Binding interactions of kistrin with platelet glycoprotein IIb-IIIa: Analysis by site-directed mutagenesis

The binding interactions between platelet fibrinogen receptor, glycoprotein (GP) IIb-IIIa, and kistrin, a snake venom disintegrin protein that contains the adhesion site recognition sequence Arg-Gly-Asp (RGD) and potently inhibits platelet aggregation, have been investigated by site-directed mutagenesis of a synthetic kistrin gene. Kistrin was expressed as a fusion protein in Escherichia coli under control of the alkaline phosphatase promoter. This construction included the stII signal sequence to direct secretion to the periplasmic space and one synthetic (Z) domain of Staphylococcal protein A to allow affinity purification using IgG Sepharose. Kistrin was cleaved from the Z-domain by site-specific proteolysis using a mutant subtilisin BPN' and purified by reverse-phase HPLC. This approach facilitated the rapid purification of a set of 43 alanine replacement mutants whose relative affinity for GP IIb-IIIa was measured by competition with immobilized kistrin and by inhibition of platelet aggregation in human platelet-rich plasma. Alanine replacements at R49, G50, and D51 led to weaker inhibitors of platelet aggregation by 90-fold, 2-fold, and >200-fold, respectively. The conservative D51E mutant was still >100-fold less potent whereas R49K had a minor effect (1.8-fold), implying the critical nature of the aspartate for high affinity binding. However, mutations outside of the RGD region led to proteins indistinguishable from kistrin, suggesting no substantial secondary binding interactions. Furthermore, reduced kistrin is not active. We therefore propose that a favorable conformation of the RGD region alone is responsible for the high affinity binding of kistrin to GP IIb-IIIa. © 1993 Wiley-Liss, Inc.     

Mambin, a potent glycoprotein IIb-IIIa antagonist and platelet aggregation inhibitor structurally related to the short neurotoxins.

The purification, complete amino acid sequence, functional activity, and structural modeling are described for mambin, a platelet glycoprotein GP IIb-IIIa antagonist and potent inhibitor of platelet aggregation from the venom of the Elapidae snake Dendroaspis jamesonii (Jameson's mamba). Mambin is 59 residues in length and contains four disulfide linkages and an RGD amino acid sequence found in protein ligands that bind to GP IIb-IIIa. Mambin inhibits ADP-induced platelet aggregation (IC50 = 172 +/- 22 nM) and inhibits the binding of purified platelet fibrinogen receptor GP IIb-IIIa to immobilized fibrinogen (IC50 = 3.1 +/- 0.8 nM). Mambin has very little sequence similarity to the Viperidae family of platelet aggregation inhibitors, except for the RGD-containing region in the protein. However, mambin does have ca. 47% similarity to the short-chain postsynaptic neurotoxins found in other Elapidae venoms, which do not contain the RGD sequence and do not act as GP IIb-IIIa antagonists. On the basis of its circular dichroism spectrum, mambin has a beta-sheet structure characteristic of the neurotoxins. Molecular modeling of the mambin sequence onto the erabutoxin b structure predicts a very similar structure within the entire protein except for the loop containing the RGD sequence. Mambin may therefore represent a genetic hybrid of neurotoxic and hemotoxic proteins found in snake venoms.     

Immunochemical and amino-terminal sequence comparison of two cytoadhesins indicates they contain similar or identical beta subunits and distinct alpha subunits

Platelet membrane glycoprotein (GP) IIb-IIIa is functionally and antigenically related to proteins present on many cell types, suggesting that it is a member of the proposed cytoadhesin family of membrane proteins. We have compared the purified tissue vitronectin receptor (VnR) with GP IIb-IIIa. Anti-VnR immunoprecipitated GP IIb-IIIa and a related endothelial cell protein. In immunoblots, GP IIIa reacted with anti-VnR and the beta subunit of the VnR reacted with poly and monoclonal anti-GP IIIa. In contrast, the alpha subunit of the VnR failed to react either with a polyclonal anti-GP IIb or with monoclonal anti-GP IIb. Furthermore, the amino-terminal sequence of GP IIIa and the beta subunit of VnR were identical at determined residues while the alpha subunit and the GP IIb were different, but showed 33% identity. These data indicate the identity or close homology of GP IIIa and the beta subunit of the VnR. In contrast, the alpha subunit and GP IIb are distinct polypeptides which may be homologous. Since GP IIb-IIIa and the VnR differ in ligand recognition specificity, the data also suggest that this specificity may be governed by the alpha subunit of cytoadhesins.     

Sequential resonance assignments as a basis for the determination of a three-dimensional structure of protein E-L30 of Escherichia coli

Nuclear Overhauser enhancement spectra of ribosomal protein E-L30 were searched for interresidual connectivities involving peptide bond amide protons in order to establish sequential neighbourships between amino acid residues. By comparing these data with the actual amino acid sequence of the protein, sequential resonance assignments became available for almost 90% for the amino acids in E-L30. With the aid of these assignments, some 30 nuclear Overhauser connectivities could be interpreted in terms of short interproton distances involving remote sites in the polypeptide chain. It turned out that these contacts between residues generated enough constraints to permit construction of a three-dimensional structure for the protein.     

Inhibition of fibrinogen binding to GP IIb-IIIa by a GP IIIa peptide.

The glycoprotein IIb-IIIa complex (GP IIb-IIIa) mediates platelet aggregation and is a member of the cytoadhesin family of receptors that bind adhesive proteins such as fibrinogen, fibronectin, and von Willebrand factor. Despite the wide range of cell-substrate interactions mediated by these receptors, ligand binding domains have not yet been identified on any of the integrins. The present study was designed to determine potential fibrinogen binding domain(s) on the GP IIb-IIIa complex. Synthetic peptides derived from residues 1-288 of the amino-terminal portion of GP IIIa were tested for their abilities to block the binding of fibrinogen to purified GP IIb-IIIa in a solid-phase microtiter assay. Two overlapping peptides encompassing residues 204-229 of GP IIIa were identified which blocked fibrinogen binding in this assay. Polyclonal antibodies to these peptides blocked fibrinogen binding to purified GP IIb-IIIa as well as platelet aggregation. The overlapping residues of these two peptides GP IIIa (211-222), SVSRNRDAPEGG-NH2, blocked the binding of fibronectin, von Willebrand factor, and vitronectin to purified GP IIb-IIIa. Finally, direct binding of GP IIIa (204-229) to fibrinogen and fibronectin was demonstrated by enzyme-linked immunosorbent assay. We conclude from these studies that the amino acid sequence 211-222 of GP IIIa is critically involved in adhesive protein binding, and may represent an important portion of the GP IIb-IIIa ligand binding domain.     

Sequence-specific 1H NMR assignments and identification of slowly exchanging amide protons in murine epidermal growth factor

Proton nuclear magnetic resonance (1H NMR) assignments for the murine epidermal growth factor (mEGF) in aqueous solution were determined by using two-dimensional NMR at pH 3.1 and 28 degrees C. The assignments are complete for all backbone hydrogen atoms, with the exception of the N-terminal amino group, and for 46 of the 53 side chains. Among the additional seven amino acid residues, three have complete assignments for all but one side-chain proton, and between two and four protons are missing for the remaining four residues. The sequential assignments by nuclear Overhauser effect spectroscopy are consistent with the chemically determined amino acid sequence. The NMR data show that the conformations of both the Tyr3-Pro4 and Cys6-Pro7 peptide bonds are trans in the predominant solution structure. Proton-deuterium exchange rate constants were also measured for 13 slowly exchanging amide protons. The information presented here has been used elsewhere to determine the three-dimensional structure of mEGF in aqueous solution.     

Generation and characterization of peptide-specific antibodies that inhibit von Willebrand factor binding to glycoprotein IIb-IIIa without interacting with other adhesive molecules. Selectivity is conferred by Pro1743 and other amino acid residues adjacent to the sequence Arg1744-Gly1745-Asp1746

We have generated antibodies against a synthetic peptide corresponding to the sequence of human von Willebrand factor (vWF) between residues Glu1737-Ser1750 which includes the Arg-Gly-Asp sequence common to several adhesive molecules. Two anti-peptide antibodies, one polyclonal, and one monoclonal reacted with native vWF and inhibited its binding to platelet glycoprotein (GP) IIb-IIIa, but showed negligible cross-reactivity with fibrinogen, fibronectin, and vitronectin, three other molecules that contain the sequence Arg-Gly-Asp and bind to platelets. The structural bases for the specificity of the two antibodies were evaluated by testing the ability of peptides homologous to the parent sequence, but with single amino acid substitutions, to neutralize the binding of the two antibodies to vWF. The substitution of Pro1743, the residue immediately adjacent to the Arg-Gly-Asp sequence on the amino-terminal side, with Phe resulted in a peptide that failed to interact with either antibody. Thus, Pro1743 is important for maintaining a peptide conformation recognized by two antibodies specific for the GP IIb-IIIa-binding domain of vWF. Other residues important for optimal peptide reactivity with the polyclonal antibody were Ser1742, Arg1744, and Gly1745, whereas Gly1741, Gly1745, and Asp1746, but not Arg1744, were important for reactivity with the monoclonal antibody. The epitopes of both antibodies, therefore, included at least 2 of the residues in the sequence Arg-Gly-Asp considered the common cell-binding site of adhesive molecules that interact with GP IIb-IIIa. Nevertheless, both antibodies reacted only with vWF. These studies demonstrate that peptide-specific antibodies, unlike the promiscuous GP IIb-IIIa receptor, can recognize distinctive structural characteristics of the cell-binding domain of adhesive molecules imposed by residues adjacent to the sequence Arg-Gly-Asp.     

Ca2+-dependent binding of a synthetic Arg-Gly-Asp (RGD) peptide to a single site on the purified platelet glycoprotein IIb-IIIa complex

The platelet glycoprotein IIb-IIIa complex (GP IIb-IIIa) is a member of the integrin receptor family that recognizes adhesive proteins containing the Arg-Gly-Asp (RGD) sequence. In the present study the binding characteristics of the synthetic hexapeptide Tyr-Asn-Arg-Gly-Asp-Ser (YNRGDS, a sequence present in the fibrinogen alpha-chain at position 570-575) to purified GP IIb-IIIa were determined by equilibrium dialysis. The binding of 125I-YNRGDS to GP IIb-IIIa was specific, saturable, and reversible. The apparent dissociation constant was 1.0 +/- 0.2 microM, and the maximal binding capacity was 0.92 +/- 0.02 mol of 125I-YNRGDS/mol of GP IIb-IIIa, indicating that GP IIb-IIIa contains a single binding site for RGD peptides. The binding of 125I-YNRGDS to purified GP IIb-IIIa showed many of the characteristics of fibrinogen binding to activated platelets: the binding was inhibited by fibrinogen, by the monoclonal antibody A2A9, and by the dodecapeptide from the C terminus of the fibrinogen gamma-chain. In addition, the binding of 125I-YNRGDS to GP IIb-IIIa was divalent cation-dependent. Our data suggest that two divalent cation binding sites must be occupied for YNRGDS to bind: one site is specific for calcium and is saturated at 1 microM free Ca2+, whereas the other site is less specific and reaches saturation at millimolar concentrations of either Ca2+ or Mg2+. The results of the present study support the hypothesis that the RGD domains within the adhesive proteins are responsible for their binding to GP IIb-IIIa.     

Characterization and cDNA cloning of a platelet aggregation inhibitor

A novel platelet aggregation inhibitor, sal-C, was purified to homogeneity from the venom of Korean snake (Agkistrodon halys brevicaudus). Several lines of experimental evidence clearly indicated that sal-C inhibits not only the collagen-induced platelet aggregation, but also the aggregation mediated by the cell surface glycoprotein IIb-IIIa (GP IIb-IIIa). We have isolated the cDNA encoding sal-C from the cDNA library of the snake venom gland and analyzed its complete nucleotide sequence. Sal-C is a single-chain polypeptide composed of 212 amino acids including 24 cysteines. The deduced polypeptide sequence of sal-C demonstrated considerable homology to previously described protein species of the collagen-induced platelet aggregation inhibitor family. Sal-C does not have the Arg-Gly-Asp (RGD) motif, but contains the Ser-Glu-Cys-Asp sequence. Interestingly, sal-C was found to inhibit GP IIb-IIIa binding to immobilized fibrinogen which is antagonized by the typical RGD motif of disintegrins.     

Sequence comparison of single-stranded DNA binding proteins and its structural implications

The primary sequences were compared among several proteins: gene product 5 protein (GP5) from phage M13; PIKE from phage Ike; gene product 32 protein (GP32) from phage T4; RecA, SSB and SSF from Escherichia coli. These proteins bind strongly and cooperatively to single-stranded DNA with no sequence specificity. GP5 is the smallest in this group and its three-dimensional structure is well-characterized. Using the entire sequence of GP5 as a template we searched for the regions in other single-stranded DNA binding proteins yielding the best alignment of aromatic and basic residues. The identified domains show alignment of five aromatic and four charged residues in these proteins. The domains in PIKE, GP32 and RecA exhibit statistically significant sequence homology with GP5. These observations strongly favor the hypothesis that the protein-single-stranded DNA complex in this class of proteins is stabilized by the stacking interaction of the aromatic residues with the bases of the DNA, and by the electrostatic interaction of the basic residues with the phosphate groups of the DNA. We also find that the DNA binding domains of these proteins have similar secondary structural preferences, mainly beta structures. The triple-stranded beta-sheet may be a common motif in the DNA binding domains of these proteins.     

1H NMR studies of plastocyanin from Scenedesmus obliquus: complete sequence-specific assignment, secondary structure analysis, and global fold

Two-dimensional 1H NMR methods have been used to make sequence-specific resonance assignments for the 97 amino acid residues of the plastocyanin from the green alga Scenedesmus obliquus. Assignments were obtained for all backbone protons and the majority of the side-chain protons. Spin system identification relied heavily on the observation of relayed connectivities to the backbone amide proton. Sequence-specific assignments were made by using the sequential assignment procedure. During this process, an extra valine residue was identified that had not been detected in the original amino acid sequence. Elements of regular secondary structure were identified from characteristic NOE connectivities between backbone protons, 3JHN alpha coupling constant values, and the observation of slowly exchanging amide protons. The protein in solution contains eight beta-strands, one short segment of helix, five reverse turns, and five loops. The beta-strands may be arranged into two beta-sheets on the basis of extensive cross-strand NOE connectivities. The chain-folding topology determined from the NMR experiments is that of a Greek key beta-barrel and is similar to that observed for French bean plastocyanin in solution and poplar plastocyanin in the crystalline state. While the overall structures are similar, several differences in local structure between the S. obliquus and higher plant plastocyanins have been identified.     

Ornatins: potent glycoprotein IIb-IIIa antagonists and platelet aggregation inhibitors from the leech Placobdella ornata.

The purification and characterization of six isoforms of ornatin, potent glycoprotein IIb-IIIa (GP IIb-IIIa) antagonists and platelet aggregation inhibitors are described. These isoforms were purified from whole leech homogenates of the leech Placobdella ornata, a North American leech commonly known as the turtle leech, by trichloroacetic acid precipitation, Sephadex G-50 size exclusion chromatography, GP IIb-IIIa affinity chromatography, and C18 reverse-phase HPLC. Each of the five completely sequenced isoforms, which range from 41 to 52 residues in length, contains the Arg-Gly-Asp (RGD) sequence, a common recognition sequence in adhesion proteins, as well as 6 cysteine residues; the positions of both of these features are conserved in the primary sequences. The amino acid sequences of ornatin isoforms B, C, D, and E are highly conserved, whereas ornatin A2 and A3 are less similar and lack 9 residues at the N-terminus. The ornatins are approximately 40% identical with decorsin, a GP IIb-IIIa antagonist isolated from the leech Macrobdella decora [Seymour, J. L., Henzel, W. J., Nevins, B., Stults, J. T. & Lazarus, R. A. (1990) J. Biol. Chem. 265, 10143-10147]; furthermore, the RGD sequence and 5 out of 6 cysteine residues are maintained in the same relative positions in both decorsin and ornatin. The ornatin isoforms do not exhibit significant similarity to any members of the snake-venom-derived family of GP IIb-IIIa antagonists [Dennis, M. S., Henzel, W. J., Pitti, R. M., Lipari, M. T., Napier, M. A., Deisher, T. A., Bunting, S. & Lazarus, R. A. (1990) Proc. Natl Acad. Sci. USA 87, 2471-2475] except in the RGD region of these proteins. The ornatin isoforms inhibit the binding of GP IIb-IIIa to immobilized fibrinogen with IC50 values ranging over 2.9-5.3 nM; ornatin isoforms A2, C, and E inhibit ADP-induced human platelet aggregation with IC50 values of about 130, 280, and 440 nM, respectively.     

Sequence-specific 1H NMR assignments and secondary structure in the sea anemone polypeptide Stichodactyla helianthus neurotoxin I

Sequence-specific assignments are reported for the 500-MHz 1H nuclear magnetic resonance (NMR) spectrum of the 48-residue polypeptide neurotoxin I from the sea anemone Stichodactyla helianthus (Sh I). Spin systems were first identified by using two-dimensional relayed or multiple quantum filtered correlation spectroscopy, double quantum spectroscopy, and spin lock experiments. Specific resonance assignments were then obtained from nuclear Overhauser enhancement (NOE) connectivities between protons from residues adjacent in the amino acid sequence. Of a total of 265 potentially observable resonances, 248 (i.e., 94%) were assigned, arising from 39 completely and 9 partially assigned amino acid spin systems. The secondary structure of Sh I was defined on the basis of the pattern of sequential NOE connectivities, NOEs between protons on separate strands of the polypeptide backbone, and backbone amide exchange rates. Sh I contains a four-stranded antiparallel beta-sheet encompassing residues 1-5, 16-24, 30-33, and 40-46, with a beta-bulge at residues 17 and 18 and a reverse turn, probably a type II beta-turn, involving residues 27-30. No evidence of alpha-helical structure was found.     

1H-NMR assignments and local environments of aromatic residues in bovine, human and guinea pig variants of alpha-lactalbumin.

1H-NMR assignments have been defined for the aromatic-ring protons of the bovine, guinea pig and human variants of alpha-lactalbumin. Spin-system networks were identified by means of double-quantum-filtered two-dimensional J-correlated spectroscopy and two-dimensional relayed coherence spectroscopy data. Analysis of two-dimensional nuclear-Overhauser-enhancement spectroscopy data of the proteins indicated that in each case two clusters of aromatic residues exist. The two clusters are also evident in the crystal structure of the human protein, and this evidence, in conjunction with sequence differences between the three proteins, permitted sequence-specific assignments to be made for the majority of aromatic residues. Remaining ambiguities in the assignments could be resolved by analysis of photochemically induced dynamic nuclear polarization (PCIDNP) effects. Comparison of the PCIDNP spectra of the three proteins indicated the presence of only minor differences in the surface exposure of conserved aromatic residues. Taken together, these results indicate that the environments of the conserved aromatic residues in bovine, guinea pig and human alpha-lactalbumin in solution are very similar to each other, and that the solution and the crystal forms of at least the human protein are similar.     

Sequence-specific assignments of the 1H nuclear magnetic resonance spectra of reduced high-potential ferredoxin (HiPIP) from Chromatium vinosum.

The 1H resonances of the high-potential [4Fe-4S]2+ ferredoxin from Chromatium vinosum have been assigned through conventional sequential methodology applied to 2D NMR spectra. Almost 80% of the residues were identified using standard 2D COSY, HOHAHA, and NOESY pulse sequences. These residues correspond to four segments of the primary structure that do not interact strongly with the iron-sulfur cluster. A minor correction to the amino acid sequence is strongly suggested by these NMR data. Additional protons more sensitive to the proximity of the cluster were assigned by a combination of NOESY experiments with fast repetition rates and short mixing times and of HOHAHA spectra recorded with reduced spin-lock duration aimed at compensating for the short relaxation rates. Hence, the contributions of 79 residues out of 85 were identified in NMR spectra, among which the assignments of 64 residues were completed. Even the fastest relaxing protons, like those of the cysteine ligands, could be correlated, partly because the strong hyperfine shifts isolate them from the crowded diamagnetic region. However, other protons, in particular those involved in NH-S hydrogen bonds with the iron-sulfur cluster, were more difficult to identify, most probably because their relatively broad signals overlapped with those of protons not or less perturbed by the active site. The availability of the major part of the 1H NMR assignments has enabled the detection and identification of many interresidue NOESY cross peaks. These data are in full agreement with the elements of secondary structure previously revealed by X-ray crystallographic analysis of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reconstitution of the purified platelet fibrinogen receptor. Fibrinogen binding properties of the glycoprotein IIb-IIIa complex

Several lines of evidence indicate that the platelet membrane glycoprotein IIb-IIIa complex (GP IIb-IIIa) is necessary for the expression of platelet fibrinogen receptors. The purpose of the present study was to determine whether purified GP IIb-IIIa retains the properties of the fibrinogen receptor on platelets. Glycoprotein IIb-IIIa was incorporated by detergent dialysis into phospholipid vesicles composed of 30% phosphatidylcholine and 70% phosphatidylserine. 125I-Fibrinogen binding to the GP IIb-IIIa vesicles, as measured by filtration, had many of the characteristics of 125I-fibrinogen binding to whole platelets or isolated platelet plasma membranes: binding was specific, saturable, reversible, time dependent, and Ca2+ dependent. The apparent dissociation constant for 125I-fibrinogen binding to GP IIb-IIIa vesicles was 15 nM, and the maximal binding capacity was 0.1 mol of 125I-fibrinogen/mol of GP IIb-IIIa. 125I-Fibrinogen binding was inhibited by amino sugars, the GP IIb and/or IIIa monoclonal antibody 10E5, and the decapeptide from the carboxyl terminus of the fibrinogen gamma chain. Furthermore, little or no 125I-fibrinogen bound to phospholipid vesicles lacking protein or containing proteins other than GP IIb-IIIa (i.e. bacteriorhodopsin, apolipoprotein A-I, or glycophorin). Also, other 125I-labeled plasma proteins (transferrin, orosomucoid) did not bind to the GP IIb-IIIa vesicles. These results demonstrate that GP IIb-IIIa contains the platelet fibrinogen receptor.     

Decorsin. A potent glycoprotein IIb-IIIa antagonist and platelet aggregation inhibitor from the leech Macrobdella decora

The discovery, purification, and characterization of decorsin, a protein isolated from the North American leech Macrobdella decora, are described. Decorsin acts as an antagonist of platelet glycoprotein IIb-IIIa (GPIIb-IIIa), and is a potent inhibitor of platelet aggregation. The protein was purified to apparent homogeneity from crude whole leech extracts by treatment with trifluoroacetic acid followed by GPIIb-IIIa affinity chromatography and C18 reverse-phase high performance liquid chromatography. Decorsin was also isolated from a solution of leech ingestate by treatment with trifluoroacetic acid followed by C18 reverse-phase high performance liquid chromatography. The primary sequence of decorsin indicates that the protein is 39 amino acids long and contains 6 cysteine and 6 proline residues, as well as the sequence Arg-Gly-Asp, (RGD), a proposed recognition site of many adhesion proteins. A molecular mass of 4379 was obtained by fast atom bombardment mass spectrometry and is consistent with the mass calculated from the observed sequence. Evidence for an N-3 isoform, lacking the first 3 amino-terminal residues is also presented. Both decorsin and the N-3 isoform inhibit GP IIb-IIIa binding to immobilized fibrinogen with an IC50 of approximately 1.5 nM. Human platelet aggregation induced by ADP is inhibited by decorsin with an IC50 of approximately 500 nM; complete inhibition was observed at less than or equal to 1 microM. Based on overall sequence homology, decorsin does not belong to the family of GPIIb-IIIa protein antagonists that is found in snake venoms (Dennis, M. S., Henzel, W. J., Pitti, R. M., Lipari, M. T., Napier, M. A., Deisher, T. A., Bunting, S., and Lazarus, R. A. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 2471-2475); however the carboxyl-terminal RGD-containing region from residues 27 to 38 of decorsin is approximately 60% homologous with the corresponding region of the snake venom proteins, suggesting that high affinity binding of these proteins to GPIIb-IIIa is defined by this epitope.     

Studies on the interaction of platelet glycoprotein IIb-IIIa and glycoprotein IV with fibrinogen and thrombospondin: a new immunochemical approach

We have designed a new binding assay based on crossed immunoelectrophoresis that allowed us to test for the relative capacities of platelet membrane glycoprotein IIb-IIIa (GP IIb-IIIa), and glycoprotein IV (GP IV) to bind purified Arg-Gly-Asp (RGD)-containing adhesive proteins. Preformed immune complexes were made by reacting a platelet lysate with murine monoclonal antibodies to GP IV (OKM5 and FA6-152) or to GP IIb-IIIa (AP-2). Upon two-dimensional electrophoretic separation in agarose gels and immunoprecipitation by a polyclonal antibody to mouse IgG, the immobilized complexes containing the desired antigen were further probed with purified 125I-labeled TSP or fibrinogen. Under these conditions, immobilized GP IV was found to specifically bind TSP, whereas it was unreactive with fibrinogen. By contrast, immobilized GP IIb-IIIa demonstrated fibrinogen binding capacity but did not demonstrate any reactivity toward TSP. These observations suggest that the overall structure of the adhesive protein may determine the accessibility of the RGD sequence to its binding site on GP IIb-IIIa.     

Modulation of platelet function through adhesion receptors. A dual role for glycoprotein IIb-IIIa (integrin alpha IIb beta 3) mediated by fibrinogen and glycoprotein Ib-von Willebrand factor.

We have found that the form of glycoprotein (GP) IIb-IIIa (integrin alpha IIb beta 3) expressed on nonstimulated platelets is a functional receptor that mediates selective and irreversible adhesion to immobilized fibrinogen. This occurs even in the presence of the elevated intracellular cAMP levels induced by prostaglandin E1 or after inhibition of protein kinase C activity by sphingosine. In the absence of inhibitors, platelets adhering to fibrinogen through GP IIb-IIIa become fully activated and aggregate with one another. Immobilized von Willebrand factor (vWF), in contrast, is recognized by nonstimulated platelets through another receptor, GP Ib. This interaction leads to a change in the ligand recognition specificity of GP IIb-IIIa that can then bind to immobilized vWF and mediate irreversible platelet adhesion and aggregation; this process, however, is inhibited by elevated intracellular cAMP levels or blockade of protein kinase C activity. Therefore, GP Ib and GP IIb-IIIa induce platelet activation through the selective recognition of immobilized vWF and fibrinogen, respectively, in the absence of exogenous agonists. Moreover, "nonactivated" and "activated" GP IIb-IIIa exhibits distinctly different reactivity toward surface-bound vWF, and the functional switch can be induced by the binding of vWF to GP Ib. These findings demonstrate the modulation of platelet function by two different adhesion receptors, GP Ib and GP IIb-IIIa, as well as the distinct dual role of the latter as the necessary common mediator of irreversible adhesion and aggregation on both fibrinogen and vWF.     

Proton NMR studies of apo-neocarzinostatin from Streptomyces carzinostaticus. Sequence-specific assignment and secondary structure

The sequence-specific resonance assignment of apo-neocarzinostatin from Streptomyces carzinostaticus was carried out from two-dimensional proton-NMR spectra. The assignments were obtained for the backbone protons of 111 of the 113 residues of the protein, missing the two C alpha H of one glycine but including 3 of the 4 prolines. The majority of side chain protons were also assigned. The secondary structure derived from the analysis of sequential connections corresponds to ten beta-strands separated by clearly identified loops and turns. Inter-strand connectivities and slowly exchanging amide protons confirm the presence of the two disulfide bridges from Cys37 to Cys47 and from Cys88 to Cys93 and indicate a global folding similar to that of the similar proteins, actinoxanthin and macromomycin, for which crystallographic data are available.