 125I-labeled chimera (Kd = 176 nM) and 125I-APC (Kd = 65 nM) both bound specifically to 293 cells stably transfected with EPCR, but both bound poorly to sham-transfected cells. The chimera also blocked APC binding to EPCR-transfected cells in a dose-dependent fashion (Ki approximately 139 nM) similarly to protein C (Ki approximately 75 nM). Chimera binding to EPCR-transfected cells was blocked by soluble EPCR, demonstrating direct protein-protein interaction between the chimera and EPCR. Consistent with this conclusion, the isolated Gla domain of protein C blocked APC binding to EPCR-transfected cells (IC50 = 2 microM). No inhibition was observed with the isolated prothrombin Gla domain. A protein C chimera with the prothrombin Gla domain and aromatic stack failed to bind to EPCR detectably. These data suggest that the Gla domain of protein C is responsible for much of the binding energy and specificity of the protein C-EPCR interaction.

The self-association and fibronectin-binding sites of fibulin-1 map to calcium-binding epidermal growth factor-like domains

Fibulin-1 is a modular glycoprotein with amino-terminal anaphylatoxin-like modules followed by nine epidermal growth factor (EGF)-like modules and, depending on alternative splicing, four possible carboxyl termini. Fibulin-1 has been shown to self-associate as well as to bind calcium, fibronectin (FN), laminin, nidogen, and fibrinogen. To map ligand-binding sites within fibulin-1, polypeptides corresponding to various regions of fibulin-1 were expressed recombinantly and evaluated for their capacity to bind calcium, FN, or fibulin-1. A calcium-binding site(s) was mapped to EGF-like modules 5-9. A fibulin-1 self-association site was localized to EGF-like modules 5 and 6 (amino acid residues 356-440), as was a binding site for FN. The self-association interaction mediated by this pair of modules involved calcium since divalent cation chelators reduced the binding affinity of the interaction. By contrast, FN binding to EGF-like modules 5 and 6 was unaffected by the presence of divalent cation chelators. It can be concluded that EGF-like modules 5 and 6 bind calcium and mediate homotypic interaction between EGF-like modules 5 and 6 present in different fibulin-1 molecules and heterotypic interaction between EGF-like modules 5 and 6 and type III repeats 13 and 14 in FN. While additional binding sites for calcium or FN were not detected, another fibulin-1 self-association site was found within amino acid residues 30-173. However, unlike the self-association site in EGF-like modules 5 and 6, which was functional in the native protein, the amino-terminal site was cryptic and revealed only after the protein was denatured.

High-affinity calcium-binding site in the gama-carboxyglutamic acid domain of bovine factor VII

The calcium-mediated interaction of factor VIIa with tissue factor is considered to be the primary trigger of blood coagulation. To determine the role of calcium ions in the action of factor VII, we prepared monoclonal antibodies whose binding to factor VII was calcium-dependent. A monoclonal antibody designated C6 strongly inhibited factor VII-induced clotting at a molar ratio of factor VII to antibody of 1:1. The half-maximal binding of factor VII to the C6 antibody was observed at a concentration of calcium ions of 80 microM. Proteolytic fragments of factor VII were assayed for their ability to inhibit competitively the binding of 125I-factor VII to immobilized C6 antibody. The binding was inhibited by increasing amounts of factor VII, by a fragment that contained the gamma-carboxyglutamic acid (Gla) domain linked to first epidermal growth factor-like domain, and by a Gla domain peptide (residues 1-41), over a range of concentration of 10(-9) to 10(-7) M. The antigenic site recognized by the monoclonal antibody C6, which was generated upon the high-affinity binding of calcium ions, was located in the Gla domain. The C6 antibody inhibited the activation of factor X and the amidolytic activity of factor VIIa in the presence of tissue factor. These results demonstrate that a high-affinity calcium-binding site(s) is located in the Gla domain of factor VII, which is concerned with the initiation of tissue factor-mediated blood coagulation by factor VIIa.

Structural changes in factor VIIa induced by Ca2+ and tissue factor studied using circular dichroism spectroscopy

Factor VIIa (fVIIa) is composed of four discrete domains, a gamma-carboxyglutamic acid (Gla)-containing domain, two epidermal growth factor (EGF)-like domains, and a serine protease domain, all of which appear to be involved, to different extents, in an optimal interaction with tissue factor (TF). All except the second EGF-like domain contain at least one Ca2+ binding site and many properties of fVIIa, e.g., TF and phospholipid binding and amidolytic activity, are Ca(2+)-dependent. A CD study was performed to characterize and locate the conformational changes in fVIIa induced by Ca2+ and TF binding. In addition to intact fVIIa, derivatives lacking the Gla domain or the protease domain were used. Assignment of the Ca(2+)-induced changes in the far-UV region of the fVIIa spectrum to the Gla domain could be made by comparing the CD spectra obtained with these fVIIa derivatives. The changes primarily appeared to reflect a Ca(2+)-induced ordering of alpha-helices existing in the apo state of fVIIa. This was corroborated by models of the apo and Ca2+ forms of fVIIa, obtained as difference spectra between fVIIa derivatives, were very similar to those of isolated Gla peptides from other vitamin K-dependent plasma proteins. The near-UV CD spectrum of fVIIa was dominated by aromatic residues residing in the protease domain and specific bands affected by Ca2+ were indicative of tertiary structural alterations. The formation of a fVIIa:TF complex led to secondary structural changes that appeared to be restricted to the catalytic domain, possibly shedding light on the mechanism by which TF induces an enhancement of fVIIa catalytic activity.

The hydrophobic nature of residue-5 of human protein C is a major determinant of its functional interactions with acidic phospholipid vesicles

We have previously proposed that a cluster of surface-exposed hydrophobic amino acids, viz., F4, L5, and L8, present at the amino-terminus of the Ca(2+)-bound form of gamma-carboxyglutamic acid domain (GD) of human protein C (PC), contributes a substantial portion of the total functional binding energy of PC and its activated form, APC, to acidic phospholipid (PL) vesicles. A deeper understanding of the importance of the hydrophobic nature of sequence position 5, and the particular relevance of leucine at that location, was sought by examination of the properties of a series of mutant proteins containing A5, V5, I5, and W5 as replacements for L5 in recombinant (r)-PC and APC. The Ca(2+)- and PL-dependent plasma-based anticoagulant activities of [L5A]r-APC, [L5V]r-APC, [L5I]r-APC, and [L5W]r-APC were determined to be approximately 28%, 51%, 98%, and 105%, respectively, of that of wild-type r-APC. A similar trend in activities of the mutant enzymes was observed in in vitro factor V/Va and factor VIII/VIIIa inactivation assays. Apparently normal Ca(2+)-dependent conformations were adopted by each of the mutant proteins, but the Ca(2+)-bound form of [L5A]r-PC was relatively the most defective of the mutants in its binding to PL. These results confirm the importance of the hydrophobic character at sequence position 5 as critical to the functional binding of PC to PL.

Tryptophans 231 and 234 in protein C report the Ca(2+)-dependent conformational change required for activation by the thrombin-thrombomodulin complex

Human protein C circulates as both single- and two-chain zymogens. Activation by the physiological activation complex, thrombin-thrombomodulin, generates the anticoagulant enzyme, activated protein C. Ca2+ binding to the protease domain of protein C is accompanied by 5.5 +/- 0.2% quenching of intrinsic fluorescence that correlates with the conformational change required for the rapid activation by the thrombin-thrombomodulin complex. To map which Trp residues report this Ca2+ binding, candidate Trp residues at positions 84, 115, 145, 205, 231, and 234 were changed individually to Phe within a protein C deletion mutant lacking the Gla domain (GDPC). Of these, the Trp to Phe mutation at position 231 (W231F) eliminated the Ca(2+)-induced fluorescence quenching, and the Trp 234 to Phe mutation (W234F) increased the maximum quenching in protein C to 9.4 +/- 0.4%. Upon Ca2+ binding, the fluorescence emission intensity of the W231F mutant was increased 3.4% +/- 0.6%. The Kd for this site (84 +/- 20 microM) was similar to that of GDPC (Kd = 39 +/- 4 microM). To compare the properties of single- and two-chain protein C, we replaced the Lys156-Arg157 dipeptide cleavage site in protein C with Thr and Gln to form GDPCKR/TQ. GDPCKR/TQ and the two-chain form of protein C were activated at the same rate with the thrombin-thrombomodulin complex, they exhibited similar Ca2+ dependence for both activation and fluorescence quenching, and these enzymes had the same chromogenic activity. In contrast to the zymogen form, activated human Gla-domainless protein C did not undergo a Ca(2+)-induced fluorescence change. These results indicate that the environment of Trp 231 and 234 within the Ca2+ binding loop of the protein C zymogen are perturbed by Ca2+ binding to the zymogen.

Cloning and characterization of the human gene encoding aspartyl beta-hydroxylase

Sequence information for aspartyl beta-hydroxylase (AspH), which specifically hydroxylates one Asp or Asn residue in certain epidermal growth factor (EGF)-like domains of a number of proteins, is so far only described for bovine species. We have isolated a 4.3-kb cDNA encoding the human AspH (hAspH) by immunoscreening of a human osteosarcoma (MG63) cDNA library in lambda ZAP with an antiserum raised against membrane fractions of these cells. Northern blot analyses revealed two transcripts with lengths of 2.6 and 4.3 kb. The deduced amino acid (aa) sequence of this cDNA encodes a protein of 757 aa (85 kDa). Comparison with the deduced bovine AspH (bAspH) aa sequence showed striking differences in the N-terminal portion of this protein. In vitro transcription and translation in the presence of canine pancreas microsomes yielded a 56-kDa protein. Western blot analyses of membrane fractions from MG63 cells with AspH-specific antibodies revealed a protein of the same M(r). These results suggest a posttranslational cleavage of the catalytic C terminus in the lumen of the endoplasmic reticulum.

Biochemistry of factor X

Factor X circulates as a serine protease which is converted to the active form at the point of convergence of the intrinsic and extrinsic coagulation pathways. Subsequently, the enzymatic species, factor Xa, is involved in macromolecular complex formation with its cofactor factor Va, a phospholipid surface and calcium to convert prothrombin into thrombin. The gene encoding factor X shares a number of structural and organisational features in common with the other vitamin K-dependent coagulation proteins, suggesting that they have evolved from a common ancestral gene. Each of the exons encoding these proteins can be considered as a module coding for a homologous domain in each protein. These structural domains in factor X are responsible for specific functional properties including gamma-carboxylase recognition, calcium binding, phospholipid surface interaction, as well as cofactor and substrate binding. Studies of recombinant proteins and proteolytic fragments continue to provide significant insight into structure-function relationships of the protein modules within factor X.

Binding of calcium to synthetic peptides containing gamma-carboxyglutamic acid

The Ca2+ binding properties of various gamma-carboxyglutamic acid (gla)-containing synthetic peptides with counterpart sequences in human protein C were investigated employing potentiometry with a Ca(2+)-selective electrode and titration calorimetric techniques. The shortest peptides, FL(gla)(gla)LR, DF(gla)(gla)AK, and the oxidized form of the cyclic hexapeptide CI(gla)(gla)IC, each of which contains one pair of gla residues, have a weak affinity for Ca2+, with some peptides probably involved in intermolecular bridging of the Ca2+. The best example of this is the oxidized form of the peptide, CI(gla)(gla)IC, where one g-atom of Ca2+ interacts with 2 mol of peptide (n = 0.5) with a Kd value of 1.6 mM. A second g-atom of Ca2+ interacts with 2 mol of this same peptide (n = 0.5) and is characterized by a Kd of 8.8 mM. A longer peptide containing this same sequence, viz. L(gla)R(gla)CI(gla)(gla)IC, possesses two binding sites (n = 2.0) for Ca2+ of Kd = 16.1 mM, as well as a tighter site (n = 1), of Kd = 0.4 mM. An increase in stoichiometry of tight binding sites as the peptide is elongated is observed from binding data obtained on a 38-residue peptide that possesses all nine of the gla-residues of protein C in their proper sequence positions. The strongest Ca2+ binding sites (n = 3-4) possess an average Kd of 0.4 mM, followed by another class of sites (n = 5-10, average Kd = 1.5-3.0 mM). The affinity and stoichiometry of these stronger sites mimic those observed for binding of Ca2+ to the gla region of prothrombin fragment 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the Ca2+ binding properties of the gamma-carboxyglutamic acid-containing module of protein Z in the intact protein and in N-terminal fragments

Protein Z is a vitamin K-dependent plasma protein of unknown function. Its modular structure is identical with those of factors VII, IX, X, and protein C. These proteins have an N-terminal gamma-carboxyglutamic acid (Gla)-containing module which binds six to ten Ca2+. In factors IX, X, and protein C, the adjacent epidermal growth factor (EGF)-like module binds one Ca2+ whereas the EGF-like module in protein Z does not. We have compared the Ca2+ binding properties of a fragment of protein Z comprising the Gla and N-terminal EGF-like modules (pZ-GlaEGFN) with those of intact protein Z and the isolated Gla module by measuring the Ca(2+)-induced quenching of the intrinsic protein fluorescence. The similar Ca2+ affinities of pZ-GlaEGFN and protein Z indicate that pZ-GlaEGFN has a native conformation and normal Ca2+ binding properties. A comparison of the Ca2+ binding to pZ-GlaEGFN with those to the corresponding fragments of factors IX, X, and protein C indicate that Ca2+ binding to the N-terminal EGF-like modules in the latter proteins does not influence the folding and Ca2+ binding properties of their Gla modules. Furthermore, the Ca(2+)-induced fluorescence enhancements of GlaEGF fragments from factors IX, X, and protein C appear to be caused by Ca2+ binding to the site in the EGF-like modules since it is not observed for pZ-GlaEGFN.

Clustering of fibrillin (FBN1) missense mutations in Marfan syndrome patients at cysteine residues in EGF-like domains

The Marfan syndrome is an autosomal dominant heritable disorder of connective tissue with prominent involvement of the ocular, skeletal, and cardiovascular systems. The gene on chromosome 15 encoding fibrillin (FBN1), a 350-kDa glycoprotein component of the extracellular microfibril, is the site of defect in most, if not all cases. Complementary DNA sequence reveals a gene composed largely of epidermal growth factor-like repeats, each containing six predictably spaced cysteine residues. To date, two FBN1 gene missense mutations have been reported. Here we describe the identification of three new missense mutations in the FBN1 gene in patients with the Marfan syndrome. All of the 5 characterized missense mutations occur within the epidermal growth factor-like repeats of the FBN1 gene. In addition, 4 of 5 involve the substitution of cysteine residues and 3 of 5 substitute the third cysteine in the epidermal growth factor-like motif consensus sequence. These data suggest that defined residues within EGF-like domains of FBN1 have particular significance and, when altered, play a pivotal role in expression of the Marfan phenotype.

Specific EGF repeats of Notch mediate interactions with Delta and Serrate: implications for Notch as a multifunctional receptor

The neurogenic loci Notch and Delta, which both encode EGF-homologous transmembrane proteins, appear to function together in mediating cell-cell communication and have been shown to interact at the cell surface in vitro. To examine the role of the EGF repeats in this interaction, we performed an extensive deletion mutagenesis of the extracellular domain of Notch. We find that of the 36 EGF repeats of Notch, only two, 11 and 12, are both necessary and sufficient to mediate interactions with Delta. Furthermore, this Delta binding ability is conserved in the corresponding two repeats from the Xenopus Notch homolog. We report a novel molecular interaction between Notch and Serrate, another EGF-homologous transmembrane protein containing a region of striking similarity to Delta, and show that the same two EGF repeats of Notch also constitute a Serrate binding domain. These results suggest that Notch may act as a multifunctional receptor whose 36 EGF repeats form a tandem array of discrete ligand-binding units, each of which may potentially interact with several different proteins during development.

Interaction of human protein Z with thrombin: evaluation of the species difference in the interaction between bovine and human protein Z and thrombin

Protein Z is a vitamin K-dependent protein of unknown function present in normal human and bovine plasma. Binding and kinetic studies showed that bovine protein Z interacts with bovine thrombin with a dissociation constant of 0.11 microM in a Ca(2+)-independent fashion and that thrombin becomes associated with phospholipid vesicles in the presence of protein Z but not in its absence (Hogg, P. J. and Stenflo, J. (1991) J. Biol. Chem., in press). In the present study the interaction of human protein Z with human thrombin and the influence of human protein Z on the association of thrombin with phospholipid vesicles was evaluated. In contrast to bovine protein Z, human protein Z bound human DIP-thrombin with a 20-fold weaker affinity at 1.5 mM Ca2+ and in a Ca(2+)-dependent fashion. Human protein Z was also less effective than bovine protein Z in promoting the association of thrombin with phospholipid vesicles. Also, bovine protein Z cleaved by thrombin at Arg-365 bound DIP-thrombin with a 10-fold weaker affinity than did native bovine protein Z. The data suggest that the species difference in the interaction between protein Z and thrombin can be explained by a difference in the COOH-terminal region of bovine protein Z versus human protein Z.

Molecular interactions between the protein products of the neurogenic loci Notch and Delta, two EGF-homologous genes in Drosophila

Genetic analyses have raised the possibility of interactions between the gene products of the neurogenic loci Notch and Delta, each of which encodes a transmembrane protein with EGF homology. To examine the possibility of intermolecular association between the products of these two genes, we studied the effects of their expression on aggregation in Drosophila S2 cells. We find that Notch-expressing cells form mixed aggregates specifically with cells that express Delta and that this process is calcium dependent. In addition, we show that Notch and Delta can associate within the membrane of a single cell, and further, that they form detergent-soluble intermolecular complexes. Our analyses suggest that Notch and Delta proteins interact at the cell surface via their extracellular domains.

Chemical and functional characterization of a fragment of C1-s containing the epidermal growth factor homology region

C1-s, one of the three subcomponents of C1-, the first component of complement, is a serine protease comprising two disulfide-linked chains, the B chain, containing the catalytic site, and the A chain, involved in Ca2+ binding and Ca2(+)-dependent interaction(s) with the other C1- subcomponents. In an attempt to identify the regions responsible for the latter functions, C1-s was submitted to limited proteolysis with plasmin, a treatment that split the A chain into three major fragments, alpha 1, alpha 2, and gamma. Fragment alpha 2, which comprised the epidermal growth factor-like (EGF-like) region of C1-s, was heterogeneous, starting at serine 97 or phenylalanine 105 and ending at lysine 195. This fragment was reduced and alkylated and then digested with elastase, and three peptides covering positions 131-135, 131-139, and 131-140 were characterized by amino acid analysis, Edman degradation, and mass spectrometry, showing that position 134 of C1-s is occupied partly by an asparagine (47%) and partly by an erythro-beta-hydroxyasparagine, in contrast with the homologous position (150) of C1-r which only contains erythro-beta-hydroxyasparagine. As measured by equilibrium dialysis, native alpha 2, like the other plasmin-cleavage fragments, did not retain the ability of intact C1-s to bind Ca2+. In the same way, plasmin cleavage abolished the ability of C1-s to dimerize or to associate with C1-r in the presence of Ca2+. In contrast, both alpha 2 and the N-terminal alpha 1 fragment, starting at serine 24 of the A chain, were able to compete significantly with intact C1s for the formation of the Ca2(+)-dependent C1-s-C1r-C1-r-C1-s tetramer.(ABSTRACT TRUNCATED AT 250 WORDS)

A further insight into the binding of blood clotting factors to membranes

The active site of factor Xa, labelled with dansylglutamylglycylarginine (DnsEGR) is sensitive to association with Ca2+, factor Va and phospholipids. When bound to factor Va, DnsEGR-factor-Xa does not change the composition of the binding site of factor Va, as shown by fluorescence energy-transfer experiments between the Trp residues of factor Va and pyrene-labelled phospholipids. Prothrombin was cleaved by alpha-chymotrypsin into two parts: N-terminal residues 1-41 (peptide 1-41) containing the gamma-carboxyglutamic acid residues (Gla), and des-(1-41)-prothrombin; their membrane association was investigated. Peptide 1-41 contains the aromatic residues Tyr and Trp in positions 24 and 41, respectively, and is suitable for fluorescence spectroscopy. The absence of fluorescence energy transfer between these residues suggests that they are more than 2.8 nm apart. Binding of Ca2+ and of phospholipids involves essentially the Tyr residue, while the C-terminal characteristics of the Trp residue remain unchanged. The conformational change which takes place on binding does not shorten the distance between Tyr and Trp beyond 2.8 nm. Our conclusion is that peptide 1-41 has an extended conformation. This result is compatible with the disordered character of the Gla region found in the crystalline structure of fragment 1 of prothrombin. Ca2+ induces a greater fluorescence energy transfer between prothrombin and membranes labelled with pyrene but has no influence on the binding of des-(1-41)-prothrombin. Moreover, the binding curves of des(1-41)-prothrombin are similar to those of prothrombin in the absence of Ca2+. It is concluded that the Ca2+-independent association of prothrombin with membranes involves essentially that part of the prothrombin molecule deleted in the Gla region.