Disruption of a Long-Range Disulfide Bond Between Residues Cys406 and Cys655 in Glycoprotein IIIa Does Not Affect the Function of Platelet Glycoprotein IIb-IIIa

Post-translational control of protein function by disulfide bond cleavage

Protein action in nature is largely controlled by the level of expression and by post-translational modifications. Post-translational modifications result in a proteome that is at least two orders of magnitude more diverse than the genome. There are three basic types of post-translational modifications: covalent modification of an amino acid side chain, hydrolytic cleavage or isomerization of a peptide bond, and reductive cleavage of a disulfide bond. This review addresses the modification of disulfide bonds. Protein disulfide bonds perform either a structural or a functional role, and there are two types of functional disulfide: the catalytic and allosteric bonds. The allosteric disulfide bonds control the function of the mature protein in which they reside by triggering a change when they are cleaved. The change can be in ligand binding, substrate hydrolysis, proteolysis, or oligomer formation. The allosteric disulfides are cleaved by oxidoreductases or by thiol/disulfide exchange, and the configurations of the disulfides and the secondary structures that they link share some recurring features. How these bonds are being identified using bioinformatics and experimental screens and what the future holds for this field of research are also discussed.

Extracellular disulfide exchange and the regulation of cellular function

An emerging concept is that disulfide bonds can act as a dynamic scaffold to present mature proteins in different conformational and functional states on the cell surface. Two examples are the conversion of the receptor, integrin alphaIIbbeta3, from a low affinity to a high affinity state, and the interaction of CD4 receptor with the HIV-1 envelope glycoprotein gp120 to promote virus-cell fusion. In both of these cases there is a remodeling of the protein disulfide bonding pattern. The formation and rearrangement of disulfide bonds is modulated by a family of enzymes known as the thiol isomerases, which include protein disulfide isomerase (PDI), ERp5, ERp57, and ERp72. While these enzymes were reported originally to be restricted in location to the endoplasmic reticulum, in some cells thiol isomerases are found on the cell surface. This may indicate a wider role for these enzymes in cell function. In platelets it has been shown that reagents that react with cell surface sulfhydryl groups are capable of blocking a number of functional responses, including integrin-mediated aggregation, adhesion, and granule secretion. Furthermore, the use of function blocking antibodies to either PDI or ERp5 causes inhibition of these functional responses. This review summarizes current knowledge of the extracellular regulation of disulfide exchange and the implications of this in the regulation of cell function.

Disruption of the long-range GPIIIa Cys(5)-Cys(435) disulfide bond results in the production of constitutively active GPIIb-IIIa (alpha(IIb)beta(3)) integrin complexes

The major platelet integrin alpha(IIb)beta(3), also known as the platelet glycoprotein (GP) IIb-IIIa complex, mediates platelet aggregation by serving as the receptor for fibrinogen and von Willebrand factor. In addition to its physiologic role, GPIIb-IIIa also bears a number of clinically important alloantigenic determinants. Previous studies have shown that disruption of the long-range Cys(5)-Cys(435) disulfide bond of the beta(3) subunit results in the production of isoforms that bind some, but not all, anti-Pl(A1) alloantibodies, suggesting that mutations in this so-called long-range disulfide bond can alter the conformation of GPIIIa. The purpose of this study was to examine the effects of either the Cys5Ala or Cys435Ala substitution of GPIIIa on the adhesive properties of the GPIIb-IIIa complex. We found that both Ala5GPIIIa and Ala435GPIIIa were capable of associating with GPIIb and were expressed normally on the cell surface when cotransfected into Chinese hamster ovary (CHO) cells. CHO cells expressing GPIIb-Ala5GPIIIa or GPIIb-Ala435IIIa bound well-characterized, conformationally sensitive ligand-induced binding site (LIBS) antibodies, and were capable of constitutively binding the fibrinogen-mimetic monoclonal antibodies Pl-55 and PAC-1, as well as soluble fibrinogen. Both GPIIb-Ala5IIIa- and GPIIb-Ala435IIIa-transfected CHO cells also bound more avidly to immobilized fibrinogen and were capable of mediating the tyrosine phosphorylation of pp125(FAK) on cell adhesion. These data are consistent with the notion that these regions of GPIIIa participate in the conformational change associated with receptor activation. Additionally, these studies may provide a molecular explanation for the previously reported ability of mild reducing agents to activate the GPIIb-IIIa complex and promote platelet aggregation.

Probing conformational changes in the I-like domain and the cysteine-rich repeat of human beta 3 integrins following disulfide bond disruption by cysteine mutations: identification of cysteine 598 involved in alphaIIbbeta3 activation

We have investigated receptor function and epitope expression of recombinant alpha(IIb)beta(3) mutated at Cys(177) or Cys(273) in the I-like domain as well as Cys(598), located in the fourth repeat of the membrane-proximal cysteine-rich region and mutated in a Glanzmann's thrombasthenia type II patient. The beta(3) mutants beta(3)C177A, beta(3)C273A, and beta(3)C598Y exhibited a decreased electrophoretic mobility in SDS-polyacrylamide gel electrophoresis under nonreducing conditions, confirming the disruption of the respective disulfide loops. Despite reduced surface expression, the alpha(IIb)beta(3)C177A, alpha(IIb)beta(3)C273A, and alpha(IIb)beta(3)C598Y receptors mediated cell adhesion to immobilized fibrinogen and translocated into focal adhesion plaques. The beta(3)C598Y mutation, but not the beta(3)C177A or beta(3)C273A mutations, induced spontaneous binding of the ligand mimetic monoclonal antibody PAC-1, while the beta(3)C177A and beta(3)C273A mutants exhibited reduced complex stability in the absence of Ca(2+). Epitope mapping of function-blocking monoclonal antibodies (mAbs) allowed the identification of two distinct subgroups; mAbs A2A9, pl2-46, 10E5, and P256 did not interact with alpha(IIb)beta(3)C273A and bound only weakly to alpha(IIb)beta(3)C177A, while mAbs AP2, LM609 and 7E3 bound normally to mutant alpha(IIb)beta(3)C273A, but interacted only weakly with mutant alpha(IIb)beta(3)C177A. Furthermore, a cryptic epitope recognized by mAb 4D10G3 and not exposed on wild type alpha(IIb)beta(3) became accessible only on mutant alpha(IIb)beta(3)C177A and was mapped to the 60-kDa chymotrypsin fragment of beta(3). Finally, the ligand-induced binding site (LIBS) epitopes AP5, D3, LIBS1, and LIBS2 were spontaneously expressed on all three mutants independent of RGDS or dithiothreitol treatment. Our results provide evidence that disruption of a single cysteine disulfide bond in the cysteine-rich repeat domain, but not in the I-like domain, activates integrin alpha(IIb)beta(3). In contrast, disruption of each of the disulfide bonds in the two long insertions of the I-like domain predicted to be in close contact with the alpha subunit beta-propeller domain affect the stability of the alpha(IIb)beta(3) heterodimer and inhibit complex-specific mAb binding without affecting the RGD binding capacity of the metal ion-dependent adhesion site-like domain.

A point mutation in the cysteine-rich domain of glycoprotein (GP) IIIa results in the expression of a GPIIb-IIIa (alphaIIbbeta3) integrin receptor locked in a high-affinity state and a Glanzmann thrombasthenia-like phenotype

This article reports a Glanzmann thrombasthenia (GT) patient, N.M., with a point mutation in the third cysteine-rich repeat of beta3-integrin or platelet glycoprotein (GP) IIIa, leading to the expression of a constitutively activated fibrinogen receptor. The diagnosis of GT was based on a severely reduced platelet-aggregation response to a series of agonists and approximately 20% of surface-expressed GPIIb-IIIa. The patient's GPIIb-IIIa constitutively expressed epitopes recognized by antibodies to ligand-induced binding sites (LIBS) and also spontaneously bound the fibrinogen-mimetic antibody, PAC-1. Furthermore, significant amounts of bound fibrinogen were detected on his platelets ex vivo. No signs of platelet activation were observed on sections of unstimulated platelets from N.M. by electron microscopy. Immunogold labeling highlighted the presence of surface-bound fibrinogen but revealed platelet heterogeneity with regard to the surface density. When the patient's platelets were stimulated by thrombin-receptor activating peptide, amounts of surface-expressed GPIIb-IIIa increased and the aggregation response improved, although it failed to normalize. Platelets from N.M. were able to adhere and spread on immobilized fibrinogen. Sequence analysis of genomic DNA from N.M. revealed a homozygous g1776T>C mutation in GPIIIa, leading to a Cys560Arg amino acid substitution. A stable Chinese hamster ovary (CHO) cell line was prepared expressing surface GPIIb-Arg560IIIa. Like platelets from the patient, GPIIb-Arg560IIIa-transfected CHO cells constitutively bound LIBS antibodies and PAC-1. They also showed an enhanced ability to adhere on surface-bound fibrinogen. Overall, these data demonstrate that a gain-of-function mutation can still be associated with a thrombasthenic phenotype even though platelets show spontaneous fibrinogen binding.

Identification of a region in glycoprotein IIIa involved in subunit association with glycoprotein IIb: further lessons from Iraqi-Jewish Glanzmann thrombasthenia

The most frequent mutation causing Glanzmann thrombasthenia in Iraqi-Jews (IJ-1) is an 11-bp deletion in exon 13 of the glycoprotein (GP) IIIa gene. This deletion predicts a frameshift that results in the elimination of the C406-C655 disulfide bond and a premature termination codon shortly before the transmembrane domain. To determine the contribution of each of these alterations to the thrombasthenic phenotype, Chinese hamster ovary or baby hamster kidney cells were cotransfected with normal GPIIb complementary DNA (cDNA) and the following GPIIIa cDNAs: normal, cDNA bearing IJ-1 mutation, 2011T>A mutated cDNA predicting C655S (single-letter amino acid codes) substitution, and 2019A>T mutated cDNA predicting Stop657. Elimination of the C406-C655 disulfide bond by C655S substitution did not affect GPIIb/IIIa surface expression or binding of the transfected cells to immobilized fibrinogen, whereas elimination of the transmembrane and cytoplasmic domains in IJ-1 and Stop657 mutants prevented both surface expression and binding of the transfected cells to immobilized fibrinogen. Immunohistochemical staining and immunoprecipitation demonstrated that the elimination of amino acids 657-762 in IJ-1 and Stop657 prevented intracellular GPIIb/IIIa complex formation, and differential immunofluorescence staining of GPIIIa and cellular organelles suggested that the truncated uncomplexed GPIIIa protein was retained in the endoplasmic reticulum. Because the use of GPIIIa Stop693 and normal GPIIb cDNAs yielded GPIIb/IIIa complex formation, though with lower efficiency, it is suggested that amino acids 657-692 of GPIIIa are essential for the intracellular association of GPIIb and GPIIIa. (Blood. 2001;98:1063-1069)

Competition between normal [674C] and mutant [674R] subunits: role of the molecular chaperone BiP in the processing of GPIIb-IIIa complexes

This work aimed at investigating the function of the [C674R] mutation in GPIIb that disrupts the intramolecular 674 to 687 disulfide bridge. Individuals heterozygous for this mutation show a platelet GPIIb-IIIa content approximately 30% of normal controls, which is less than expected from one normal functioning allele. Coexpression of normal [674C]GPIIb and mutant [674R]GPIIb with normal GPIIIa produced a [674R]GPIIb concentration-dependent inhibition of surface exposure of GPIIb-IIIa complexes in Chinese hamster ovary (CHO) cells, suggesting that [674R]GPIIb interferes with the association and/or intracellular trafficking of normal subunits. Mutation of either 674C or 687C had similar effects in reducing the surface exposure of GPIIb-IIIa. However, substitution of 674C for A produced a much lesser inhibition than R, suggesting that a positive-charged residue at that position renders a less efficient subunit conformation. The mutant [674R]GPIIb but not normal GPIIb was found associated with the endoplasmic reticulum chaperone BiP in transiently transfected CHO cells. BiP was also found associated with [674R]GPIIb-IIIa heterodimers, but not with normal GPIIIa or normal heterodimers. Overexpression of BiP did not increase the surface exposure of [674R]GPIIb-IIIa complexes, indicating that its availability was not a limiting step. Platelets from the thrombasthenic patient expressing [674R]GPIIb-IIIa were found to bind soluble fibrinogen in response to physiologic agonists or dithiothreitol treatment. Thus, the [674R]GPIIb mutation leads to a retardation of the secretory pathway, most likely related to its binding to the molecular chaperone BiP, with the result of a defective number of functional GPIIb-IIIa receptors in the cell surface.

The platelet integrin alpha IIbbeta 3 has an endogenous thiol isomerase activity

Integrins are cysteine-rich heterodimeric cell-surface adhesion molecules that alter their affinity for ligands in response to cellular activation. The molecular mechanisms involved in this activation of integrins are not understood. Treatment with the thiol-reducing agent, dithiothreitol, can induce an activation-like state in many integrins suggesting that cysteine-cysteine dithiol bonds are important for the receptor's tertiary structure and may be involved in activation-induced conformational changes. Here we demonstrate that the platelet-specific integrin, alpha(IIb)beta(3), contains an endogenous thiol isomerase activity, predicted from the presence of the tetrapeptide motif, CXXC, in each of the cysteine-rich repeats of the beta(3) polypeptide. This motif comprises the active site in enzymes involved in disulfide exchange reactions, including protein-disulfide isomerase (EC ) and thioredoxin. Intrinsic thiol isomerase activity is also observed in the related integrin, alpha(v)beta(3), which shares a common beta-subunit. Thiol isomerase activity within alpha(IIb)beta(3) is time-dependent and saturable, and is inhibited by the protein-disulfide isomerase inhibitor, bacitracin. Furthermore, this activity is calcium-sensitive and is regulated in the EDTA-stabilized conformation of the integrin. This novel demonstration of an enzymatic activity associated with an integrin subunit suggests that altered thiol bonding within the integrin or its substrates may be locally modified during alpha(IIb)beta(3) activation.

Molecular Genetic Analysis of a Compound Heterozygote for the Glycoprotein (GP) IIb Gene Associated With Glanzmann’s Thrombasthenia: Disruption of the 674-687 Disulfide Bridge in GPIIb Prevents Surface Exposure of GPIIb-IIIa Complexes

This work was aimed at elucidating the molecular genetic lesion(s) responsible for the thrombasthenic phenotype of a patient whose low platelet content of glycoprotein (GP) IIb-IIIa indicated that it was a case of type II Glanzmann’s thrombasthenia (GT). The parents did not admit consanguinity and showed a reduced platelet content of GPIIb-IIIa. Polymerase chain reaction (PCR)–single-stranded conformational polymorphism analysis of genomic DNA showed no mutations in the patient’s GPIIIa and two novel mutations in the GPIIb gene: one of them was a heterozygous splice junction mutation, a C→A transversion, at position +2 of the exon 5-intron 5 boundary [IVS5(+2)C→A] inherited from the father. The predicted effect of this mutation, insertion of intron 5 (76 bp) into the GPIIb-mRNA, was confirmed by reverse transcription-PCR analysis of platelet mRNA. The almost complete absence of this mutated form of GPIIb-mRNA suggests that it is very unstable. Virtually all of the proband’s GPIIb-mRNA was accounted for by the allele inherited from the mother showing a T2113→C transition that changes Cys674→Arg674 disrupting the 674-687 intramolecular disulfide bridge. The proband showed a platelet accumulation of proGPIIb and minute amounts of GPIIb and GPIIIa. Moreover, transfection and immunoprecipitation analysis demonstrated that [Arg674]GPIIb is capable of forming a heterodimer complex with GPIIIa, but the rate of subunit maturation and the surface exposure of GPIIb-IIIa are strongly reduced. Thus, the intramolecular 674-687 disulfide bridge in GPIIb is essential for the normal processing of GPIIb-IIIa complexes. The additive effect of these two GPIIb mutations provides the molecular basis for the thrombasthenic phenotype of the proband.

Molecular Genetic Analysis of a Compound Heterozygote for the Glycoprotein (GP) IIb Gene Associated With Glanzmann’s Thrombasthenia: Disruption of the 674-687 Disulfide Bridge in GPIIb Prevents Surface Exposure of GPIIb-IIIa Complexes

This work was aimed at elucidating the molecular genetic lesion(s) responsible for the thrombasthenic phenotype of a patient whose low platelet content of glycoprotein (GP) IIb-IIIa indicated that it was a case of type II Glanzmann’s thrombasthenia (GT). The parents did not admit consanguinity and showed a reduced platelet content of GPIIb-IIIa. Polymerase chain reaction (PCR)–single-stranded conformational polymorphism analysis of genomic DNA showed no mutations in the patient’s GPIIIa and two novel mutations in the GPIIb gene: one of them was a heterozygous splice junction mutation, a C→A transversion, at position +2 of the exon 5-intron 5 boundary [IVS5(+2)C→A] inherited from the father. The predicted effect of this mutation, insertion of intron 5 (76 bp) into the GPIIb-mRNA, was confirmed by reverse transcription-PCR analysis of platelet mRNA. The almost complete absence of this mutated form of GPIIb-mRNA suggests that it is very unstable. Virtually all of the proband’s GPIIb-mRNA was accounted for by the allele inherited from the mother showing a T2113→C transition that changes Cys674→Arg674 disrupting the 674-687 intramolecular disulfide bridge. The proband showed a platelet accumulation of proGPIIb and minute amounts of GPIIb and GPIIIa. Moreover, transfection and immunoprecipitation analysis demonstrated that [Arg674]GPIIb is capable of forming a heterodimer complex with GPIIIa, but the rate of subunit maturation and the surface exposure of GPIIb-IIIa are strongly reduced. Thus, the intramolecular 674-687 disulfide bridge in GPIIb is essential for the normal processing of GPIIb-IIIa complexes. The additive effect of these two GPIIb mutations provides the molecular basis for the thrombasthenic phenotype of the proband.

Truncation of Glycoprotein (GP) IIIa (▵ 616-762) Prevents Complex Formation With GPIIb: Novel Mutation in Exon 11 of GPIIIa Associated With Thrombasthenia

This work reports the molecular genetic study of a patient who suffered from Glanzmann thrombasthenia (GT). Structural analysis of the glycoprotein (GP) IIb and GPIIIa genes showed the presence of a homozygous G1846→T transversion in exon 11 of GPIIIa that changes Glu616→Stop. Cytometric and immunochemical analysis indicated that platelet GPIIb-IIIa was absent in the proband but present at normal levels in the heterozygous relatives. The following observations indicate that this mutation is responsible for the thrombasthenic phenotype of the proband. (1) We failed to detect mutations other than [T1846]GPIIIa in the coding region of both GPIIb and GPIIIa genes. (2) The G1846→T mutation was observed in either parent and a brother of the proband, but none of 100 unrelated individuals carried this defect. (3) Pulse-chase and immunoprecipitation analysis of GPIIb-IIIa complexes in cells transiently cotransfected with cDNAs encoding normal GPIIb and [T1846]GPIIIa showed neither maturation of GPIIb nor complex formation and surface exposure of GPIIb-▵GPIIIa. These observations indicate that the sequence from Glu616 to Thr762 in GPIIIa is essential for heterodimerization with GPIIb. Polymerase chain reaction-based analysis demonstrated the presence of normal levels of full-length GPIIIa-mRNA in the proband and in heterozygous relatives. In addition, a shortened transcript, with a 324-nucleotide deletion, resulting from in-frame skipping of exons 10 and 11, was detectable upon reamplification of the DNA. Thus, unlike other nonsense mutations, [T1846]GPIIIa does not lead to abnormal processing or reduction in the number of transcripts with the termination codon.