Molecular characterization of a unique von Willebrand disease variant. A novel mutation affecting von Willebrand factor/factor VIII interaction

von Willebrand factor (vWF) plays a central role in blood coagulation, mediating the adhesion of the initial platelet plug to the subendothelium, and serving as the carrier for factor VIII (FVIII) in the circulation. In previous studies, we have mapped the epitope for an anti-vWF monoclonal antibody which inhibits the interaction between FVIII and vWF to a region spanning Thr78 to Thr96 of the mature protein (Bahou, W.F., Ginsburg, D., Sikkink, R., Litwiller, R., and Fass, D. N. (1989) J. Clin. Invest. 84, 56-61). We now report the identification of a mutation within this region of vWF that results in decreased FVIII binding. Sequence analysis of polymerase chain reaction amplified platelet vWF mRNA from a von Willebrand disease (vWD) patient with a disproportionately low FVIII level identified a single nucleotide substitution (G----A), resulting in the conversion of Arg91----Gln. Recombinant vWF carrying this substitution showed decreased binding to FVIII compared with wild-type vWF or vWF carrying a polymorphic substitution in the same region (Arg89----Gln). These observations suggest a critical role for Arg91 in the interaction of vWF with FVIII and identify the molecular mechanism for a variant of vWD associated with unusually low FVIII levels.     

Platelet glycoprotein Ib beta/IX mediates glycoprotein Ib alpha localization to membrane lipid domain critical for von Willebrand factor interaction at high shear

The localization of the platelet glycoprotein GP Ib-IX complex (GP Ibα, GP Ibβ, and GP IX) to membrane lipid domain, also known as glycosphingolipid-enriched membranes (GEMs or raft) lipid domain, is essential for the GP Ib-IX complex mediated platelet adhesion to von Willebrand factor (vWf) and subsequent platelet activation. To date, the mechanism for the complex association with the GEMs remains unclear. Although the palmitate modifications of GP Ibβ and GP IX were thought to be critical for the complex presence in the GEMs, we found that the removal of the putative palmitoylation sites of GP Ibβ and GP IX had no effects on the localization of the GP Ib-IX complex to the GEMs. Instead, the disruption of GP Ibα disulfide linkage with GP Ibβ markedly decreased the amount of the GEM-associated GP Ibα without altering the GEM association of GP Ibβ and GP IX. Furthermore, partial dissociation with the GEMs greatly inhibited GP Ibα interaction with vWf at high shear instead of in static condition or under low shear stress. Thus, for the first time, we demonstrated that GP Ibβ/GP IX mediates the disulfide-linked GP Ibα localization to the GEMs, which is critical for vWf interaction at high shear.     

Full-length von Willebrand factor (vWF) cDNA encodes a highly repetitive protein considerably larger than the mature vWF subunit.

Full-length human von Willebrand factor (vWF) cDNA was assembled from partial, overlapping vWF cDNAs. This cDNA construct includes a coding sequence of 8439 nucleotides which encode a single-chain precursor of 2813 amino-acid residues, representing a putative signal peptide, a prosequence and mature vWF of 22, 741 and 2050 amino acids, respectively. This represents the longest coding sequence determined to date. In-vitro expression of full-length vWF cDNA revealed the synthesis of a polypeptide with a mol. wt corresponding with that of the unglycosylated precursor. The precursor is a highly repetitive protein which consists of two duplicated (B, C), a triplicated (A), a quadruplicated (D) and a partly duplicated domain (D'), in the following order: H-D1-D2-D'-D3-A1-A2-A3-D4-B1-B2-C1-C2-OH. Both the prosequence, composed of two D domains (D1, D2), and mature vWF harbor an arg-gly-asp ('R-G-D') sequence which has been implicated in cell-attachment functions. It is argued that the pro-sequence is equivalent to von Willebrand Antigen II (vW AgII).     

Full-length von Willebrand factor (vWF) cDNA encodes a highly repetitive protein considerably larger than the mature vWF subunit

[This corrects the article on p. 1839 in vol. 5, PMID: 3019665.].     

The lymphoma transmembrane glycoprotein GP85 (CD44) is a novel guanine nucleotide-binding protein which regulates GP85 (CD44)-ankyrin interaction.

In this study, we have used photoaffinity labeling by [32P]azido-GTP as well as [32P]ADP-ribosylation by pertussis toxin (PT) and cholera toxin (CT) to identify GTP-binding proteins associated with mouse T-lymphoma plasma membranes. Our results indicate that GP85 (CD44) can be photoaffinity labeled by [32P] azido-GTP and [32P]ADP-ribosylated by both PT and CT. Using purified GP85 (CD44) obtained by Triton X-100 extraction, wheat germ agglutinin-Sepharose, and anti-GP85 (CD44) antibody affinity chromatographies, we have further characterized GP85 (CD44) as a GTP-binding protein. GP85 (CD44) is found to bind guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) in a time- and dose-dependent manner with a dissociation constant of 0.83 nM. Importantly, GP85 (CD44) appears to display a GTPase activity which hydrolyzes [gamma-32P]GTP at a rate of 0.011 mol of Pi released/mol of GP85 (CD44)/min. This GTPase activity can be readily inhibited by PT- or CT-mediated ribosylation of GP85 (CD44). Most interestingly, GTP binding significantly enhances the interaction of purified GP85 (CD44) with ankyrin, whereas ADP-ribosylation of GP85 (CD44) by PT or CT inhibits the GTP-induced increase in ankyrin binding to GP85 (CD44). In addition to GP85 (CD44) being the first reported transmembrane GTP-binding protein, these results suggest that GTP plays an important role in promoting the interaction between GP85 (CD44) and its underlying membrane cytoskeleton through ankyrin.     

Molecular mapping of the chloride-binding site in von Willebrand factor (VWF): energetics and conformational effects on the VWF/ADAMTS-13 interaction

Physiological concentrations of NaCl inhibit the hydrolysis of von Willebrand factor (VWF) by ADAMTS-13. This effect is because of the specific binding of chloride ions to VWF. Urea-induced unfolding was measured in the presence of NaCl, CH3COONa, and NaClO4 at pH 8.0, 25 degrees C, for multimeric VWF, the recombinant A1-A2-A3 VWF domains, and the A1 domain. Chloride stabilizes the folded conformation of the A1-A2-A3 and A1 domains more efficiently than acetate but less strongly than perchlorate. Spectroscopic evidence showed that chloride binds to both the A1 and A1-A2 domain but not to the isolated A2 domain. Binding of Cl- to both wild type (WT) and the natural mutant p.R1306W A1-A2-A3 domains of VWF has a large heat capacity change equal to -1 and -0.4 kcal mol(-1) K(-1) for WT and p.R1306W A1-A2-A3 domains, respectively. This result implies that a burial of a vast apolar surface area is caused by conformational transitions linked to chloride binding. At any temperature, chloride affinity was higher for WT than for the mutant p.R1306W form. Chloride ions inhibit hydrolysis by ADAMTS-13 of the A1-A2-A3 and A1-A2 domains in the presence of either urea or high shear stress, whereas this effect was either absent or negligible in experiments using A2 and A2-A3 domains. These findings show that the A1 domain contains the binding site of chloride ions that control allosterically the proteolysis by ADAMTS-13 of the Tyr1605-Met1606 bond in the A2 domain and that the R1306W mutation of type 2B VWD quenches the binding of chloride ion to the A1 domain.     

Expression of variant von Willebrand factor (vWF) cDNA in heterologous cells: requirement of the pro-polypeptide in vWF multimer formation.

Von Willebrand factor (vWF) is a multimeric plasma glycoprotein synthesized by vascular endothelial cells as a pre-pro-polypeptide with a highly repetitive domain structure, symbolized by the formula: (H)-D1-D2-D'-D3-A1-A2-A3-D4-B1-B2-B3-C1-C2-(OH) A heterologous expression system for the synthesis of recombinant vWF protein was developed, consisting of a monkey kidney cell line (COS-1), transfected with full-length vWF cDNA. This system was shown to mimic the constitutive secretory pathway of vWF in endothelial cells, since dimerization and multimerization occur similarly. To determine whether the pro-polypeptide, composed of the domains D1 and D2, is involved in vWF multimerization, a vWF cDNA was constructed that lacked the coding sequence for the pro-polypeptide. The mutant vWF protein, expressed by COS-1 cells transfected with this cDNA, did not assemble beyond the dimer stage. From this observation, we conclude that (i) dimerization does not involve the pro-polypeptide of pro-vWF and (ii) the presence of the pro-polypeptide, as part of pro-vWF, is obligatory for multimerization. It is argued that the interactions, required for interchain binding, are mediated by the D domains.     

Functional analysis of a recombinant glycoprotein Ia/IIa (Integrin alpha(2)beta(1)) I domain that inhibits platelet adhesion to collagen and endothelial matrix under flow conditions

The interaction of platelets with collagen plays an important role in primary hemostasis. Glycoprotein Ia/IIa (GPIa/IIa, integrin alpha(2)beta(1)) is a major platelet receptor for collagen. The binding site for collagen has been mapped to the I domain within the alpha(2) subunit (GPIa). In order to assess the role of the alpha(2)-I domain structure in GPIa/IIa binding to collagen, a recombinant I domain (amino acids 126-337) was expressed in Escherichia coli. The alpha(2)-I protein bound human types I and III collagen in a saturable and divalent cation-dependent manner and was blocked by the alpha(2)beta(1) function blocking antibody 6F1. The alpha(2)-I protein inhibited collagen-induced platelet aggregation (IC(50) = 600 nM). Unexpectedly, 6F1, an antibody that fails to inhibit platelet aggregation in platelet-rich plasma, blocked the inhibitory effect of the alpha(2)-I protein. The alpha(2)-I protein was able to prevent platelet adhesion to a collagen surface exposed to flowing blood under low shear stress. Interestingly, it inhibited platelet adhesion to extracellular matrix at high shear stress. These results, taken together, provide firm evidence that GPIa/IIa directly mediates the first contact of platelets with collagen under both stirring and flow conditions.     

Integrin-associated protein (CD47/IAP) contributes to T cell arrest on inflammatory vascular endothelium under flow.

Integrin-associated protein (CD47/IAP) is a pentaspan molecule that regulates integrin functions. We prepared a CD47-deficient Jurkat T cell line to assess its role in the arrest of T cells on inflammatory endothelium. Under flow conditions, constitutive arrest of CD47-deficient cells is strongly decreased as compared to the original cell line, whereas reexpression of CD47 reestablishes their ability to stop. Moreover, cells transfected with a chimera made with the extracellular portion of CD47 and the transmembrane domain of CD7 or several truncated forms of CD47 show that the first transmembrane domain and a short cytoplasmic loop are sufficient for this process. CD47 effect is indirect and depends mainly on the alpha4beta1/VCAM-1 pathway, as shown by blocking antibodies. We detected on endothelium the two CD47 counter receptors known to date: thrombospondin and SIRP1alpha. Blocking experiments show that both are involved. Overall, CD47 participates in the constitutive arrest of T lymphocytes on inflamed vascular endothelium by up-regulating alpha 4beta1 integrins.     

The human leukocyte adhesion glycoprotein Mac-1 (complement receptor type 3, CD11b) alpha subunit. Cloning, primary structure, and relation to the integrins, von Willebrand factor and factor B

Mac-1 (CD 11b/CD18) is a leukocyte adhesion heterodimeric glycoprotein which functions both as a receptor for iC3b (CR3) and in several cell-cell and cell-substrate adhesive interactions. We describe full-length cDNA clones for the alpha subunit of Mac-1. Mac-1 alpha subunit message was detected in blood monocytes and phorbol-12-myristate acetate-induced myeloid cell lines, but not in cells of the T or B lineages, correlating with Mac-1 protein surface expression. The alpha subunit of Mac-1 is a transmembrane protein of 1137 residues with a long extracellular domain (1092 residues) and a 19-amino acid cytoplasmic tail. The extracellular domain contains three putative divalent cation-binding sequences and 19 potential N-glycosylation sites. The amino acid sequence of Mac-1 alpha shows that it is a member of the integrin superfamily; Mac-1 alpha shows 63% identity to the alpha subunit of the leukocyte adhesion glycoprotein p150.95 and 25% to the alpha subunits of the extracellular matrix receptors platelet glycoprotein IIb/IIIa, the fibronectin receptor, and the vitronectin receptor. The Mac-1 alpha subunit putative divalent cation-binding sites and the flanking regions exhibit a high degree of identity both to the p150.95 alpha subunit (87% identity at the amino acid level) and to the rest of the integrin alpha subunits (38%). The alpha subunit of Mac-1, like the p150.95 alpha subunit, contains a domain of 187 amino acids in the extracellular region which is absent in other integrins. This leukocyte or "L" domain is homologous to the A domains of von Willebrand factor, which in turn are homologous to regions of the C3-binding proteins factor B and C2. These findings draw attention to this region of Mac-1 as a potential binding site for iC3b.     

A Bethlem myopathy Gly to Glu mutation in the von Willebrand factor A domain N2 of the collagen alpha3(VI) chain interferes with protein folding.

A single G1679E mutation in the amino-terminal globular domain N2 of the alpha3 chain of type VI collagen was found in a large family affected with Bethlem myopathy. Recombinant production of N2 ( approximately 200 residues) in transfected mammalian cells has now been used to examine the possibility that the mutation interfered with protein folding. The wild-type form and a G1679A mutant were produced at high levels and shown to fold into a stable globular structure. Only a small amount of secretion was observed for mutants G1679E and G1679Q, which apparently were efficiently degraded within the cells. Homology modeling onto the related von Willebrand factor A1 structure indicated that substitution of G1679 by the bulky E or Q cannot be accommodated without considerable changes in the folding pattern. This suggests protein misfolding as a molecular basis for this particular mutation in Bethlem myopathy, in agreement with radioimmunoassay data showing reduced levels of domain N2 in cultured fibroblasts from two patients.     

CD40 signaling activates CD11a/CD18 (LFA-1)-mediated adhesion in B cells.

Cell-cell adhesion events play critical roles in the sequential migrations and multiple specific cell-cell interactions which B cells undergo during normal development and function. We have observed that mAb to several B cell-associated molecules, including mAb to CD19, CD37, and CD40, induce homotypic aggregation of freshly isolated human B cells. The aggregation of B cells induced by CD40 mAb was due to activation of a cell-cell adhesion system, and not due to agglutination by mAb, because 1) in addition to being energy dependent and cation dependent, the aggregation was blocked by inhibitors of messenger RNA and protein synthesis; and 2) a mouse B cell line transformed with intact human CD40 aggregated in response to CD40 mAb, whereas a line expressing surface CD40, but lacking the cytoplasmic tail and previously shown incapable of transmitting a signal from the cell surface, did not aggregate. The aggregation, although of slow onset, was persistent and of high avidity. In addition, CD40 mAb induced increased surface expression of intercellular adhesion molecule-1 (CD54), a ligand for CD11a/CD18 (LFA-1), and CD18 mAb blocked aggregation. CD40 mAb also augmented the ability of dense B cells to stimulate the proliferation of allogeneic T cells via a CD18-dependent process. We conclude that signaling through CD40, elicited by cross-linking the CD40 protein on the cell surface, activates the CD18/intercellular adhesion molecule adhesion system; in addition, CD40 cross-linking may activate a second adhesion system since CD40 mAb induced aggregation of the B cell line Ramos, which does not express surface CD18. B cell adhesion may be triggered by signaling through multiple surface proteins, thereby lending specificity of activation to adhesion systems which are broadly expressed.     

Characterization of SAF-A, a novel nuclear DNA binding protein from HeLa cells with high affinity for nuclear matrix/scaffold attachment DNA elements.

We identified four proteins in nuclear extracts from HeLa cells which specifically bind to a scaffold attachment region (SAR) element from the human genome. Of these four proteins, SAF-A (scaffold attachment factor A), shows the highest affinity for several homologous and heterologous SAR elements from vertebrate cells. SAF-A is an abundant nuclear protein and a constituent of the nuclear matrix and scaffold. The homogeneously purified protein is a novel double stranded DNA binding protein with an apparent molecular weight of 120 kDa. SAF-A binds at multiple sites to the human SAR element; competition studies with synthetic polynucleotides indicate that these sites most probably reside in the multitude of A/T-stretches which are distributed throughout this element. In addition we show by electron microscopy that the protein forms large aggregates and mediates the formation of looped DNA structures.     

Cross-linking of a monomeric 39/34-kDa dispase fragment of von Willebrand factor (Leu-480/Val-481-Gly-718) to the N-terminal region of the alpha-chain of membrane glycoprotein Ib on intact platelets with bis(sulfosuccinimidyl) suberate

A 39/34-kilodalton (kDa) monomeric dispase fragment of von Willebrand factor (vWF) has been purified by heparin affinity chromatography. Detailed structural analysis of the individual 39- and 34-kDa fragments indicated that they had identical amino acid sequences extending from Leu-480/Val-481 to Gly-718 with an intramolecular disulfide bond between Cys-509 and Cys-695. In addition to the binding site for heparin, the 39/34-kDa fragment also contained binding sites for collagen and for platelet membrane glycoprotein (GP) Ib. Unlike native vWF, the 39/34-kDa fragment bound to GP Ib without the requirement for a modulator but showed increased binding in the presence of botrocetin. The 39/34-kDa vWF fragment was cross-linked to intact human platelets by using the membrane-impermeable, homobifunctional cross-linking reagent bis(sulfosuccinimidyl) suberate. Two distinct cross-linked species of similar molecular weight (220/200 kDa, nonreduced; 190/175 kDa, reduced) were identified by SDS-polyacrylamide gel electrophoresis and autoradiography, consistent with the cross-linking of the 125I-labeled 39/34-kDa vWF fragment to GP Ib. The formation of these cross-linked species was enhanced 1.5-2.5-fold in the presence of the modulator botrocetin. The platelet membrane protein involved in cross-linking was shown unequivocally to be GP Ib since (i) neither cross-linked species was formed with Bernard-Soulier syndrome platelets, which genetically lack the GP Ib-IX complex, (ii) both cross-linked species were specifically immunoprecipitated by anti-GP Ib polyclonal and monoclonal antibodies, and (iii) the formation of the cross-linked species was completely inhibited only by those anti-GP Ib-IX complex monoclonal antibodies that inhibited vWF-GP Ib-IX complex interaction. Proteolysis of cross-linked platelets with endoproteinase Lys-C, which preferentially cleaves off the N-terminal peptide domain on the alpha-chain of GP Ib, indicated that the 39/34-kDa vWF fragment was cross-linked exclusively to this region of the GP Ib-IX complex.