Alternative proteolytic processing of platelet membrane glycoprotein IIb

Posttranslational modifications of platelet adhesion receptors

Platelets play a key role in normal hemostasis, whereas pathological platelet adhesion is involved in various cardiovascular events. The underlying cause in cardiovascular events involves plaque rupture leading to subsequent platelet adhesion, activation, release, and eventual thrombosis. Traditional antithrombotic drugs often target the signal transduction process of platelet adhesion receptors by influencing the synthesis of some key molecules, and their effects are limited. Posttranslational modifications (PTMs) of platelet adhesion receptors increase the functional diversity of the receptors and affect platelet physiological and pathological processes. Antithrombotic drugs targeting PTMs of platelet adhesion receptors may represent a new therapeutic idea. In this review, various PTMs, including phosphorylation, glycosylation, ubiquitination, nitrosylation, methylation, lipidation, and proteolysis, of three platelet adhesion receptors, glycoprotein Ib-IX-V (GPIb-IX-V), glycoprotein VI (GPVI), and integrin α_IIbβ_3, are reviewed. It is important to comprehensively understand the PTMs process of platelet adhesion receptors.

Glanzmann Thrombasthenia Associated with a 21-Amino Acid Deletion (Leu817-Gln837) in Glycoprotein IIb due to Abnormal Splicing in Exon 25

We report a novel genetic defect in a Japanese patient with type I Glanzmann thrombasthenia. The glycoprotein (GP) Ilb complementary DNA (cDNA) from platelet messenger RNA had a 63-base pair deletion in the 5' boundary of exon 25, resulting in an in-frame deletion of 21 amino acid residues (Leu817-Gln837) in the calf-2 domain. The deleted region was present in the genomic DNA, but the splice acceptor site (AG) of exon 25 was mutated to AC, leading to the use of an AG sequence in the middle of exon 25 as an abnormal cryptic splice acceptor site. The effect of this deletion on protein synthesis was further analyzed. Mutant GPIIb-IIIa complexes were not detected on the surfaces of cells cotransfected with cDNAs of mutant GPIIb and normal GPIIIa. Mutant pro-GPIIb was detected in cell lysates and was coimmunoprecipitated with an anti-GPIIb-IIIa complex antibody. Immunostaining demonstrated that the mutant pro-GPIIb colocalized with an endoplasmic reticulum protein, calnexin, within the cells. These results indicate that complex formation was not completely prevented and that impairment of the subsequent transport was the major reason for the defect in cell surface expression. The data suggest that the GPIIb calf-2 domain is important for intracellular transport of GPIIIb-IIIa complexes.

Two genetic defects in alphaIIb are associated with type I Glanzmann's thrombasthenia in a Great Pyrenees dog: a 14-base insertion in exon 13 and a splicing defect of intron 13

Glannzmann's thrombasthenia (GT) is an autosomal recessive bleeding disorder caused by qualitative or quantitative deficiencies of the platelet membrane glycoprotein alphaIIbbeta3. This is the first report of a molecular genetic basis for type I GT in dogs. As previously reported, a thrombasthenic Great Pyrenees dog (dog No. 1) experienced uncontrolled epistaxis despite results of coagulation screening tests, platelet quantitation, and von Willebrand factor quantitation that were within reference ranges. Platelet aggregation was minimal in response to agonists. Flow cytometry, autoradiography, and immunoblot experiments demonstrated either marked reduction or absence of glycoproteins alphaIIb and beta3. In this study, we report the presence of a 14-base insertion in exon 13 and defective splicing of intron 13 in the alphaIIb gene of two thrombasthenic dogs (Nos. 1 and 8). The insertion disrupted the fourth alphaIIb calcium-binding domain, caused a shift in the reading frame and resulted in a premature termination codon. Possible consequences of this mutation include decreased alphaIIb mRNA stability and production of truncated alphaIIb protein that lacks the transmembrane and cytoplasmic domains and a large portion of the extracellular domain. We identified the dam, sire, and three littermates of dog No. 8 as carriers of the alphaIIb mutation. Canine alphaIIb and beta3 genes share significant homology with the genes in human beings, making canine GT an excellent translational model for human GT. A defined molecular basis for canine GT will enhance ongoing gene therapy research and increase the understanding of structure-function relationships of this integrin.

Probing chemical and conformational differences in the resting and active conformers of platelet integrin alpha(IIb)beta(3)

Integrin alpha(IIb)beta(3) is the fibrinogen receptor that mediates platelet adhesion and aggregation. The ligand binding function of alpha(IIb)beta(3) is "activated" on the platelet surface by physiologic stimuli. Two forms of alpha(IIb)beta(3) can be purified from platelet lysates. These forms are facsimiles of the resting (Activation State-1 or AS-1) and the active (Activation State-2 or AS-2) conformations of the integrin found on the platelet surface. Here, the differences between purified AS-1 and AS-2 were examined to gain insight into the mechanism of activation. Four major findings are put forth. 1) The association rate (k(1)) between fibrinogen and the integrin is a key difference between AS-1 and AS-2. 2) Although the divalent ion Mn(2+) enhances the ligand binding function of AS-1, this ion is unable to convert AS-1 to AS-2. Therefore, its effect on integrin is unrelated to activation. 3) Peptide mass fingerprints indicate that the chemical structure of AS-1 and AS-2 are virtually identical, calling into question the idea that post-translational modifications are necessary for activation. 4) The two forms of alpha(IIb)beta(3) have significant conformational differences at three positions. These include the junction of the heavy and light chain of alpha(IIb), the divalent ion binding sites on alpha(IIb), and at a disulfide-bonded knot linking the amino terminus of beta(3) to the cysteine-rich domain. These observations indicate that integrin is activated by a series of specific conformational rearrangements in the ectodomain that increase the rate of ligand association.

Bidirectional transmembrane modulation of integrin alphaIIbbeta3 conformations

Activation of blood platelets by physiological stimuli (e.g. thrombin, ADP) at sites of vascular injury induces inside-out signaling, resulting in a conformational change of the prototype integrin alphaIIbbeta3 from an inactive to an active state competent to bind soluble fibrinogen. Furthermore, ligand occupancy of alphaIIbbeta3 initiates outside-in signaling and additional conformational changes of the receptor, leading to the exposure of extracellular neoepitopes termed ligand-induced binding sites (LIBS), which are recognized by anti-LIBS monoclonal antibodies. To date, the mechanism of bidirectional transmembrane signaling of alphaIIbbeta3 has not been established. In this study, using our newly developed anti-LIBScyt1 monoclonal antibody, we showed that extracellular ligand binding to alphaIIbbeta3 on blood platelets induces a transmembrane conformational change in alphaIIbbeta3, thereby exposing the LIBScyt1 epitope in the alphaIIb cytoplasmic sequence between Lys994 and Asp1003. In addition, a point mutation at this site (P998A/P999A) renders alphaIIbbeta3 constitutively active to bind extracellular ligands, resulting in fibrinogen-dependent cell-cell aggregation. Taken collectively, these results demonstrated that the extracellular ligand-binding site and a cytoplasmic LIBS epitope in integrin alphaIIbbeta3 are conformationally and functionally coupled. Such bidirectional modulation of alphaIIbbeta3 conformation across the cell membrane may play a key role in inside-out and outside-in signaling via this integrin.

A Recombinant Soluble Form of the Integrin IIbβ3 (GPIIb-IIIa) Assumes an Active, Ligand-Binding Conformation and Is Recognized by GPIIb-IIIa–Specific Monoclonal, Allo-, Auto-, and Drug-Dependent Platelet Antibodies

The IIb-IIIa glycoprotein complex is a favored target for allo-, auto-, and drug-dependent antibodies associated with immune thrombocytopenia. A soluble, recombinant form of the GPIIb-IIIa heterodimer that could be produced in large quantities and maintained in solution without detergent could provide a useful experimental tool for the study of platelet-reactive antibodies, but previous attempts to produce such a construct have yielded only small quantities of the end product. Using a baculovirus expression system and the dual-promoter transfer vector P2Bac, we were able to express soluble GPIIb-IIIa complex (srGPIIb-IIIa) lacking cytoplasmic and transmembrane domains in quantities of about 1,000 μg/L, about 40 times greater than reported previously. The high yield achieved may be related to inclusion of the entire extracellular region of the GPIIb light chain in the construct. srGPIIb-IIIa reacts spontaneously with fibrinogen, and this interaction is totally inhibited by the peptide RGDS. Reactions of 24 GPIIb-IIIa–specific antibodies evaluated (12 monoclonal, 3 allo-specific, 3 auto-specific, and 6 drug-dependent) with srGPIIb-IIIa were indistinguishable from reactions with platelet GPIIb-IIIa. Thus, srGPIIb-IIIa spontaneously assumes an active, ligand-binding conformation and contains epitopes for all monoclonal and human antibodies tested to date. srGPIIb-IIIa can be produced in large quantities, can readily be modified by site-directed mutagenesis, and should facilitate identification of epitopes recognized by GPIIb-IIIa–specific antibodies, study of the mechanism(s) by which certain drugs promote antibody binding to GPIIb-IIIa in drug-induced thrombocytopenia and structure-function relationships of GPIIb-IIIa.

© 1998 by The American Society of Hematology.

A Recombinant Soluble Form of the Integrin IIbβ3 (GPIIb-IIIa) Assumes an Active, Ligand-Binding Conformation and Is Recognized by GPIIb-IIIa–Specific Monoclonal, Allo-, Auto-, and Drug-Dependent Platelet Antibodies

The IIb-IIIa glycoprotein complex is a favored target for allo-, auto-, and drug-dependent antibodies associated with immune thrombocytopenia. A soluble, recombinant form of the GPIIb-IIIa heterodimer that could be produced in large quantities and maintained in solution without detergent could provide a useful experimental tool for the study of platelet-reactive antibodies, but previous attempts to produce such a construct have yielded only small quantities of the end product. Using a baculovirus expression system and the dual-promoter transfer vector P2Bac, we were able to express soluble GPIIb-IIIa complex (srGPIIb-IIIa) lacking cytoplasmic and transmembrane domains in quantities of about 1,000 μg/L, about 40 times greater than reported previously. The high yield achieved may be related to inclusion of the entire extracellular region of the GPIIb light chain in the construct. srGPIIb-IIIa reacts spontaneously with fibrinogen, and this interaction is totally inhibited by the peptide RGDS. Reactions of 24 GPIIb-IIIa–specific antibodies evaluated (12 monoclonal, 3 allo-specific, 3 auto-specific, and 6 drug-dependent) with srGPIIb-IIIa were indistinguishable from reactions with platelet GPIIb-IIIa. Thus, srGPIIb-IIIa spontaneously assumes an active, ligand-binding conformation and contains epitopes for all monoclonal and human antibodies tested to date. srGPIIb-IIIa can be produced in large quantities, can readily be modified by site-directed mutagenesis, and should facilitate identification of epitopes recognized by GPIIb-IIIa–specific antibodies, study of the mechanism(s) by which certain drugs promote antibody binding to GPIIb-IIIa in drug-induced thrombocytopenia and structure-function relationships of GPIIb-IIIa.

© 1998 by The American Society of Hematology.

A novel alpha integrin subunit associates with betaPS and functions in tissue morphogenesis and movement during Drosophila development

We have identified a novel alpha integrin subunit in Drosophila, that associates with betaPS integrin. We report the temporal expression of the gene encoding this integrin subunit, which we have called alphaPS3, throughout development and the localization of its expression during embryogenesis. AlphaPS3 RNA was localized to tissues undergoing invagination, tissue movement and morphogenesis such as salivary gland, trachea, midgut, dorsal vessel, midline of the ventral nerve cord, amnioserosa and the amnioproctodeal invagination. AlphaPS3 DNA localized to the chromosomal vicinity of scab (scb), previously identified by a failure of dorsal closure. Embryos homozygous for the 119 allele of scb had no detectable alphaPS3 RNA and the 1035 allele of scb contains a Pelement inserted just 5′ of the coding region for the shorter of the gene's two transcripts. Furthermore, mutations in the scb locus exhibit additional defects corresponding to sites of alphaPS3 transcription, including abnormal salivary glands, mislocalization of the pericardial cells and interrupted trachea. Removal of both maternal and zygotic betaPS produced similar defects, indicating that these two integrin subunits associate in vivo and function in the movement and morphogenesis of tissues during development in Drosophila. Phenotypic similarities suggest that laminin A is a potential ligand for this integrin, at least in some tissues.

Catfish thrombocytes express an integrin-like CD41/CD61 complex

A thrombocyte-specific antigen was identified in two closely related catfish, Ictalurus punctatus and Ictalurus furcatus, by monoclonal antibodies 4-20 and 7-2. The antibodies immunoprecipitate two noncovalently associated glycoprotein chains of Mr 180,000 and Mr 95,000. Under reducing conditions the Mr 180,000 chain is resolved into Mr 150,000 and 32,000 subcomponents. Analysis of N-terminal amino acid sequences indicates homology of the Mr 95,000 chain with the beta3 integrin subunit and homology of the Mr 150,000 chain with the alphaIIb integrin subunit. These antibodies induce catfish thrombocyte aggregation and alteration of cell shape. The data indicate conservation of the megakaryocyte/platelet-restricted CD41/CD61 complex in bony fish.

Direct binding of the platelet integrin alphaIIbbeta3 (GPIIb-IIIa) to talin. Evidence that interaction is mediated through the cytoplasmic domains of both alphaIIb and beta3

As a consequence of platelet activation and fibrinogen binding, glycoprotein (GP)IIb-IIIa (integrin alphaIIbbeta3) becomes associated with the cytoskeleton. Although talin has been suggested to act as a linkage protein mediating the attachment of GPIIb-IIIa to actin filaments, direct binding of GPIIb-IIIa to this cytoskeletal protein has not been demonstrated. In the present study, we examined the interaction of GPIIb-IIIa with purified talin using a solid-phase binding assay. Soluble GPIIb-IIIa bound in a time- and dose-dependent manner to microtiter wells coated with talin but not with BSA. Time course studies demonstrated that steady-state binding was achieved after 4-5 h incubation at 37 degrees C. Binding isotherms with varying concentrations of GPIIb-IIIa showed that half-saturation binding was achieved at approximately 15 nM GPIIb-IIIa. At saturation, there was 211 +/- 8 fmol of GPIIb-IIIa bound per well containing 117 +/- 10 fmol of immobilized talin. Besides binding to immobilized talin, GPIIb-IIIa also bound to talin captured by the anti-talin monoclonal antibody 8d4. Moreover, the interaction of GPIIb-IIIa to 8d4-captured talin was blocked by mAb10B2, a monoclonal antibody raised against a synthetic peptide encompassing the entire cytoplasmic sequence of GPIIb. The interaction of talin with the cytoplasmic domain of GPIIb-IIIa was further investigated using peptide-coated wells. Purified talin was found to bind to both synthetic peptides corresponding to the cytoplasmic sequences of GPIIb (P2b) and GPIIIa (P3a). As expected, the binding of talin to P2b-coated wells was specifically blocked by mAb10B2. Thus, these results demonstrate direct binding of GPIIb-IIIa to talin and suggest a role of the cytoplasmic sequences of both GPIIb and GPIIIa in mediating this interaction.

The cytoplasmic domain of alphaIIb beta3. A ternary complex of the integrin alpha and beta subunits and a divalent cation

Peptides corresponding to the cytoplasmic tails of the alphaIIb (alphaIIb (985-1008)) and beta3 (beta3 (713-762)) subunits of the integrin receptor alphaIIb beta3 (glycoprotein IIb-IIIa) were synthesized and used to characterize their interaction with cations and with one another. alphaIIb (985-1008) was found to contain a functional cation binding site as assessed by both terbium luminescence and electrospray ionization mass spectroscopy. The binding of Tb3+ to alphaIIb (985-1008) was of high affinity (Kd = 8.8 +/- 5.2 nM), occurred with a 1:1 stoichiometry, and was mediated by its acidic carboxy] terminus (alphaIIb (999-1008), PLEEDDEEGE). The affinity of this site for divalent cations was in the micromolar range, suggesting that this site would be constitutively occupied in the intracellular environment. Incubation of alphaIIb (999-1008) with beta3 (713-762) resulted in the formation of a complex, both in the presence and absence of cations. The interactive site for alphaIIb (999-1008) in beta3 was mapped to beta3 (721-740), and complex formation was associated with a stabilization of secondary structure as assessed by circular dichroism. Both a binary (alphaIIb (985-1008).beta3 (721-740)) and a ternary (Tb3+.alphaIIIb (999-1008).beta3 (721-740)) complex were detected by mass spectroscopy, but the distribution and intensity of the mass/charge peaks were distinct. These difference may reflect the involvement of distinct cation coordination sites and the formation of salt bridges in stabilizing the ternary complex. These data demonstrate the formation of a novel entity composed of the cytoplasmic tails of alphaIIb and beta3 and a cation which may constitute a functional intracellular domain.

Calpain cleavage of the cytoplasmic domain of the integrin beta 3 subunit

The cytoplasmic domains of integrin beta subunits are involved in bidirectional transmembrane signaling. We report that the cytoplasmic domain of the integrin beta 3 subunit undergoes limited proteolysis by calpain, an intracellular calcium-dependent protease. Calpain cleavage occurs during platelet aggregation induced by agonists such as thrombin. Five cleavage sites have been identified. Four of these sites (C-terminal to Thr741, Tyr747, Phe754, and Tyr759) are utilized in intact platelets and flank two NXXY motifs (Asn744-Pro-Leu-Tyr747 and Asn756-Ile-Thr-Tyr759). The fifth site (Ala735) is accessible to calpain after EDTA treatment of the alpha IIb beta 3 heterodimer. The NXXY motif is critical to the bidirectional signaling functions of beta 3 integrins and their association with the cytoskeleton. Thus, calpain cleavage of the beta 3 cytoplasmic domain may provide a means to regulate integrin signaling functions.

Anti-peptide antibodies against the human blood platelet thromboxane A2/prostaglandin H2 receptor. Production, purification and characterization

Two anti-peptide antibodies have been raised against the human blood platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor. Based on the published sequence of the placental TXA2/PGH2 receptor, two decapeptide segments were selected as potential antigens: one in the first extracellular loop corresponding to residue 89 through 98, and the other in the C-terminal region of the intracellular domain corresponding to residue 314 through 323. Rabbits were immunized with each peptide, and the antisera were subjected to a two-step purification procedure. The IgG fraction was purified using a DEAE Affi-Gel Blue column, and the peptide-specific IgG was further purified by affinity chromatography employing each peptide as the immobilized ligand. The combined purification factor for both procedures was approximately 60-fold. By ELISA, both antibodies displayed immunoreactivity toward their synthetic antigens, solubilized platelet membranes and affinity-purified TXA2/PGH2 receptor protein. Furthermore, Western blot analysis revealed that: (1) each antibody reacted with the purified platelet TXA2/PGH2 receptor protein (55 kDa); and (2) each antibody recognized a single band (55 kDa) in solubilized platelet membranes. These findings establish antibody specificity for the human platelet TXA2/PGH2 receptor protein. Functional analysis demonstrated that neither antibody interfered with ADP- or U46619-induced platelet aggregation of [3H]SQ29,548 binding to the solubilized receptor. These results suggest that the antibody epitopes are separate from the TXA2/PGH2 binding domain. In summary, two specific anti-peptide antibodies have been raised against the human platelet TXA2/PGH2 receptor. These antibodies should prove to be of value in the further investigation of the platelet TXA2/PGH2 receptor.

Integrins and their ligands

Integrins are expressed on almost every cell type and are responsible for the linkage of the extracellular matrix with the cytoskeleton. In this review I have focused on the intra- and extracellular proteins that bind to integrins. Although many integrins bind to the same extracellular ligand, they mostly recognize different sites on these ligands. Some integrins interact with the same site but then there are requirements for different additional sequences to obtain high affinity. By modulating the expression and activity of integrins in the plasma membrane, cells can adapt their capacity of binding to the matrix. How integrins become activated is as yet not clear, but interaction with other proteins or lipids may be critical. Binding to ligands could also be modulated by alternative splicing of mRNAs for ligand binding sites in the extracellular domain. In Drosophila, the mRNA for the extracellular domain of the PS2 integrin is spliced near a site implicated in ligand binding. In humans, however, there are no indications that alternative splicing contributes to the regulation of function of the extracellular domain of integrins. The only splice variant of the extracellular domain of an integrin identified so far concerns are alpha subunit of the alpha IIb beta 3 complex, but the splicing occurs in a region that has not been implicated in cell adhesion. There is also no evidence as yet that integrin function can be modulated by alternative splicing of mRNA for the cytoplasmic domain of integrin subunits. However, the loss of function seen with some deletion mutants of the cytoplasmic domains of integrin subunits suggests that such a mechanism may well exist. In a different way the binding capacity of a given cell can be influenced by regulating the expression of its ligand or by alternative mRNA splicing of sequences encoding the cell binding domain in their ligands. In the case of fibronectin, the mRNA for one of the integrin binding sites is subject to alternative splicing. The mRNAs for the three chains of laminin appear not to be subject to alternative splicing but, by combining different variant chains of laminin, isoforms can be generated which may have different affinities for integrins. Binding of cells to the matrix therefore does not only depend on the expression and activity of the correct integrin but also of the correct variant of the ligand.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of the affinity of integrin alpha IIb beta 3 (GPIIb-IIIa) by the cytoplasmic domain of alpha IIb

Intracellular signaling alters integrin adhesive functions in inflammation, immune responses, hemostasis, thrombosis, and retinal development. By truncating the cytoplasmic domain of alpha IIb, the affinity of integrin alpha IIb beta 3 for ligand was increased. Reconstitution with the cytoplasmic domain from integrin alpha 5 did not reverse the increased affinity. Thus, the cytoplasmic domain of the alpha subunit of GPIIb-IIIa controls ligand binding affinity, which suggests mechanisms for inside-out transmembrane signaling through integrins. These findings imply the existence of hitherto unappreciated hereditary and acquired thrombotic disorders in humans.

Activation of the fibrinogen receptor on human platelets exposed to alpha chymotrypsin. Relationship with a major proteolytic cleavage at the carboxyterminus of the membrane glycoprotein IIb heavy chain

The serine proteinase alpha chymotrypsin from bovine pancreas (CT) is known to expose fibrinogen binding sites on the surface of human platelets in the absence of cell activation and granular secretion. This is accompanied by the appearance of membrane-bound chymotryptic fragments of both glycoprotein (GP) IIb and GPIIIa, the two subunits of the platelet fibrinogen receptor, the GPIIb-IIIa complex. However, no clear relationship between discrete proteolytic event(s) within GPIIb-IIIa and fibrinogen-binding-site expression has yet been established. We have now evaluated the proteolysis of GPIIb-IIIa by CT by Western blot analyses using a panel of polyclonal and monoclonal antibodies against GPIIb or GPIIIa. The different proteolytic events were then correlated with the kinetics of the expression of active fibrinogen binding sites on platelets, as measured through the binding of 125I-labelled purified fibrinogen and to the capacity of CT-treated platelets to aggregate. Treatment of platelets with CT at 22 degrees C resulted in the expression of fibrinogen binding sites prior to cleavage of GPIIIa (Mr approximately 90,000) into a previously described, major membrane-bound fragment with Mr 60,000. In contrast, fibrinogen receptor expression closely paralleled a proteolytic cleavage at the carboxy terminus of the GPIIb heavy chain (Mr approximately 120,000), which was converted into a faster migrating species with Mr approximately 115,000). This proteolysis resulted in the release of a soluble peptide with an expected molecular mass of less than 3.7 kDa. Quantitation of this peptide using a competitive immunoenzymatic assay, confirmed that its release from the platelet surface correlated with the expression of fibrinogen binding sites and aggregability. When platelets were exposed to CT at 37 degrees C, a prompt increase in fibrinogen binding sites and platelet aggregability was observed, whereas the GPIIb heavy chain was rapidly converted into the carboxy-terminal-cleaved form. However, incubation at 37 degrees C for longer than 10 min resulted in extensive and simultaneous degradation of both the GPIIb heavy and light chains and of GPIIIa, with the latter being converted into the 60-kDa fragment. These later events were associated with a sharp decline of platelet aggregability and a reduction in the number of fibrinogen binding sites. These data allow us to propose that an early and limited proteolytic processing of the GPIIb component of the platelet fibrinogen receptor is associated with a shift of this receptor complex into a state which expresses specific binding sites for fibrinogen. Further cleavage of GPIIIa to generate the 60-kDa fragment results in loss of receptor activity.

Molecular cloning of the human alpha 6 integrin subunit. Alternative splicing of alpha 6 mRNA and chromosomal localization of the alpha 6 and beta 4 genes

We have isolated cDNAs encoding the alpha 6 subunit from a lambda gt11 expression library from human keratinocytes by combined screening with a rabbit polyclonal anti-alpha 6 antibody and the polymerase chain reaction. The alpha 6 subunit encoded by this cDNA consists of 1050 amino acids with a 991-amino-acid extracellular, a 23-amino-acid transmembrane and a 36-amino-acid cytoplasmic domain. The extracellular domain contains three putative divalent cation-binding sites and nine potential N-linked glycosylation sites. From a cDNA library from normal human mammary gland cells two different cDNAs for alpha 6 were isolated, one of which is identical to the above cDNA. The two alpha 6 subunits, called alpha 6A and alpha 6B, encoded by the two cDNAs each have a unique cytoplasmic domain, that of alpha 6B being 18 amino acids longer than that of alpha 6A. Different carcinoma cell lines contain transcripts for both alpha 6 subunits. K562 leukemic cells have little alpha 6A or alpha 6B mRNAs. The overall level of expression varies in the carcinoma cell lines, but reflects alpha 6 cell surface expression. In A375 melanoma cells, however, cell surface expression of alpha 6 was low in spite of a high level of mRNA. This suggest that other mechanisms may be involved in regulating the expression of alpha 6 on the surface of these cells. The mRNA for both alpha 6 subunits is around 6 kb. The alpha 6 subunits are similar to other alpha subunits (26-31% identity with cleaved alpha subunits) of the integrin family but they are more similar to the alpha 3 subunit (40% identity). This high degree of similarity may be the basis for their functional resemblance since both alpha 3 and alpha 6 subunits, when associated with beta 1, function as laminin receptors and bind to the long arm of laminin. The genes for alpha 6 and beta 4, the alternative beta subunit with which alpha 6 combines on certain epithelial cells, were mapped to chromosome 2 and 17q11-qter, respectively.

Analysis of a variant form of platelet glycoprotein (GP) IIb: a second patient with abnormal molecular weight GPIIb

In 1988, we reported a thrombasthenic patient whose platelets contained an abnormal molecular weight GPIIb (Blood, 71, 915, 1988). Here we describe another patient whose platelets contain a variant GPIIb with properties similar to the previously reported abnormal GPIIb. This patient has a small amount of the abnormal GPIIb and was suggested to be a heterozygote characterized by deficient and abnormal GPIIb genes. The abnormal GPIIb was suggested to be related to the precursor form of GPIIb because the molecular weights of both proteins are the same. The abnormal GPIIb may lack a peptide region around the cleavage site of the two chains, as suggested from its lack of reactivity towards antibodies against these epitopes.

Characterization of the beta-chain N-terminus heterogeneity and the alpha-chain C-terminus of human platelet GPIIb. Posttranslational cleavage sites

Human platelet glycoprotein IIb (GPIIb) and IIIa (GPIIIa) form a Ca2(+)-dependent heterodimer, the integrin GPIIb/IIIa, which functions as the fibrinogen receptor at the surface of activated platelets. GPIIB and GPIIIa are synthesized as single polypeptides from single messages and their amino acid sequences were derived from their cDNAs. The GPIIb precursor is proteolytically processed to yield the known disulphide-bonded two-chain (GPIIb alpha and GPIIb beta) covalent structure found in mature GPIIb. Our present protein chemical and mass spectrometric analyses indicate that the GPIIb precursor is proteolytically cleaved at two or three sites, to give rise to an homogeneous alpha-chain (GPIIb 1-856) single disulphide-bonded to one of the two beta-chains, which are present in a nearly 1:1 ratio: GPIIb beta 1 (860-1008), with pyroglutamic acid as its blocked N-terminal residue: and GPIIb beta 2 (872-1008), with the already known N-terminal sequence. These results satisfy the previously observed electrophoretic size-residue: and GPIIb beta 2 (872-1008), with the already known N-terminal sequence. These results satisfy the previously observed electrophoretic size-heterogeneity of the beta-chain, confirmed the potential cleavage sites in the junction region, and indicate a probable dual proteolytic processing of GPIIb, which may be relevant to the rest of the two-chain alpha-subunits of the integrin family.

Decreased stability and structural heterogeneity of the residual platelet glycoprotein IIb/IIIa complex in a variant of Glanzmann's thrombasthenia

A patient is described with a disturbance of platelet function comparable to that in Glanzmann's thrombasthenia. Platelet aggregation and binding of fibrinogen to the patient's platelets were defective and thrombin-induced clot retraction was absent. The platelet fibrinogen content was only moderately reduced. As measured by monoclonal antibody binding in the presence of divalent cations, the platelets contained about 15% of the normal amount of GPIIb and GPIIIa and only 6% of the normal amount of intact GPIIb/IIIa complex. The residual GPIIb/IIIa complex exhibited a decreased stability as shown by the lack of binding of a complex-dependent anti-GPIIb/IIIa antibody to platelets incubated with ethylene diamine tetraacetic acid (EDTA) at 22 degrees C. Crossed immunoelectrophoresis (CIE) in the presence of divalent cations showed partial dissociation of GPIIb/IIIa as well as the presence of two forms of the residual intact GPIIb/IIIa complex. In addition, both CIE in the presence of the EDTA and two-dimensional sodium dodecyl sulphate (SDS) gel electrophoresis showed the presence of two forms of GPIIb. This form of thrombasthenia is characterized by a defective platelet function, a marked reduction of GPIIb and GPIIIa, decreased stability of the residual GPIIb/IIIa complex and structural heterogeneity of GPIIb.

Developmentally regulated alternative splicing of Drosophila integrin PS2 alpha transcripts

We report the characterization of a chromosomal integrin gene that encodes the Drosophila PS2 alpha subunit. The gene is composed of 12 exons spanning 31 kb. By employing a novel method for directed cDNA cloning, we have analyzed over 300 independent cDNA clones for the existence of alternate RNA products. Two forms of PS2 alpha mRNA are frequently observed: a canonical (C) form and a form lacking the 75 nucleotide exon 8 (m8). The relative ratio of these two forms varies widely during development. Although region A, derived from exon 8 and the adjacent 25 amino acids, shows weak conservation among the sequences of alpha subunits that bind to different ligands, it is highly conserved in the homologous PS2 alpha gene of the distantly related Mediterranean fruitfly. We suggest that the variable region A may be important in determining the specificity and affinity of integrin receptors for their ligands.

Immunoblot analysis of platelet glycoprotein IIb in patients with Glanzmann thrombasthenia in Israel

Recent studies have indicated that severe ('type I') Glanzmann thrombasthenia is a heterogeneous hereditary disorder caused by quantitative and/or qualitative abnormalities of platelet membrane glycoproteins (GP) IIb and IIIa. Immunoblot analysis of sodium dodecyl sulphate (SDS)-solubilized platelets was carried out on controls and 18 patients (12 Iraqi-Jews, two Iranian Jews and four Arabs) employing three antibodies (one monoclonal and two polyclonal) directed at different sites on GPIIb. Nonreduced control platelet samples contained a major Mr approximately 140k immunoreactive protein that was split into an Mr approximately 120k (alpha) and an Mr approximately 25k (beta) band after reduction with mercaptoethanol. The nonreduced samples from all 18 patients tested had trace amounts of Mr approximately 140k band corresponding to normal GPIIb; the intensity of this band was estimated to be less than 1% of the normal amount. Unlike the control samples, however, this Mr approximately 140k band did not change electrophoretic mobility following reduction. Since GPIIb originates from a single chain precursor molecule of Mr approximately 140k that comprises both the alpha and beta chains, and which does not change mobility with reduction, our data suggest that the platelets of these patients contain small amounts of this precursor.