Regulation of von Willebrand factor binding to the platelet glycoprotein Ib-IX by a membrane skeleton-dependent inside-out signal

The platelet receptor for von Willebrand factor (vWF), glycoprotein Ib-IX (GPIb-IX), mediates initial platelet adhesion and activation. We show here that the receptor function of GPIb-IX is regulated intracellularly via its link to the filamin-associated membrane skeleton. Deletion of the filamin binding site in GPIb(alpha) markedly enhances ristocetin- (or botrocetin)-induced vWF binding and allows GPIb-IX-expressing cells to adhere to immobilized vWF under both static and flow conditions. Cytochalasin D (CD) that depolymerizes actin also enhances vWF binding to wild type GPIb-IX. Thus, vWF binding to GPIb-IX is negatively regulated by the filamin-associated membrane skeleton. In contrast to native vWF, binding of the isolated recombinant vWF A1 domain to wild type and filamin binding-deficient mutants of GPIb-IX is comparable, suggesting that the membrane skeleton-associated GPIb-IX is in a state that prevents access to the A1 domain in macromolecular vWF. In platelets, there is a balance of membrane skeleton-associated and free forms of GPIb-IX. Treatment of platelets with CD increases the free form and enhances vWF binding. CD also reverses the inhibitory effects of prostaglandin E1 on vWF binding to GPIb-IX. Thus, GPIb-IX-dependent platelet adhesion is doubly controlled by vWF conformation and a membrane skeleton-dependent inside-out signal.     

Interaction of purified actin-binding protein with the platelet membrane glycoprotein Ib-IX complex

The interaction of platelet membrane glycoprotein (GP) Ib-IX complex with the cytoplasmic membrane skeleton is potentially of major importance in regulating platelet function. Indirect evidence suggested that this interaction is mediated by actin-binding protein, but it is not known whether GP Ib-IX and actin-binding protein associate directly. To examine more closely the nature of this association, purified GP Ib-IX complex was specifically bound and oriented on the surface of impermeable polymer beads via a monoclonal antibody, AK 2, directed against the extracytoplasmic domain of GP Ib alpha (glycocalicin). Binding was specific since 1) it was abolished by excess unlabeled actin-binding protein; 2) there was no detectable specific binding of radiolabeled actin-binding protein to beads coated with glycocalicin, the major extracytoplasmic proteolytic fragment of GP Ib alpha; and 3) unlike actin-binding protein, there was no specific binding of bovine serum albumin or human platelet vinculin to the GP Ib-IX complex-coated beads. Binding of actin-binding protein to the GP Ib-IX complex-coated beads, but not to the glycocalicin-coated beads, was saturable and reversible (apparent Kd = 1 x 10(-7) M). These experiments provide direct evidence that actin-binding protein can bind to the cytoplasmic domain of a membrane glycoprotein. Because actin-binding protein is found submembranously in cells other than the platelet, it is possible that this protein may link actin filaments to the plasma membrane in those cells.     

Functional analysis of a recombinant glycoprotein Ib alpha polypeptide which inhibits von Willebrand factor binding to the platelet glycoprotein Ib-IX complex and to collagen.

By deletion mutagenesis and transient expression in COS cells, a 96-amino acid hydrophilic sequence in the glycoprotein Ib alpha polypeptide located between L220 and L318 was identified which appeared to contain its von Willebrand factor- (vWF) binding site. The cDNA encoding this fragment was then expressed in Escherichia coli and purified from the bacterial cell lysate. The recombinant polypeptide, rGpIb alpha Q221-L318, was monomeric and had an apparent molecular weight of 14,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It inhibited both ristocetin-induced binding of 125I-vWF to fixed washed platelets and ristocetin-induced platelet agglutination. The recombinant polypeptide also inhibited the binding of 125I-vWF to immobilized type I and III collagen. Inhibition of 125I-vWF binding to platelets and collagen was dose-dependent, with IC50 values of 500 and 200 nM rGpIb alpha Q221-L318, respectively. Fifty % inhibition of ristocetin-induced platelet agglutination required 500 nM rGpIb alpha Q221-L318. Although rGpIb alpha Q221-L318 inhibited vWF binding to collagen it did not, itself, bind to collagen-coated surfaces. Reduction of the disulfide bond between C248 and C264 abolished activity. 125I-rGpIb alpha Q221-L318 bound directly to GpIb/IX sites on multimeric vWF. These studies document that a portion of the sequence between Q221 and L318 is needed for recognition and binding to vWF and that binding requires an intact disulfide bond between C248 and C264. The binding of this recombinant polypeptide to vWF multimers inhibits vWF interaction with two important substrates, platelet GpIb/IX and collagen.     

Purification of botrocetin from Bothrops jararaca venom. Analysis of the botrocetin-mediated interaction between von Willebrand factor and the human platelet membrane glycoprotein Ib-IX complex

Interaction of von Willebrand factor (vWF) with its platelet receptor only occurs in vitro in the presence of a modulator such as ristocetin. We have recently confirmed that the human platelet membrane glycoprotein (GP) Ib-IX complex is the receptor involved in the ristocetin-dependent binding of vWF by reconstitution with the purified components [Berndt, M.C., Du, X., & Booth, W.J. (1988) Biochemistry 27, 633-640]. We have now developed a similar solid-phase reconstitution assay using an alternate modulator, botrocetin, for the competitive analysis of functional domains in both vWF and the GP Ib-IX complex. Botrocetin was purified from Bothrops jararaca venom by ammonium sulfate fractionation and subsequent DEAE-cellulose and hydroxylapatite chromatography. The purified protein was a 25-kilodalton (kDa) disulfide-linked dimer with apparent subunit molecular weights of 14,000 and 14,500. Binding studies with immobilized botrocetin demonstrated that botrocetin bound to vWF and to a 52/48-kDa region of vWF that contains the GP Ib binding domain, but not to glycocalicin, a proteolytic fragment of GP Ib that contains the vWF binding site. Binding of 125I-labeled vWF to GP Ib-IX complex coated beads and to platelets was strictly botrocetin-dependent with half-maximal binding at a botrocetin concentration of congruent to 0.27 microM. Botrocetin-dependent binding of vWF was specific, saturable, and comparable to that observed with ristocetin. An anti-vWF monoclonal antibody, 3F8, inhibited ristocetin- but not botrocetin-dependent binding of vWF, suggesting the presence of distinct ristocetin and botrocetin modulator sites on vWF. The botrocetin reconstitution assay was at least an order of magnitude more sensitive than the corresponding ristocetin assay for the competitive analysis of functional domains on both vWF and the GP Ib-IX complex and has confirmed the localization of the vWF-binding domain to the 45-kDa N-terminal region of GP Ib.     

Dynamic force spectroscopy of glycoprotein Ib-IX and von Willebrand factor

The first stage in hemostasis is the binding of the platelet membrane receptor glycoprotein (GP) Ib-IX complex to the A1 domain of von Willebrand factor in the subendothelium. A bleeding disorder associated with this interaction is platelet-type von Willebrand disease, which results from gain-of-function (GOF) mutations in amino acid residues 233 or 239 of the GP Ibalpha subunit of GP Ib-IX. Using optical tweezers and a quadrant photodetector, we investigated the binding of A1 to GOF and loss-of-function mutants of GP Ibalpha with mutations in the region containing the two known naturally occurring mutations. By dynamically measuring unbinding force profiles at loading rates ranging from 200-20,000 pN/s, we found that the bond strengths between A1 and GP Ibalpha GOF mutants (233, 235, 237, and 239) were significantly greater than the A1/wild-type GP Ib-IX bond at all loading rates examined (p < 0.05). In addition, mutants 231 and 232 exhibited significantly lower bond strengths with A1 than the wild-type receptors (p < 0.05). We computed unloaded dissociation rate constant (k(off)(0)) values for interactions involving mutant and wild-type GP Ib-IX receptors with A1 and found the A1/wild-type GP Ib-IX k(off)(0) value of 5.47 +/- 0.25 s(-1) to be significantly greater than the GOF k(off)(0) values and significantly less than the loss-of-function k(off)(0) values. Our data illustrate the importance of the bond kinetics associated with the VWF/GP Ib-IX interaction in hemostasis and also demonstrate the drastic changes in binding that can occur when only a single amino acid of GP Ibalpha is altered.     

Identification of discontinuous von Willebrand factor sequences involved in complex formation with botrocetin. A model for the regulation of von Willebrand factor binding to platelet glycoprotein Ib

We have used proteolytic fragments and overlapping synthetic peptides to define the domain of von Willebrand factor (vWF) that forms a complex with botrocetin and modulates binding to platelet glycoprotein (GP) Ib. Both functions were inhibited by the dimeric 116-kDa tryptic fragment and by its constituent 52/48-kDa subunit, comprising residues 449-728 of mature vWF, but not by the dimeric fragment III-T2 which lacks amino acid residues 512-673. Three synthetic peptides, representing discrete discontinuous sequences within the region lacking in fragment III-T2, inhibited vWF-botrocetin complex formation; they corresponded to residues 539-553, 569-583, and 629-643. The 116-kDa domain, with intact disulfide bonds, exhibited greater affinity for botrocetin than did the reduced and alkylated 52/48-kDa molecule, and both fragments had significantly greater affinity than any of the inhibitory peptides. Thus, conformational attributes, though not strictly required for the interaction, contribute to the optimal functional assembly of the botrocetin-binding site. Accordingly, 125I-labeled botrocetin bound to vWF and to the 116-kDa fragment immobilized onto nitrocellulose but not to equivalent amounts of the reduced and alkylated 52/48-kDa fragment; it also bound to the peptide 539-553, but only when the peptide was immobilized onto nitrocellulose at a much greater concentration than vWF or the proteolytic fragments. These studies demonstrate that vWF interaction with GP Ib may be modulated by botrocetin binding to a discontinuous site located within residues 539-643. The finding that single point mutations in Type IIB von Willebrand disease are located in the same region of the molecule supports the concept that this domain may contain regulatory elements that modulate vWF affinity for platelets at sites of vascular injury.     

Identification of a region in the cytoplasmic domain of the platelet membrane glycoprotein Ib-IX complex that binds to purified actin-binding protein.

The platelet membrane glycoprotein (GP) Ib-IX complex is a major site of attachment of the platelet membrane skeleton to the plasma membrane. This association is mediated by the interaction of actin-binding protein with the GP Ib-IX complex. The aim of the present work was to identify domains on the GP Ib-IX complex that interact with actin-binding protein. Synthetic peptides corresponding to sequences of the GP Ib alpha-chain and beta-chain cytoplasmic domains were analyzed for their ability to bind to purified actin-binding protein. Two overlapping peptides encompassing a sequence (Thr-536-Phe-568) from the central region of the cytoplasmic domain of GP Ib alpha were the most effective in binding 125I-actin-binding protein, as assessed by a microtiter well approach and peptide affinity chromatography. One of the active peptides (Thr-536-Leu-554) was chosen to evaluate the likelihood that the central region of the cytoplasmic domain of GP Ib alpha is involved in binding of the intact complex to actin-binding protein. This peptide could be specifically cross-linked to purified actin-binding protein in solution. Rabbit polyclonal antibody against this peptide inhibited the binding of purified actin-binding protein to the purified GP Ib-IX complex. Finally, as in intact platelets, the calpain-induced hydrolytic fragments of purified actin-binding protein (M(r) = 200,000 and M(r) = 91,000) showed little binding to the GP Ib alpha peptide. Taken together, these results provided evidence that a region between Thr-536 and Phe-568 of the cytoplasmic domain of GP Ib alpha participates in the interaction of the GP Ib-IX complex with actin-binding protein.     

Trigramin: primary structure and its inhibition of von Willebrand factor binding to glycoprotein IIb/IIIa complex on human platelets

Trigramin, a naturally occurring peptide purified from Trimeresurus gramineus (T. stejnegeri formosensis) snake venom, inhibits platelet aggregation and the binding of 125I-fibrinogen to ADP-stimulated platelets (Ki = 2 X 10(-8) M) without affecting the platelet-release reaction. 125I-trigramin binds to ADP-stimulated and to chymotrypsin-treated normal platelets but not to thrombasthenic platelets. 125I-trigramin binding to platelets is blocked by monoclonal antibodies directed against the glycoprotein IIb/IIIa complex and by Arg-Gly-Asp-Ser (RGDS) [Huang et al. (1987) J. Biol. Chem. 262, 161]. We determined the primary structure of trigramin, which is composed of a single polypeptide chain of 72 amino acid residues and six disulfide bridges. The molecular weight of trigramin calculated on the basis of amino acid sequence was 7500, and the average pI was 5.61. An RGD sequence appeared in the carboxy-terminal domain of trigramin. An amino-terminal fragment (7-33) of trigramin showed 39% homology with a region (1555-1581) of von Willebrand factor (vWF). Trigramin also showed 36% identity in a 42 amino acid overlap and 53% identity in a 15 amino acid overlap when compared with two adhesive proteins, collagen alpha 1 (I) and laminin B1, respectively. Trigramin blocked binding of human vWF to the glycoprotein IIb/IIIa complex in thrombin-activated platelets in a dose-dependent manner. Reduction of trigramin resulted in a marked decrease in its ability to block vWF binding to human platelets.(ABSTRACT TRUNCATED AT 250 WORDS)     

Targeting of liposomes carrying recombinant fragments of platelet membrane glycoprotein Ibalpha to immobilized von Willebrand factor under flow conditions

Liposomes with covalently bound recombinant fragments of platelet membrane glycoprotein Ibalpha that retain the von Willebrand factor (vWf)-binding function (rGPIbalpha-liposomes) were prepared. Their interactions with an immobilized vWf surface under flow conditions were evaluated with a recirculating flow chamber, mounted on an epifluorescence microscope, which allows real-time visualization of fluorescence-labeled liposomes interacting with the surface. The interaction of rGPIbalpha-liposomes with the vWf surface was directly related to shear rate. At high densities of rGPIbalpha and vWf, rGPIbalpha-liposomes establishing contact with the vWf surface exhibited continuous displacement with decreased velocity relative to the hydrodynamic flow, depending on receptor density and matrix concentration. At lower densities of rGPIbalpha and vWf, rGPIbalpha-liposomes stopped only transiently, in the millisecond range, on the surface. This is the first study to demonstrate that the targeting of rGPIbalpha-liposomes is specific to the vWf surface under flow conditions.     

Reconstitution of the platelet glycoprotein Ib-IX complex in phospholipid bilayer Nanodiscs

The glycoprotein Ib-IX (GPIb-IX) complex expressed on platelet plasma membrane is involved in thrombosis and hemostasis via the initiation of adhesion of platelets to von Willebrand factor (VWF) exposed at the injured vessel wall. While most of the knowledge of the GPIb-IX complex was obtained from studies on platelets and transfected mammalian cells expressing the GPIb-IX complex, there is not an in vitro membrane system that allows systematic analysis of this receptor. The phospholipid bilayer Nanodisc composed of a patch of phospholipid surrounded by membrane scaffold protein is an attractive tool for membrane protein study. We show here that the GPIb-IX complex purified from human platelets has been reconstituted into the Nanodisc. The Nanodisc-reconstituted GPIb-IX complex was able to bind various conformation-sensitive monoclonal antibodies. Furthermore, it bound to VWF in the presence of botrocetin with an apparent K(d) of 0.73 ± 0.07 nM. The binding to VWF was inhibited by anti-GPIbα antibodies with epitopes overlapping with the VWF-binding site, but not by anti-GPIbβ monoclonal antibody RAM.1. Finally, the Nanodisc-reconstituted GPIb-IX complex exhibited ligand binding activity similar to that of the isolated extracellular domain of GPIbα. In conclusion, the GPIb-IX complex in Nanodiscs adopts a native-like conformation and possesses the ability to bind its natural ligands, thus making a Nanodisc a suitable in vitro platform for further investigation of this hemostatically important receptor complex.     

Identification of aspartic acid 514 through glutamic acid 542 as a glycoprotein Ib-IX complex receptor recognition sequence in von Willebrand factor. Mechanism of modulation of von Willebrand factor by ristocetin and botrocetin.

As the first step in hemostasis, the binding of von Willebrand factor (vWF) to the platelet membrane glycoprotein (GP) Ib-IX complex is essential for platelet adhesion at high-shear blood flow. This interaction in vivo requires the prior binding of vWF to the subendothelial matrix, a process which exposes a normally cryptic binding site on vWF for the GP Ib-IX complex. This process can be mimicked in vitro by modulators such as ristocetin or the snake venom protein botrocetin or by desialation of vWF. We have previously localized the GP Ib binding site on vWF to a monomeric dispase fragment which extends from Leu-480/Val-481 to Gly-718 in the primary sequence of mature vWF [Andrews, R. K., Gorman, J. J., Booth, W. J., Corino, G. L., Castaldi, P. A., & Berndt, M. C. (1989) Biochemistry 28, 8326-8336]. This fragment also contains a distinct binding site for botrocetin. Analysis of synthetic peptides corresponding to hydrophilic stretches of sequence within this fragment indicated that the sequence Asp-514-Glu-542 represents a major adhesive sequence involved in receptor recognition. This peptide inhibited both the ristocetin- and botrocetin-mediated binding of vWF to either platelets or purified GP Ib-IX complex (IC50 approximately 50-200 microM) as well as the asialo-vWF- and bovine vWF-dependent agglutination of platelets. Both the N- and C-terminal halves of the peptide were inhibitory but less so than the intact peptide. This peptide also inhibited botrocetin binding to vWF, suggesting that botrocetin modulates vWF-GP Ib interaction by binding in close proximity to the vWF adhesion sequence.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of peptide antagonists to glycoprotein Ibalpha that selectively inhibit von Willebrand factor dependent platelet aggregation

GPIbalpha is an integral membrane protein of the GPIb-IX-V complex found on the platelet surface that interacts with the A1 domain of von Willebrand factor (vWF-A1). The interaction of GPIbalpha with vWF-A1 under conditions of high shear stress is the first step in platelet-driven thrombus formation. Phage display was used to identify peptide antagonists of the GPIbalpha-vWF-A1 interaction. Two nine amino acid cysteine-constrained phage display libraries were screened against GPIbalpha revealing peptides that formed a consensus sequence. A peptide with sequence most representative of the consensus, designated PS-4, was used as the basis for an optimized library. The optimized selection identified additional GPIbalpha binding peptides with sequences nearly identical to the parent peptide. Surface plasmon resonance of the PS-4 parent and two optimized synthetic peptides, OS-1 and OS-2, determined their equilibrium dissociation GPIbalpha binding constants ( K Ds) of 64, 0.74, and 31 nM, respectively. Isothermal calorimetry corroborated the K D of peptide PS-4 with a resulting affinity value of 68 nM. An ELISA demonstrated that peptides PS-4, OS-1, and OS-2 competitively inhibited the interaction between the vWF-A1 domain and GPIbalpha-Fc in a concentration-dependent manner. All three peptides inhibited GPIbalpha-vWF-mediated platelet aggregation induced under high shear conditions using the platelet function analyzer (PFA-100) with full blockade observed at 150 nM for OS-1. In addition, OS-1 blocked ristocetin-induced platelet agglutination of human platelets in plasma with no influence on platelet aggregation induced by several agonists of alternative platelet aggregation pathways, demonstrating that this peptide specifically disrupted the GPIbalpha-vWF-A1 interaction.     

Identification of a site in the alpha chain of platelet glycoprotein Ib that participates in von Willebrand factor binding

The binding of von Willebrand factor (vWF) to the platelet receptor glycoprotein (GP) Ib-IX complex is a key event in hemostasis and may participate in the development of thrombotic vascular occlusion. We present here evidence that residues Ser251-Tyr279 in the GP Ib alpha-chain participate in this function. Initial studies suggested that the modality of vWF interaction with GP Ib depended on the conditions used for induction of binding, either in the presence of ristocetin, or botrocetin, or with asialo-vWF. In fact, only the 45-kDa amino-terminal fragment of GP Ib alpha inhibited the vWF-GP Ib interaction under all conditions tested, while the 84-kDa macroglycopeptide was significantly effective only in the presence of ristocetin. Moreover, the 45-kDa fragment with reduced disulfide bonds still inhibited ristocetin-induced binding but had no effect, at the concentrations tested, on botrocetin-mediated or direct asialo-vWF binding. In order to localize in more detail the functional site, the entire sequence of the 45-kDa fragment was reproduced in 27 overlapping synthetic peptides that were then used in inhibition of binding assays. This led to the identification of a linear GP Ib alpha sequence (residues Ser251-Tyr279) that effectively inhibited platelet interaction with vWF mediated by ristocetin and, at higher concentration, also by botrocetin. A shorter peptide overlapping with the longer one (residues Gly271-Glu285) was the second most active inhibitory species. This region of the molecule contains several residues with a high surface probability index, as expected for a site involved in ligand binding. Thus, while native conformation of GP Ib alpha appears to be important for optimal interaction with vWF, the results obtained with short synthetic peptides may help in defining the amino acid residues participating in this essential function.     

Human factor XII binding to the glycoprotein Ib-IX-V complex inhibits thrombin-induced platelet aggregation

Factor XII deficiency has been postulated to be a risk factor for thrombosis suggesting that factor XII is an antithrombotic protein. The biochemical mechanism leading to this clinical observation is unknown. We have previously reported high molecular weight kininogen (HK) inhibition of thrombin-induced platelet aggregation by binding to the platelet glycoprotein (GP) Ib-IX-V complex. Although factor XII will bind to the intact platelet through GP Ibalpha (glycocalicin) without activation, we now report that factor XIIa (0. 37 microm), but not factor XII zymogen, is required for the inhibition of thrombin-induced platelet aggregation. Factor XIIa had no significant effect on SFLLRN-induced platelet aggregation. Moreover, an antibody to the thrombin site on protease-activated receptor-1 failed to block factor XII binding to platelets. Inhibition of thrombin-induced platelet aggregation was demonstrated with factor XIIa but not with factor XII zymogen or factor XIIf, indicating that the conformational exposure of the heavy chain following proteolytic activation is required for inhibition. However, inactivation of the catalytic activity of factor XIIa did not affect the inhibition of thrombin-induced platelet aggregation. Factor XII showed displacement of biotin-labeled HK (30 nm) binding to gel-filtered platelets and, at concentrations of 50 nm, was able to block 50% of the HK binding, suggesting involvement of the GP Ib complex. Antibodies to GP Ib and GP IX, which inhibited HK binding to platelets, did not block factor XII binding. However, using a biosensor, which monitors protein-protein interactions, both HK and factor XII bind to GP Ibalpha. Factor XII may serve to regulate thrombin binding to the GP Ib receptor by co-localizing with HK, to control the extent of platelet aggregation in vivo.     

Specific heteromeric association of four transmembrane peptides derived from platelet glycoprotein Ib-IX complex

As the receptor on the platelet surface for von Willebrand factor, glycoprotein (GP) Ib-IX complex is critically involved in hemostasis and thrombosis. How the complex is assembled from GP Ibα, GP Ibβ and GP IX subunits, all of which are type I transmembrane proteins, is not entirely clear. Genetic and mutational analyses have identified the transmembrane (TM) domains of these subunits as active participants in assembly of the complex. In this study, peptides containing the transmembrane domain of each subunit have been produced and their interaction with one another characterized. Only the Ibβ TM sequence, but not the Ibα and IX counterparts, can form homo-oligomers in SDS-PAGE and TOXCAT assays. Following up on our earlier observation that a Ibβ-Ibα-Ibβ peptide complex (αβ₂) linked through native juxtamembrane disulfide bonds could be produced from isolated Ibα and Ibβ TM peptides in detergent micelles, we show here that addition of the IX TM peptide facilitates formation of the native αβ₂ complex, reproducing the same effect by the IX subunit in cells expressing the GP Ib-IX complex. Specific fluorescence resonance energy transfer was observed between donor-labeled αβ₂ peptide complex and acceptor-conjugated IX TM peptide in micelles. Finally, the mutation D135K in the IX TM peptide could hamper both the formation of the αβ₂ complex and the energy transfer, consistent with its reported effect in the full-length complex. Overall, our results have demonstrated directly the native-like heteromeric interaction among the isolated Ibα, Ibβ and IX TM peptides, which provides support for the four-helix bundle model of the TM domains in the GP Ib-IX complex and paves the way for further structural analysis. The methods developed in this study may be applicable to other studies of heteromeric interaction among multiple TM helices.     

Efficient plasma membrane expression of a functional platelet glycoprotein Ib-IX complex requires the presence of its three subunits.

The glycoprotein (GP) Ib-IX complex of the platelet plasma membrane mediates the adhesion of platelets to damaged blood vessel wall. The complex is composed of three membrane-spanning polypeptides, GP Ib alpha, GP Ib beta, and GP IX, all of which are absent from the platelets of patients with the hereditary bleeding disorder Bernard-Soulier syndrome. In this study we report stable expression of the recombinant receptor in three cell lines and demonstrate that the three subunits of the complex are necessary for its efficient expression on the plasma membrane. The expressed complex associates with the cytoskeleton of the transfected cells through an interaction with actin-binding protein and binds its ligand, von Willebrand factor. These data suggest that the lack of plasma membrane GP Ib-IX complex in the Bernard-Soulier syndrome could potentially arise from mutations affecting any one of its three subunits.     

Fibronectin-binding properties of the purified platelet glycoprotein IIb-IIIa complex

Fibronectin binds to specific receptors on the surface of washed, thrombin-activated platelets. Evidence suggests that these receptors are closely associated with the platelet glycoprotein IIb-IIIa complex (GP IIb-IIIa). To determine whether GP IIb-IIIa itself can form a platelet receptor for fibronectin, we used a filtration assay to examine the interaction of purified fibronectin with purified GP IIb-IIIa incorporated into phospholipid vesicles. 125I-Fibronectin binding to the phospholipid vesicles required the presence of incorporated GP IIb-IIIa and was specific, time-dependent, reversible, saturable, and divalent cation-dependent (Mg2+ greater than Ca2+). The dissociation constant for 125I-fibronectin binding to the GP IIb-IIIa-containing vesicles in the presence of 2 mM MgCl2 was 87 nM. Proteins or peptides that inhibit 125I-fibronectin binding to whole platelets also inhibited 125I-fibronectin binding to the GP IIb-IIIa vesicles. Thus, specific 125I-fibronectin binding was inhibited by excess unlabeled fibrinogen or fibronectin, the anti-GP IIb-IIIa monoclonal antibody 10E5, the decapeptide from the carboxyl terminus of the fibrinogen gamma-chain, and the tetrapeptide Arg-Gly-Asp-Ser from the cell-binding domain of fibronectin. In contrast to results obtained using whole platelets, unlabeled fibronectin inhibited 125I-fibronectin binding to the GP IIb-IIIa vesicles. These results show that 125I-fibronectin binds directly to purified GP IIb-IIIa with most of the previously reported properties of 125I-fibronectin binding to washed, thrombin-stimulated platelets. Thus, GP IIb-IIIa has the potential to function as a platelet receptor for fibronectin as well as for fibrinogen.     

The transmembrane domain of glycoprotein Ibbeta is critical to efficient expression of glycoprotein Ib-IX complex in the plasma membrane

Lack of expression of glycoprotein (GP) Ib-IX-V complex in platelets often results from mutations in its three subunits: GP Ibalpha, GP Ibbeta, or GP IX. The requirement of all three subunits in the efficient surface expression of the receptor complex has been reproduced in Chinese hamster ovary cells. Here, we probed the role of the transmembrane domains in expression of the GP Ib-IX complex and potential interactions between these domains. Replacing the transmembrane domains of either GP Ibalpha or GP Ibbeta, but not that of GP IX, with unrelated sequences markedly diminished surface expression of the GP Ib-IX complex in transiently transfected Chinese hamster ovary cells. Replacement of the Ibbeta transmembrane domain produced the largest effect. Furthermore, several single-site mutations in the Ibbeta transmembrane domain were found to significantly decrease overall expression as well as surface expression of GP Ibalpha, probably by perturbing the interaction between the Ibalpha and Ibbeta transmembrane domains and in turn reducing the stability of GP Ibalpha in the cell. Mutations S503V and S503L in the Ibalpha transmembrane domain partly reversed the expression-decreasing effect of mutation H139L, but not the others, in the Ibbeta transmembrane domain, suggesting a specific interaction between these two polar residues. Together, our results have demonstrated the importance of the Ibbeta transmembrane domain, through its interaction with the Ibalpha counterpart, to the proper assembly and efficient surface expression of the GP Ib-IX complex.     

Modulation of an RGDS binding site on the platelet membrane glycoprotein IIb-IIIa complex

Fibronectin, von Willebrand factor, and fibrinogen each bind to the glycoprotein IIb-IIIa complex on activated platelets via an arg-gly-asp-ser (RGDS) sequence present within the adhesive proteins. Both the IIb and IIIa polypeptides of the IIb-IIIa complex on thrombin activated platelets are specifically and extensively labeled by a radiolabeled, photoactivatable arylazide derivative of the RGDS sequence when the labeling is performed in the presence of concentrations of Ca++ or Mg++ approaching 0.5 mM. In contrast, labeling of unactivated platelets, ADP activated platelets, or thrombin activated platelets in the presence of low concentrations of divalent cations resulted in restriction of labeling to the IIb polypeptide of the complex.