Ligand binding to integrin alphaIIbbeta3 is dependent on a MIDAS-like domain in the beta3 subunit

Development of the Integrated Computer Simulation Model of the Intracellular, Transmembrane, and Extracellular Domain of Platelet Integrin α IIb β 3 (Platelet Membrane Glycoprotein: GPIIb-IIIa)

Background  The structure and functions of the extracellular domain of platelet integrin α IIb β 3 (platelet membrane glycoprotein: GPIIb-IIIa) change substantially upon platelet activation. However, the stability of the integrated model of extracellular/transmembrane/intracellular domains of integrin α IIb β 3 with the inactive state of the extracellular domain has not been clarified. Methods  The integrated model of integrin α IIb β 3 was developed by combining the extracellular domain adopted from the crystal structure and the transmembrane and intracellular domain obtained by Nuclear Magnetic Resonace (NMR). The transmembrane domain was settled into the phosphatidylcholine (2-oleoyl-1-palmitoyl-sn-glycerol-3-phosphocholine (POPC)) lipid bilayer model. The position coordinates and velocity vectors of all atoms and water molecules around them were calculated by molecular dynamic (MD) simulation with the use of Chemistry at Harvard Macromolecular Mechanics force field in every 2 × 10 -15 seconds. Results  The root-mean-square deviations (RMSDs) of atoms constructing the integrated α IIb β 3 model apparently stabilized at approximately 23 Å after 200 ns of calculation. However, minor fluctuation persisted during the entire calculation period of 650 ns. The RMSDs of both α IIb and β 3 showed similar trends before 200 ns. The RMSD of β 3 apparently stabilized approximately at 15 Å at 400 ns with persisting minor fluctuation afterward, while the structural fluctuation in α IIb persisted throughout the 650 ns calculation period. Conclusion  In conclusion, the integrated model of the intracellular, transmembrane, and extracellular domain of integrin α IIb β 3 suggested persisting fluctuation even after convergence of MD calculation.

An update on mechanism of entry of white spot syndrome virus into shrimps

Host-parasite relationships can be best understood at the level of protein-protein interaction between host and pathogen. Such interactions are instrumental in understanding the important stages of life cycle of pathogen such as adsorption of the pathogen on host surface followed by effective entry of pathogen into the host body, movement of the pathogen across the host cytoplasm to reach the host nucleus and replication of the pathogen within the host. White Spot Disease (WSD) is a havoc for shrimps and till date no effective treatment is available against the disease. Moreover information regarding the mechanism of entry of White Spot Syndrome Virus (WSSV) into shrimps, as well as knowledge about the protein interactions occurring between WSSV and shrimp during viral entry are still at very meagre stage. A cumulative and critically assessed information on various viral-shrimp interactions occurring during viral entry can help to understand the exact pathway of entry of WSSV into the shrimp which in turn can be used to device drugs that can stop the entry of virus into the host. In this context, we highlight various WSSV and shrimp proteins that play role in the entry mechanism along with the description of the interaction between host and pathogen proteins.

Nuclear Localization of Integrin Cytoplasmic Domain-associated Protein-1 (ICAP1) Influences β1 Integrin Activation and Recruits Krev/Interaction Trapped-1 (KRIT1) to the Nucleus

Binding of ICAP1 (integrin cytoplasmic domain-associated protein-1) to the cytoplasmic tails of β1 integrins inhibits integrin activation. ICAP1 also binds to KRIT1 (Krev interaction trapped-1), a protein whose loss of function leads to cerebral cavernous malformation, a cerebrovascular dysplasia occurring in up to 0.5% of the population. We previously showed that KRIT1 functions as a switch for β1 integrin activation by antagonizing ICAP1-mediated inhibition of integrin activation. Here we use overexpression studies, mutagenesis, and flow cytometry to show that ICAP1 contains a functional nuclear localization signal and that nuclear localization impairs the ability of ICAP1 to suppress integrin activation. Moreover, we find that ICAP1 drives the nuclear localization of KRIT1 in a manner dependent upon a direct ICAP1/KRIT1 interaction. Thus, nuclear-cytoplasmic shuttling of ICAP1 influences both integrin activation and KRIT1 localization, presumably impacting nuclear functions of KRIT1.

Clinical, Biochemical, and Molecular Aspects of Glanzmann's Thrombasthenia in Humans and Dogs

Glanzmann's thrombasthenia (GT) is an inherited, intrinsic platelet function defect that involves the platelet glycoprotein complex IIb–IIIa, also known as the fibrinogen receptor and the integrin αIIbβ3. The defect was originally described by Dr. Glanzmann in humans in 1918 as a bleeding disorder that differed clinically from other known coagulopathies. Over the decades that followed, researchers determined the biochemical and molecular basis for the disease in humans. Otterhounds with thrombasthenic thrombopathia, described in the 1960s, were the only animal model that closely resembled the disease described in humans until 1996. At that time, a Great Pyrenees dog was identified with unequivocal clinical and biochemical features of Type I GT. The cDNA encoding for glycoproteins IIb and IIIa were sequenced in normal dogs in 1999, allowing for identification of specific mutations causing Type I GT in both Otterhounds and Great Pyrenees dogs. Knowing the molecular basis for Type I GT in dogs as well as the cDNA sequences in normal dogs should enhance the understanding of structure/function relationships of the αIIbβ3 integrin and provide an excellent animal model for studies aimed at correction of GT in humans. The following review focuses on the structure and function of this platelet receptor and reviews the molecular, biochemical, and clinical aspects of Glanzmann's thrombasthenia in humans and dogs.

αIIbβ3: structure and function

During the past decade, advanced techniques in structural biology have provided atomic level information on the platelet integrin αIIbβ3 activation mechanism that results in it adopting a high-affinity ligand-binding conformation(s). This review focuses on advances in imaging intact αIIbβ3 in a lipid bilayer in the absence of detergent and new structural insights into the changes in the ligand-binding pocket with receptor activation and ligand binding. It concludes with descriptions of novel therapeutic αIIbβ3 antagonists being developed based on an advanced knowledge of the receptor's structure.

Different biochemical expression pattern of platelet surface glycoproteins suggests molecular diversity of Glanzmann's thrombasthenia in Iran

Glanzmann's thrombasthenia is a rare congenital bleeding disorder characterized by lack of platelet aggregation induced by most agonists. The disease is caused by different mutations in either GPIIb or GPIIIa genes that lead to a lack or dysfunction of the αIIbβ3. Western blot analysis was performed on the platelet lysates of 95 patients with Glanzmann's thrombasthenia who were referred to the Iranian Blood Transfusion and Hemophilia Clinic. Glanzmann's thrombasthenia was diagnosed based on clinical findings and were classified according to Glanzmann's Thrombasthenia Italian Team (GLATIT) protocol. The platelet glycoprotein expression pattern in Iranian patients with Glanzmann's thrombasthenia was studied and the relationship between the platelet glycoprotein expression levels and clinical symptoms were investigated. Loss or severe reduction of platelet GpIIb and GpIIIa were observed in majority of patients (78%). The remaining patients (22%) showed a relatively sharp decline to the normal amounts of the glycoproteins. None of the patients showed expression of CD41 without CD61. Statistical analysis showed no significant relationship between clinical symptoms and expression of platelet glycoproteins. Different patterns of platelet glycoproteins expression suggest that there is variety of mutations in the patients. Unlike the universal Glanzmann's thrombasthenia database, the majority of Iranian patients may suffer from a GpIIIa gene mutation. This study also confirmed a lack of correlation between clinical manifestation and GPIIb/IIIa expression in the patients.

Evolution of integrin I domains

In humans, an ~200-residue "inserted" I domain, a von Willebrand factor A domain (vWFA), buds out from the β-propeller domain in 9 of 18 integrin α subunits. The vWFA domain is not unique to the α subunit as it is an integral part of all integrin β subunits and many other proteins. The βI domain has always been a component of integrins but the αI domain makes its appearance relatively late, in early chordates, since it is found in tunicates and later diverging species. The tunicate αI domains are distinct from the human collagen and leukocyte recognizing integrin α subunits, but fragments of integrins from agnathastomes suggest that the human-type αI domains arose in an ancestor of the very first vertebrate species. The rise of integrins with αI domains parallels the enormous changes in body plan and systemic development of the chordate line that began some 550 million or more years ago.

Recessive and dominant mutations in COL12A1 cause a novel EDS/myopathy overlap syndrome in humans and mice

Collagen VI-related myopathies are disorders of connective tissue presenting with an overlap phenotype combining clinical involvement from the muscle and from the connective tissue. Not all patients displaying related overlap phenotypes between muscle and connective tissue have mutations in collagen VI. Here, we report a homozygous recessive loss of function mutation and a de novo dominant mutation in collagen XII (COL12A1) as underlying a novel overlap syndrome involving muscle and connective tissue. Two siblings homozygous for a loss of function mutation showed widespread joint hyperlaxity combined with weakness precluding independent ambulation, while the patient with the de novo missense mutation was more mildly affected, showing improvement including the acquisition of walking. A mouse model with inactivation of the Col12a1 gene showed decreased grip strength, a delay in fiber-type transition and a deficiency in passive force generation while the muscle seems more resistant to eccentric contraction induced force drop, indicating a role for a matrix-based passive force-transducing elastic element in the generation of the weakness. This new muscle connective tissue overlap syndrome expands on the emerging importance of the muscle extracellular matrix in the pathogenesis of muscle disease.

C-type lectin binds to β-integrin to promote hemocytic phagocytosis in an invertebrate

Phagocytosis is a conserved cellular response among metazoans. Opsonins are some molecules that label targets to increase their susceptibility to phagocytosis. Opsonins are usually captured by receptors on the surface of phagocytes. Our previous study found the C-type lectin FcLec4 from Chinese white shrimp Fenneropenaeus chinensis might function as an opsonin to facilitate bacterial clearance. In the present study we purified the native FcLec4 protein and confirmed its opsonic activity in the near relation, kuruma shrimp Marsupenaeus japonicus. The possible receptor of FcLec4 was identified as β-integrin by panning a T7 phage display library of shrimp hemocytes and then confirmed by co-immunoprecipitation assay. We further proved that the interaction between FcLec4 and β-integrin did not rely on the carbohydrate recognition domain but on the N terminus of FcLec4. In addition, inhibition of FcLec4 expression using RNAi delayed bacterial clearance, and β-integrin knockdown suppressed the opsonic activity of FcLec4. This study is the first to show the direct interaction between an opsonin and its receptor in crustaceans. Our study provides new insights into invertebrate phagocytosis and the functions of C-type lectins.

Variation in one residue associated with the metal ion-dependent adhesion site regulates αIIbβ3 integrin ligand binding affinity

The Asp of the RGD motif of the ligand coordinates with the β I domain metal ion dependent adhesion site (MIDAS) divalent cation, emphasizing the importance of the MIDAS in ligand binding. There appears to be two distinct groups of integrins that differ in their ligand binding affinity and adhesion ability. These differences may be due to a specific residue associated with the MIDAS, particularly the β3 residue Ala²⁵² and corresponding Ala in the β1 integrin compared to the analogous Asp residue in the β2 and β7 integrins. Interestingly, mutations in the adjacent to MIDAS (ADMIDAS) of integrins α4β7 and αLβ2 increased the binding and adhesion abilities compared to the wild-type, while the same mutations in the α2β1, α5β1, αVβ3, and αIIbβ3 integrins demonstrated decreased ligand binding and adhesion. We introduced a mutation in the αIIbβ3 to convert this MIDAS associated Ala²⁵² to Asp. By combination of this mutant with mutations of one or two ADMIDAS residues, we studied the effects of this residue on ligand binding and adhesion. Then, we performed molecular dynamics simulations on the wild-type and mutant αIIbβ3 integrin β I domains, and investigated the dynamics of metal ion binding sites in different integrin-RGD complexes. We found that the tendency of calculated binding free energies was in excellent agreement with the experimental results, suggesting that the variation in this MIDAS associated residue accounts for the differences in ligand binding and adhesion among different integrins, and it accounts for the conflicting results of ADMIDAS mutations within different integrins. This study sheds more light on the role of the MIDAS associated residue pertaining to ligand binding and adhesion and suggests that this residue may play a pivotal role in integrin-mediated cell rolling and firm adhesion.

Glanzmann thrombasthenia: state of the art and future directions

Glanzmann thrombasthenia (GT) is the principal inherited disease of platelets and the most commonly encountered disorder of an integrin. GT is characterized by spontaneous mucocutaneous bleeding and an exaggerated response to trauma caused by platelets that fail to aggregate when stimulated by physiologic agonists. GT is caused by quantitative or qualitative deficiencies of αIIbβ3, an integrin coded by the ITGA2B and ITGB3 genes and which by binding fibrinogen and other adhesive proteins joins platelets together in the aggregate. Widespread genotyping has revealed that mutations spread across both genes, yet the reason for the extensive variation in both the severity and intensity of bleeding between affected individuals remains poorly understood. Furthermore, although genetic defects of ITGB3 affect other tissues with β3 present as αvβ3 (the vitronectin receptor), the bleeding phenotype continues to dominate. Here, we look in detail at mutations that affect (i) the β-propeller region of the αIIb head domain and (ii) the membrane proximal disulfide-rich epidermal growth factor (EGF) domains of β3 and which often result in spontaneous integrin activation. We also examine deep vein thrombosis as an unexpected complication of GT and look at curative procedures for the diseases, including allogeneic stem cell transfer and the potential for gene therapy.

Identification of small molecules that inhibit the interaction of TEM8 with anthrax protective antigen using a FRET assay

Tumor marker endothelial 8 (TEM8) is a receptor for the protective antigen (PA) component of anthrax toxin. TEM8 is upregulated on endothelial cells lining the blood vessels within tumors, compared with normal blood vessels. A number of studies have demonstrated a pivotal role for TEM8 in developmental and tumor angiogenesis. We have also shown that targeting the anthrax receptors with a mutated form of PA inhibits angiogenesis and tumor formation in vivo. Here we describe the development and testing of a high-throughput fluorescence resonance energy transfer assay to identify molecules that strongly inhibit the interaction of PA and TEM8. The assay we describe is sensitive and robust, with a Z' value of 0.8. A preliminary screen of 2310 known bioactive library compounds identified ebselen and thimerosal as inhibitors of the TEM8-PA interaction. These molecules each contain a cysteine-reactive transition metal, and complementary studies indicate that their inhibition of interaction is due to modification of a cysteine residue in the TEM8 extracellular domain. This is the first demonstration of a high-throughput screening assay that identifies inhibitors of TEM8, with potential application for antianthrax and antiangiogenic diseases.

The regulation of integrin function by divalent cations

Integrins are a family of α/β heterodimeric adhesion metalloprotein receptors and their functions are highly dependent on and regulated by different divalent cations. Recently advanced studies have revolutionized our perception of integrin metal ion-binding sites and their specific functions. Ligand binding to integrins is bridged by a divalent cation bound at the MIDAS motif on top of either α I domain in I domain-containing integrins or β I domain in α I domain-less integrins. The MIDAS motif in β I domain is flanked by ADMIDAS and SyMBS, the other two crucial metal ion binding sites playing pivotal roles in the regulation of integrin affinity and bidirectional signaling across the plasma membrane. The β-propeller domain of α subunit contains three or four β-hairpin loop-like Ca(2+)-binding motifs that have essential roles in integrin biogenesis. The function of another Ca(2+)-binding motif located at the genu of α subunit remains elusive. Here, we provide an overview of the integrin metal ion-binding sites and discuss their roles in the regulation of integrin functions.

Aquaporin 2 promotes cell migration and epithelial morphogenesis

The aquaporin 2 (AQP2) water channel, expressed in kidney collecting ducts, contributes critically to water homeostasis in mammals. Animals lacking or having significantly reduced levels of AQP2, however, have not only urinary concentrating abnormalities but also renal tubular defects that lead to neonatal mortality from renal failure. Here, we show that AQP2 is not only a water channel but also an integrin-binding membrane protein that promotes cell migration and epithelial morphogenesis. AQP2 expression modulates the trafficking and internalization of integrin β1, facilitating its turnover at focal adhesions. In vitro, disturbing the interaction between AQP2 and integrin β1 by mutating the RGD motif led to reduced endocytosis, retention of integrin β1 at the cell surface, and defective cell migration and tubulogenesis. Similarly, in vivo, AQP2-null mice exhibited significant retention of integrin β1 at the basolateral membrane and had tubular abnormalities. In summary, these data suggest that the water channel AQP2 interacts with integrins to promote renal epithelial cell migration, contributing to the structural and functional integrity of the mammalian kidney.

Calcium modulates force sensing by the von Willebrand factor A2 domain

von Willebrand factor (VWF) multimers mediate primary adhesion and aggregation of platelets. VWF potency critically depends on multimer size, which is regulated by a feedback mechanism involving shear-induced unfolding of the VWF-A2 domain and cleavage by the metalloprotease ADAMTS-13. Here we report crystallographic and single-molecule optical tweezers data on VWF-A2 providing mechanistic insight into calcium-mediated stabilization of the native conformation that protects A2 from cleavage by ADAMTS-13. Unfolding of A2 requires higher forces when calcium is present and primarily proceeds through a mechanically stable intermediate with non-native calcium coordination. Calcium further accelerates refolding markedly, in particular, under applied load. We propose that calcium improves force sensing by allowing reversible force switching under physiologically relevant hydrodynamic conditions. Our data show for the first time the relevance of metal coordination for mechanical properties of a protein involved in mechanosensing.

Regulation of integrin αIIbβ3 ligand binding and signaling by the metal ion binding sites in the β I domain

The ability of αIIbβ3 to bind ligands and undergo outside-in signaling is regulated by three divalent cation binding sites in the β I domain. Specifically, the metal ion-dependent adhesion site (MIDAS) and the synergistic metal binding site (SyMBS) are thought to be required for ligand binding due to their synergy between Ca(2+) and Mg(2+). The adjacent to MIDAS (ADMIDAS) is an important ligand binding regulatory site that also acts as a critical link between the β I and hybrid domains for signaling. Mutations in this site have provided conflicting results for ligand binding and adhesion in different integrins. We have mutated the β3 SyMBS and ADMIDAS. The SyMBS mutant abolished ligand binding and outside-in signaling, but when an activating glycosylation mutation in the αIIb Calf 2 domain was introduced, the ligand binding affinity and signaling were restored. Thus, the SyMBS is important but not absolutely required for integrin bidirectional signaling. The ADMIDAS mutants showed reduced ligand binding affinity and abolished outside-in signaling, and the activating glycosylation mutation could fully restore integrin signaling of the ADMIDAS mutant. We propose that the ADMIDAS ion stabilizes the low-affinity state when the integrin headpiece is in the closed conformation, whereas it stabilizes the high-affinity state when the headpiece is in the open conformation with the swung-out hybrid domain.

Bactrian camel (Camelus bactrianus) integrins alphavbeta3 and alphavbeta6 as FMDV receptors: molecular cloning, sequence analysis and comparison with other species

Integrins are heterodimeric adhesion receptors that participate in a variety of cell-cell and cell-extracellular matrix protein interactions. Many integrins recognize RGD sequences displayed on extracellular matrix proteins and the exposed loops of viral capsid proteins. Four members of the alphav integrin family of cellular receptors, alphavbeta3, alphavbeta6, alphavbeta1 and alphavbeta8, have been identified as receptors for foot-and-mouth disease virus (FMDV) in vitro, and integrins are believed to be the receptors used to target epithelial cells in the infected animals. To analyse the roles of the alphav integrins from a susceptible species as viral receptors, we have cloned Bactrian camel alphav, beta3 and beta6 integrin cDNAs and compared them to those of other species. The coding sequences for Bactrian camel integrin alphav, beta3 and beta6 were found to be 3165, 2289 and 2367 nucleotides in length, encoding 1054, 762 and 788 amino acids, respectively. The Bactrian camel alphav, beta3 and beta6 subunits share many structural features with homologues of other species, including the ligand binding domain and cysteine-rich region. Phylogenetic trees and similarity analyses showed the close relationships of integrin genes from Bactrian camels, pigs and cattle, which are each susceptible to FMDV infection, that were distinct from the orders Rodentia, Primates, Perissodactyla, Carnivora, Galliformes and Xenopus. We postulate that host tropism of FMDV may in part be related to the divergence in integrin subunits among different species.

Distinct roles of beta1 metal ion-dependent adhesion site (MIDAS), adjacent to MIDAS (ADMIDAS), and ligand-associated metal-binding site (LIMBS) cation-binding sites in ligand recognition by integrin alpha2beta1

Integrin-ligand interactions are regulated in a complex manner by divalent cations, and previous studies have identified ligand-competent, stimulatory, and inhibitory cation-binding sites. In collagen-binding integrins, such as alpha2beta1, ligand recognition takes place exclusively at the alpha subunit I domain. However, activation of the alphaI domain depends on its interaction with a structurally similar domain in the beta subunit known as the I-like or betaI domain. The top face of the betaI domain contains three cation-binding sites: the metal-ion dependent adhesion site (MIDAS), the ADMIDAS (adjacent to MIDAS), and LIMBS (ligand-associated metal-binding site). The role of these sites in controlling ligand binding to the alphaI domain has yet to be elucidated. Mutation of the MIDAS or LIMBS completely blocked collagen binding to alpha2beta1; in contrast mutation of the ADMIDAS reduced ligand recognition but this effect could be overcome by the activating monoclonal antibody TS2/16. Hence, the MIDAS and LIMBS appear to be essential for the interaction between alphaI and betaI, whereas occupancy of the ADMIDAS has an allosteric effect on the conformation of betaI. An activating mutation in the alpha2 I domain partially restored ligand binding to the MIDAS and LIMBS mutants. Analysis of the effects of Ca(2+), Mg(2+), and Mn(2+) on ligand binding to these mutants showed that the MIDAS is a ligand-competent site through which Mn(2+) stimulates ligand binding, whereas the LIMBS is a stimulatory Ca(2+)-binding site, occupancy of which increases the affinity of Mg(2+) for the MIDAS.

Glanzmann's thrombasthenia: an overview

Glanzmann's thrombasthenia (GT) is an autosomal recessive inherited bleeding disorder due to a defect in platelet function. The hallmark of this disease is severely reduced/absent platelet aggregation in response to multiple physiological agonists. Bleeding signs in GT include epistaxis, bruising, gingival hemorrhage, gastrointestinal hemorrhage, hematuria, menorrhagia, and hemarthrosis. Homozygous or compound heterozygous mutations in the genes of GPIIb and GPIIIa lead to GT. A patient with GT, with no possible causative mutations in GPIIb and GPIIIa genes, may harbor defects in a regulatory element affecting the transcription of these 2 genes. GT occurs in high frequency in certain ethnic populations with an increased incidence of consanguinity such as in Indians, Iranians, Iraqi Jews, Palestinian and Jordanian Arabs, and French Gypsies. Carrier detection in GT is important to control the disorder in family members. Carrier detection can be done both by protein analysis and direct gene analysis.

The GPIIb/IIIa (integrin alphaIIbbeta3) odyssey: a technology-driven saga of a receptor with twists, turns, and even a bend

Starting 90 years ago with a clinical description by Glanzmann of a bleeding disorder associated with a defect in platelet function, technologic advances helped investigators identify the defect as a mutation(s) in the integrin family receptor, alphaIIbbeta3, which has the capacity to bind fibrinogen (and other ligands) and support platelet-platelet interactions (aggregation). The receptor's activation state was found to be under exquisite control, with activators, inhibitors, and elaborate inside-out signaling mechanisms controlling its conformation. Structural biology has produced high-resolution images defining the ligand binding site at the atomic level. Research on alphaIIbbeta3 has been bidirectional, with basic insights resulting in improved Glanzmann thrombasthenia carrier detection and prenatal diagnosis, assays to identify single nucleotide polymorphisms responsible for alloimmune neonatal thrombocytopenia, and the development of alphaIIbbeta3 antagonists, the first rationally designed antiplatelet agents, to prevent and treat thrombotic cardiovascular disease. The future looks equally bright, with the potential for improved drugs and the application of gene therapy and stem cell biology to address the genetic abnormalities. The alphaIIbbeta3 saga serves as a paradigm of rigorous science growing out of careful clinical observations of a rare disorder yielding both important new scientific information and improved diagnosis, therapy, and prevention of other disorders.

Functional and computational studies of the ligand-associated metal binding site of beta3 integrins

A combination of experimental and computational approaches was used to provide a structural context for the role of the beta3 integrin subunit ligand-associated metal binding site (LIMBS) in the binding of physiological ligands to beta3 integrins. Specifically, we have carried out (1) adhesion assays on cells expressing normal alphaIIbeta3, normal alphaVbeta3, or the corresponding beta3 D217A LIMBS mutants; and (2) equilibrium and nonequilibrium (steered) molecular dynamics (MD) simulations of eptifibatide in complex with either a fully hydrated normal alphaIIbeta3 integrin fragment (alphaIIb beta-propeller and the beta3 betaA (I-like), hybrid, and PSI domains) or the equivalent beta3 D217A mutant. Normal alphaIIbeta3 expressing cells adhered to immobilized fibrinogen and echistatin, whereas cells expressing the alphaIIbeta3 D217A LIMBS mutant failed to adhere to either ligand. Similarly, the equivalent alphaVbeta3 mutant was unable to support adhesion to vitronectin or fibrinogen. The alphaIIbeta3 D217A mutation increased the binding of mAb AP5, which recognizes a ligand-induced binding site (LIBS) in the beta3 PSI domain, indicating that this mutation induced allosteric changes in the protein. Steered MD simulating the unbinding of eptifibatide from either normal alphaIIbeta3 or the equivalent beta3 D217A mutant suggested that the reduction in ligand binding caused by the LIMBS mutant required the loss of both the LIMBS and the metal ion-dependent adhesion site (MIDAS) metal ions. Our computational results indicate that the LIMBS plays a crucial role in ligand binding to alphaIIbeta3 by virtue of its effects on the coordination of the MIDAS.

A novel Phytophthora infestans haustorium-specific membrane protein is required for infection of potato

Phytophthora infestans causes late-blight, a devastating and re-emerging disease of potato crops. During the early stages of infection, P. infestans differentiates infection-specific structures such as appressoria for host epidermal cell penetration, followed by infection vesicles, and haustoria to establish a biotrophic phase of interaction. Here we report the cloning, from a suppression subtractive hybridization library, of a P. infestans gene called Pihmp1 encoding a putative glycosylated protein with four closely spaced trans-membrane helices. Pihmp1 expression is upregulated in germinating cysts and in germinating cysts with appressoria, and significantly upregulated throughout infection of potato. Transient gene silencing of Pihmp1 led to loss of pathogenicity and indicated involvement of this gene in the penetration and early infection processes of P. infestans. P. infestans transformants expressing a Pihmp1::monomeric red fluorescent protein (mRFP) fusion demonstrated that Pihmp1 was translated in germinating sporangia, germinating cysts and appressoria, accumulated in the appressorium, and was located at the haustorial membrane during infection. Furthermore, we discovered that haustorial structures are formed over a 3 h period, maturing for up to 12 h, and that their formation is initiated only at sites on the surface of intercellular hyphae where Pihmp1::mRFP is localized. We propose that Pihmp1 is an integral membrane protein that provides physical stability to the plasma membrane of P. infestans infection structures. We have provided the first evidence that the surface of oomycete haustoria possess proteins specific to these biotrophic structures, and that formation of biotrophic structures (infection vesicles and haustoria) is essential to successful host colonization by P. infestans.

Tyr178 of beta3 is critical for alphaIIb maturation and macromolecular ligand binding to alphaIIbbeta3

To explore the structural basis of ligand binding to alphaIIbbeta3, we conducted a site-directed mutagenesis of Y178, which is located in the ligand-specificity region (C177-C184) of the beta3 subunit. Two mutant beta3 constructs, Y178A and Y178I, were transfected into CHO cells and co-expressed with human alphaIIb subunit on the cell surface. Our results showed that the Y178A mutation affected processing and cell surface exposure of recombinant alphaIIbbeta3 receptor, abrogated the binding of PAC-1, a ligand-mimetic antibody, to alphaIIbbeta3 pre-treated with the known activator DTT. The Y178A mutation also resulted in reduced adhesion of alphaIIbbeta3 on immobilized fibrinogen. In contrast, the interaction of alphaIIbbeta3 with the small molecular ligand RGDS was unaffected by Y178A mutation, as evidenced by the elevated LIBS-1 epitope expression following RGDS addition. Interestingly however, Y178I mutation did not affect the receptor synthesis and function at all. As for post-receptor occupancy, neither Y178A nor Y178I prevented alphaIIbbeta3 translocation to focal adhesion contacts. These results suggest that Y178 is involved in alphaIIb maturation and alphaIIbbeta3 complex expression. This residue is also critical for alphaIIbbeta3 interaction with its macromolecular ligand or ligand-mimetic mAb, possibly due to its involvement in other ligand-binding sites distinct from the RGD-binding pocket. We also propose that a residue with appropriate side-chain size and hydrophobicity at position 178 is spatially required for formation of the correct tertiary structure of the site.

Molecular characterization and expression analysis of the gene coding for the porcine beta(3) integrin subunit (CD61)

Integrins are heterodimeric cell adhesion molecules with major roles in a variety of biological processes ranging from cell migration to tissue organization, immune and non-immune defense mechanisms and oncogenic transformation. Members of the beta(3) integrin subfamily are composed of a beta(3) subunit (CD61) non-covalently associated with two alpha subunits, alpha(IIb) (CD41) and alpha(v) (CD51), to constitute a group of transmembrane glycoproteins that participate in many physiologically important events. This investigation has focused on the molecular characterization of the cDNA encoding the porcine beta(3) integrin subunit. The deduced 762-amino acid sequence was 93, 92, 91, 89, 79 and 73% homologous to human, dog, rabbit, mouse, chicken and Xenopus laevis CD61 protein, respectively. Porcine CD61 molecule shares many structural features with human CD61, including a region containing a metal ion-dependent adhesion site (MIDAS) folding into an I domain-like structure. Through PCR-SSCP analysis and sequencing, six polymorphic positions were detected in the cDNA sequence of porcine CD61, and their frequencies were observed from a collection of 47 pigs. Expression analysis was done at two different levels: expression of the CD61 mRNA by RT-PCR and localization of the protein by immunohistochemistry. Our results show that CD61 transcripts were detected mainly in platelets and hematopoietic tissues. The immunohistochemical tissue localization of CD61 protein by a specific monoclonal antibody against CD61 recombinant protein showed that CD61 was expressed on vascular and non-vascular smooth muscle, epithelium and myeloid cells, being undetectable in cells of the lymphoid lineage. Furthermore, pulmonary intravascular macrophages (PIM), a subpopulation of macrophages which seem to play an important role in blood clearance, expressed much more CD61 when compared to pulmonary alveolar macrophages (PAM). The knowledge of the structure and distribution of the CD61 provides insight into the physiological function of the porcine beta(3) integrins and should be of importance in understanding the role of this integrin family in biological processes.

Adhesion molecules in the nervous system: structural insights into function and diversity

The unparalleled complexity of intercellular connections in the nervous system presents requirements for high levels of both specificity and diversity for the proteins that mediate cell adhesion. Here we describe recent advances toward understanding the molecular mechanisms that underlie adhesive binding, specificity, and diversity for several well-characterized families of adhesion molecules in the nervous system. Although many families of adhesion proteins, including cadherins and immunoglobulin superfamily members, are utilized in neural and nonneural contexts, nervous system-specific diversification mechanisms, such as precisely regulated alternative splicing, provide an important means to enable their function in the complex context of the nervous system.

Beta-integrin mediates WSSV infection

White Spot Syndrome Virus (WSSV) is a virulent and widespread dsDNA virus with a wide range of hosts. Although remarkable progress has been made on virus characterization, however, its mechanism of infection is poorly understood. In this study, by analyzing the phage display library of the WSSV genome, a WSSV envelope protein VP187 (wsv209) was found to interact with shrimp integrin. VP187 possesses the RGD motif. The interaction between integrin and VP187 was confirmed with coimmunoprecipitation. These results demonstrate for the first time an interaction between the WSSV envelope protein and a cell surface molecule. Soluble integrin, integrin-specific antibody and an RGD-containing peptide were found to block the WSSV infection in vivo and in vitro. Gene silencing using a sequence-specific dsRNA targeting beta-integrin effectively inhibited the virus infection. These findings suggest that beta-integrin may function as a cellular receptor for WSSV infection.

The Specificity and Function of the Metal-binding Sites in the Integrin β3 A-domain

The A-domains within integrin beta subunits contain three metal sites termed the metal ion-dependent adhesion site (MIDAS), site adjacent to the metal ion-dependent adhesion site (ADMIDAS), and ligand-induced metal-binding site (LIMBS), and these sites are involved in ligand engagement. The selectivity of these metal sites and their role in ligand binding have been investigated by expressing a fragment corresponding to the beta3 A-domain, beta3-(109-352), and single point mutants in which each of the cation-binding sites has been disabled. Equilibrium dialysis experiments identified three Mn2+- and two Ca2+-binding sites with the LIMBS being the site that did not bind Ca2+. Although the ADMIDAS could bind Ca2+, it did not bind Mg2+. These results indicate that the Ca2+-specific site that inhibits ligand binding is the ADMIDAS. Two different assay systems, surface plasmon resonance and a microtiter plate assay, demonstrated that the beta3 A-domain fragment bound fibrinogen in the presence of 0.1 mm Ca2+ but not in 3 mm Ca2+. This behavior recapitulated the effects of Ca2+ on fibrinogen binding to alphavbeta3 but not alphaIIbbeta3. Disabling any of the three cation-binding sites abrogated fibrinogen binding. These results indicate that the specificities of the three metal-binding sites for divalent cations are distinct and that each site can regulate the ligand binding potential of the beta3 A-domain.

Differences in integrin-dependent phagocytosis among three hemocyte subpopulations of the Pacific oyster "Crassostrea gigas"

Integrins play a key role in immunoresponses such as attachment, spreading, and phagocytosis in invertebrate hemocytes. This study was designed to identify integrin expression patterns at the hemocyte subpopulation level, and correlate the expression levels with phagocytic ability. First, we cloned a beta integrin from Crassostreagigas hemocytes and used real-time RT-PCR to analyze the quantitative expression level of its encoding mRNA. The expression level in hyalinocytes was significantly higher than that in granulocytes and agranulocytes. Subsequently, we investigated the phagocytic ability of each subpopulation using anti-alpha(5)beta(1) integrin antibody, and found that phagocytosis of hyalinocytes was inhibited by neutralization with the antibody but enhanced against the antibody-conjugated microspheres. In contrast, phagocytic abilities of granulocytes and agranulocytes showed high and zero levels, respectively, regardless of the antibody. These results suggest that phagocytosis of hyalinocytes is regulated by an integrin-dependent mechanism and that of granulocytes is elicited by other functional receptors.

Angiogenesis and rhodopsin-like receptors: a role for N-terminal acidic residues?

Numerous rhodopsin-like G-protein coupling receptors induce or inhibit angiogenesis. The active human receptors include several chemokine receptors, apelin APJ receptor, neuropeptide Y Y2 receptor, Duffy antigen, and herpes virus-8 receptor. A common and striking feature of these receptors is the large fraction (up to 42%) of residues with anionic sidechains (Asp, Glu, and benzene anions Tyr, Trp, and Phe) in the N-terminal extracellular domain. These residues (which are frequently clustered) can assist the binding of ligand peptides, but should also support interactions that help tubular arraying of cells, e.g., via cationic bridges and/or hydrogen bonding with cell-connecting receptors such as integrins, or with proteins of the extracellular matrix.

Structural basis for allostery in integrins and binding to fibrinogen-mimetic therapeutics

Integrins are important adhesion receptors in all Metazoa that transmit conformational change bidirectionally across the membrane. Integrin alpha and beta subunits form a head and two long legs in the ectodomain and span the membrane. Here, we define with crystal structures the atomic basis for allosteric regulation of the conformation and affinity for ligand of the integrin ectodomain, and how fibrinogen-mimetic therapeutics bind to platelet integrin alpha(IIb)beta3. Allostery in the beta3 I domain alters three metal binding sites, associated loops and alpha1- and alpha7-helices. Piston-like displacement of the alpha7-helix causes a 62 degrees reorientation between the beta3 I and hybrid domains. Transmission through the rigidly connected plexin/semaphorin/integrin (PSI) domain in the upper beta3 leg causes a 70 A separation between the knees of the alpha and beta legs. Allostery in the head thus disrupts interaction between the legs in a previously described low-affinity bent integrin conformation, and leg extension positions the high-affinity head far above the cell surface.

Sequence variation in the I-like domain of the beta1 integrin subunit in human oral squamous cell carcinomas

We recently identified a heterozygous mutation in the beta1 integrin subunit of a squamous cell carcinoma (SCC) that maps to the I-like domain and activates ligand binding. To investigate the frequency of such mutations we screened 124 human oral SCCs. We identified six single nucleotide changes, all of which were also present in normal tissue, suggestive of polymorphisms. Two were in non-coding intronic sequences. Three were silent changes in exons. One caused a change in amino acid (A239V) that is unlikely to disturb integrin structure. We conclude that mutations in the beta1 I-like domain are uncommon in SCCs. However, population based studies of the polymorphisms we found may reveal an association with SCC development or prognosis.

Distinct roles for the alpha and beta subunits in the functions of integrin alphaMbeta2

Integrin alphaMbeta2 (Mac-1, CD11b/CD18) is a noncovalently linked heterodimer of alphaM and beta2 subunits on the surface of leukocytes, where it plays a pivotal role in the adhesion and migration of these cells. Using HEK293 cells expressing alphaMbeta2 or the individual constituent chains on their surface, we analyzed the contributions of the alphaM or beta2 subunits to functional responses mediated by the integrin. In cells expressing only alphaM or beta2, the individual subunits were not associated with the endogenous integrins of the cells, and other partners for the subunits were not detected by surface labeling and immunoprecipitation under a variety of conditions. The alphaM cells mediated adhesion and spreading on a series of alphaMbeta2 ligands (fibrinogen, Factor X, iC3b, ICAM-1 (intercellular adhesion molecule-1), and denatured ovalbumin) but could not support cell migration to any of these. The spreading of the alphaM cells suggested an unanticipated linkage of this subunit to the cytoskeleton. The beta2 cells supported migration and attachment but not spreading on a subset of the alphaMbeta2 ligands. The heterodimeric receptor and its individual subunits were purified from the cells by affinity chromatography and recapitulated the ligand binding properties of the corresponding cell lines. These data indicate that each subunit of alphaMbeta2 contributes distinct properties to alphaMbeta2 and that, in most but not all cases, the response of the integrin is a composite of the functions of its individual subunits.

Combining pharmacophore search, automated docking, and molecular dynamics simulations as a novel strategy for flexible docking. Proof of concept: docking of arginine-glycine-aspartic acid-like compounds into the alphavbeta3 binding site

A novel and highly efficient flexible docking approach is presented where the conformations (internal degrees of freedom) and orientations (external degrees of freedom) of the ligands are successively considered. This hybrid method takes advantage of the synergistic effects of structure-based and ligand-based drug design techniques. Preliminary antagonist-derived pharmacophore determination provides the postulated bioactive conformation. Subsequent docking of this pharmacophore to the receptor crystal structure results in a postulated pharmacophore/receptor binding mode. Pharmacophore-oriented docking of antagonists is subsequently achieved by matching ligand interacting groups with pharmacophore points. Molecular dynamics in water refines the proposed complexes. To validate the method, arginine-glycine-aspartic acid (RGD) containing peptides, pseudopeptides, and RGD-like antagonists were docked to the crystal structure of alphavbeta3 holoprotein and apoprotein. The proposed directed docking was found to be more accurate, faster, and less biased with respect to the protein structure (holo and apoprotein) than DOCK, Autodock, and FlexX docking methods. The successful docking of an antagonist recently cocrystallized with the receptor to both apo and holoprotein is particularly appealing. The results summarized in this report illustrated the efficiency of our light CoMFA/rigid body docking hybrid method.

The mechanism of alphaIIbbeta3 antagonism by savignygrin and its implications for the evolution of anti-hemostatic strategies in soft ticks

Savignygrin, a alphaIIbbeta3 antagonist presents the RGD sequence on the substrate-binding loop of the (BPTI-fold). This study investigated whether this is the only integrin-targeting motif associated with its mechanism. It forms a tight-binding complex with alphaIIbbeta3 that is resistant to SDS dissociation under reducing and non-reducing conditions, but not to temperature or EDTA. The same complex is formed on resting and activated platelets, as well as aggregated platelets that have been disaggregated with savignygrin. Binding of FITC labeled savignygrin to platelets show that the binding kinetics and affinity of savignygrin is similar for resting and activated platelets (Kd approximately 50-70 nM). Binding to resting or activated platelets was significantly inhibited by two savignygrin peptide fragments, S2 (GSRGDEDATFG) and S3 (FDREDGGSRQG) that correspond with two specific loop-like areas in the structure of savignygrin that together form a continuous binding interface. The inability of S3 to inhibit platelet aggregation indicates that it targets a novel ligand-binding site. A model of alphaIIbbeta3 based on the recent crystal structure of alphavbeta3 into which the RGD sequence of savignygrin was docked shows that savignygrin lies along the interface formed by the two subunits. A novel mode of integrin antagonism is indicated that includes the targeting of distinct sites on the alphaIIbbeta3 subunits. The S2 and S3 loops are not involved in the mechanisms of the related soft tick blood coagulation inhibitors and suggest that this allowed their evolution as integrin targeting motifs.

Pharmacology of platelet adhesion and aggregation

At the injured vessel wall, blood platelets become activated and adhere to the subendothelial surface as well as to each other. These cellular adhesion processes are required for primary hemostasis, but can also lead to thrombosis. Considerable progress has been made during recent years in understanding the molecular mechanisms underlying platelet activation and adhesion. This knowledge will drive future efforts towards the development of new antiplatelet drugs for the prevention and treatment of cardiovascular diseases.

Docking studies on alphavbeta3 integrin ligands: pharmacophore refinement and implications for drug design

Starting from the first crystal structure of the extracellular segment of the alpha(v)beta(3) integrin receptor with a cyclic RGD ligand bound to the active site, structural models for the interactions of known ligands with the alpha(v)beta(3) integrin receptor were generated by automated computational docking. The obtained complexes were evaluated for their consistency with structure-activity relationships and site-directed mutagenesis data. A comparison between the calculated interaction free energies and the experimental biological activities was also made. All the possible interactions of the investigated compounds at the active site and the probable ligand binding conformations provide an improved basis for structure-based rational ligand design. Additionally, our docking results allow a further validation and refinement of the pharmacophore model previously postulated by us.

Integrins: bidirectional, allosteric signaling machines

In their roles as major adhesion receptors, integrins signal across the plasma membrane in both directions. Recent structural and cell biological data suggest models for how integrins transmit signals between their extracellular ligand binding adhesion sites and their cytoplasmic domains, which link to the cytoskeleton and to signal transduction pathways. Long-range conformational changes couple these functions via allosteric equilibria.

Agonist-specific structural rearrangements of integrin alpha IIbbeta 3. Confirmation of the bent conformation in platelets at rest and after activation

Concrete structural features of integrin alpha(IIb)beta(3) on the surface of platelets (at rest and after activation) have been obtained from epitope maps based on cross-competition among monoclonal antibodies directed against the alpha(IIb) subunit calf-2 domain and the beta(3) subunit betaA domain of alpha(IIb)beta(3). At rest, the observed intersubunit interface is formed by the sequence stretches beta(3)-(150-216), alpha(IIb) light chain-(1-92), and alpha(IIb) heavy chain-(826-856); and the alpha(IIb) interchain interface is formed by the two latter sequence stretches, disulfide-bonded between alpha(IIb) heavy chain Cys(826) and alpha(IIb) light chain Cys(9). These structural features agree with those observed in the alpha(IIb)beta(3) rudimentary connectivity map in solution and with the alpha(v)beta(3) V-shaped crystal structure (Xiong, J.-P., Zhang, R., Dunker, R., Scott, D. L., Joachimiak, A., Goodman, S. L., and Arnaout, M. A. (2001) Science 294, 339-345), but they disagree with the domain disposition suggested by the actual ultrastructural model. The epitope maps in platelets activated by ADP, thrombin receptor activation peptide, and arachidonic acid differ not only from those in platelets at rest, but also among themselves. The structural rearrangements observed confirm the presence in activated platelets of the crystallographically observed knee and argue against the switchblade mechanism proposed for activation (Beglova, N., Blacklow, S. C., Takagi, J., and Springer, T. A. (2002) Nat. Struct. Biol. 9, 282-287), demonstrate the existence of alpha(IIb)beta(3) agonist-specific activation states, explain the specificity for ligand binding and functional inhibition for some agonists, and predict the existence of agonist-specific final effectors and receptor activation mechanisms. The distinct non-reciprocal competition patterns observed at rest and after activation support the agonist-specific activation states and the existence of intrasubunit and intersubunit allosteric effects, previously proposed as the mechanism for alpha(IIb)beta(3) transmembrane activation.

Distribution and evolution of von Willebrand/integrin A domains: widely dispersed domains with roles in cell adhesion and elsewhere

The von Willebrand A (VWA) domain is a well-studied domain involved in cell adhesion, in extracellular matrix proteins, and in integrin receptors. A number of human diseases arise from mutations in VWA domains. We have analyzed the phylogenetic distribution of this domain and the relationships among approximately 500 proteins containing this domain. Although the majority of VWA-containing proteins are extracellular, the most ancient ones, present in all eukaryotes, are all intracellular proteins involved in functions such as transcription, DNA repair, ribosomal and membrane transport, and the proteasome. A common feature seems to be involvement in multiprotein complexes. Subsequent evolution involved deployment of VWA domains by Metazoa in extracellular proteins involved in cell adhesion such as integrin beta subunits (all Metazoa). Nematodes and chordates separately expanded their complements of extracellular matrix proteins containing VWA domains, whereas plants expanded their intracellular complement. Chordates developed VWA-containing integrin alpha subunits, collagens, and other extracellular matrix proteins (e.g., matrilins, cochlin/vitrin, and von Willebrand factor). Consideration of the known properties of VWA domains in integrins and extracellular matrix proteins allows insights into their involvement in protein-protein interactions and the roles of bound divalent cations and conformational changes. These allow inferences about similar functions in novel situations such as protease regulators (e.g., complement factors and trypsin inhibitors) and intracellular proteins (e.g., helicases, chelatases, and copines).

Identification of the principal binding site for RGD-containing ligands in the alpha(V)beta(3) integrin: a photoaffinity cross-linking study

By superimposing data obtained by photo-cross-linking RGD-containing ligands to the human alpha(V)beta(3) integrin onto the crystal structure of the ectopic domain of this receptor (Xiong et al. (2001) Science 294, 339-345), we have identified the binding site for the RGD triad within this integrin. We synthesized three novel analogues of the 49-amino acid disintegrin, echistatin: [Bpa(21),Leu(28)]-, [Bpa(23),Leu(28)]-, and [Bpa(28)]echistatin. Each contains a photoreactive p-benzoyl-phenylalanyl (Bpa) residue in close proximity to the RGD motif which spans positions 24-26; together, the photoreactive positions flank the RGD motif. The analogues bind with high affinity to the purified recombinant alpha(V)beta(3) integrin, but very poorly to the closely related human alpha(IIb)beta(3) platelet integrin. While echistatin analogues containing Bpa in either position 23 or 28 cross-link specifically and almost exclusively to the beta(3) subunit of alpha(V)beta(3), [Bpa(21),Leu(28)]echistatin cross-links to both the alpha(V) and the beta(3) subunits, with cross-linking to the former favored. [Bpa(23),Leu(28)]echistatin cross-links 10-30 times more effectively than the other two analogues. We identified beta(3)[109-118] as the domain that encompasses the contact site for [Bpa(28)]echistatin. This domain is included in beta(3)[99-118] (Bitan et al. (2000) Biochemistry 39, 11014-11023), a previously identified contact domain for a cyclic RGD-containing heptapeptide with a benzophenone moiety in a position that is similar to the placement of the benzophenone in [Bpa(28)]echistatin relative to the RGD triad. Recently, we identified beta(3)[209-220] as the contact site for an echistatin analogue with a photoreactive group in position 45, near the C-terminus of echistatin (Scheibler et al. (2001) Biochemistry 40, 15117-14126). Taken together, these results support the hypothesis that the very high binding affinity of echistatin to alpha(V)beta(3) results from two distinct epitopes in the ligand, a site including the RGD triad and an auxiliary epitope at the C-terminus of echistatin. Combining our results from photoaffinity cross-linking studies with data now available from the recently published crystal structure of the ectopic domain of alpha(V)beta(3), we characterize the binding site for the RGD motif in this receptor.

A new platelet polymorphism Duv(a+), localized within the RGD binding domain of glycoprotein IIIa, is associated with neonatal thrombocytopenia

We report here the identification and characterization of a new platelet alloantigen, Duv(a+), implicated in a case of neonatal thrombocytopenia. Immunochemical studies demonstrated that the epitope was localized on glycoprotein (GP) IIIa. Sequencing of the exons 2 to 15 of GP IIIa gene polymerase chain reaction products from both parents revealed a single base substitution 517C>T (complementary DNA) present in a heterozygous state in DNA from the father leading to amino acid substitution Thr140Ile (ACC>ATC) within the Arg-Gly-Asp binding domain of GP IIIa. Flow cytometry and immunoprecipitation studies of IIb-C517 or T517 IIIa transfected Cos cells allowed us to demonstrate this mutation was responsible for expression of the Duv(a+) epitope. By polymerase chain reaction-single-strand conformational-polymorphism analysis, the mutated allele could not be detected in a population of 100 healthy unrelated donors, indicating a low frequency of occurrence. The Thr140/Ile dimorphism, localized 3 amino acids upstream from the Arg143 involved in the expression of HPA-4a, did not interfere with the binding of an anti-HPA-4a antibody in flow cytometry. Results of functional analysis of wild-type or mutated transfected CHO cells-(1) aggregation in the presence of Ca(++) and soluble fibrinogen after complex activation by dithiothreitol, (2) adhesion on coated fibrinogen, (3) binding of monoclonal antibody PAC-1 or LIBS antibody D3, and (4) outside-in signaling-all suggest that the Thr140Ile polymorphism localized in the Arg-Gly-Asp binding domain of GP IIIa does not affect significantly, if at all, the integrin function. We have shown that the anti-Duv(a+) antibody may inhibit platelet GP IIb-IIIa function.

Chimeras of the integrin beta subunit mid-region reveal regions required for heterodimer formation and for activation

A central region of the beta2 integrin subunit, RN (residues D300 to C459), was replaced by the equivalent sequences from beta1 and beta7 to give the chimeras beta2RN1 and beta2RN7. Whilst the former construct failed to form heterodimer at the cell surface with alphaL, the later of these could be expressed together with the alphaL subunit to form a variant LFA-1. Based on recent modelling work, the RN region consists of two parts, one is the C-terminal end of the putative A-domain (RB, residues D300 to A359), and the other the mid-region (BN, residues Y360 to C459). Chimeras exchanging the two component regions were made. Of the four resultant chimeras, only the beta2RB1 chimera failed to support LFA-1 expression. Thus the beta1 specific residues of this region affect the interaction with the alphaL subunit. Whereas the alphaL/beta2RB7 LFA-1 variant is wildtype like with respect to ICAM-1 adhesion, the alphaLbeta2BN1 and alphaLbeta2BN7, as well as the alphaLbeta2RN7, variants are more adhesive than the wildtype. These results suggest that an authentic beta2 mid-region is, in part, required for maintaining the LFA-1 in a resting state.

Differential regulation of Rho GTPases by β1 and β3 integrins: the role of an extracellular domain of integrin in intracellular signaling

Integrins mediate cell adhesion and signal transduction at focal adhesions. Here we investigate the roles of integrin β subunits in the regulation of actin cytoskeletal structure and the activities of Rho and Rac. The overexpression of β3 integrin in Chinese hamster ovary cells enhances Rho activity and stress fiber formation, whereas the overexpression of β1 integrin increases Rac activity and lamellipodia formation. The overexpression of a mutant β1-3-1 integrin, in which the extracellular I-domain-like sequence of β1 integrin has been replaced with the corresponding sequence of β3 integrin, also enhances Rho activity and the formation of stress fibers. Our results demonstrate that β1 and β3 integrins differentially regulate the activities of Rho family GTPases and that the extracellular domains of integrin β subunits play a critical role in transducing the extracellular ligand-binding information into specific intracellular signaling events.

Protein structure prediction and biomolecular recognition: from protein sequence to peptidomimetic design with the human beta3 integrin

Computational tools can bridge the gap between sequence and protein 3D structure based on the notion that information is to be retrieved from the databases and that knowledge-based methods can help in approaching a solution of the protein-folding problem. To this aim our group has implemented neural network-based predictors capable of performing with some success in different tasks, including predictions of the secondary structure of globular and membrane proteins, the topology of membrane proteins and porins and stable alpha-helical segments suited for protein design. Moreover we have developed methods for predicting contact maps in proteins and the probability of finding a cysteine in a disulfide bridge, tools which can contribute to the goal of predicting the 3D structure starting from the sequence (the so called ab initio prediction). All our predictors take advantage of evolution information derived from the structural alignments of homologous (evolutionary related) proteins and taken from the sequence and structure databases. When it is necessary to build models for proteins of unknown spatial structure, which have very little homology with other proteins of known structure, non-standard techniques need to be developed and the tools for protein structure predictions may help in protein modeling. The results of a recent simulation performed in our lab highlights the role of high performing computing technology and of tools of computational biology in protein modeling and peptidomimetic design.