Experimental support for a beta-propeller domain in integrin alpha-subunits and a calcium binding site on its lower surface

CRTAC1 has a Compact β-propeller-TTR Core Stabilized by Potassium Ions

Cartilage acidic protein-1 (CRTAC1) is a secreted glycoprotein with roles in development, function and repair of the nervous system. It is linked to ischemic stroke, osteoarthritis and (long) COVID outcomes, and has suppressive activity in carcinoma and bladder cancer. Structural characterization of CRTAC1 has been complicated by its tendency to form disulfide-linked aggregates. Here, we show that CRTAC1 is stabilized by potassium ions. Using x-ray crystallography, we determined the structure of CRTAC1 to 1.6 Å. This reveals that the protein consists of a three-domain fold, including a previously-unreported compact β-propeller-TTR combination, in which an extended loop of the TTR plugs the β-propeller core. Electron density is observed for ten bound ions: six calcium, three potassium and one sodium. Low potassium ion concentrations lead to changes in tryptophan environment and exposure of two buried free cysteines located on a β-blade and in the β-propeller-plugging TTR loop. Mutating the two free cysteines to serines prevents covalent intermolecular interactions, but not aggregation, in absence of potassium ions. The potassium ion binding sites are located between the blades of the β-propeller, explaining their importance for the stability of the CRTAC1 fold. Despite varying in sequence, the three potassium ion binding sites are structurally similar and conserved features of the CRTAC protein family. These insights into the stability and structure of CRTAC1 provide a basis for further work into the function of CRTAC1 in health and disease.

Genome-wide characterization of integrin (ITG) gene family and their expression profiling in half-smooth tongue sole (Cynoglossus semilaevis) upon Vibrio anguillarum infection

Integrins (ITGs) are transmembrane heterodimer receptors with ITGα subunit and ITGβ subunit, participating in various physiological processes, including immunity. At present, systematic research on ITGs in teleost is scarce, especially in half-smooth tongue sole (Cynoglossus semilaevis). In this study, a set of 28 ITG genes in half-smooth tongue sole have been identified and characterized. The phylogenetic analysis showed that ITGα and ITGβ subunits were respectively classified into five and two clusters, consistent with previous studies. The selection pressure analysis indicated that most of ITG genes were under purifying selection, except for ITGα11b and ITGαL with positive selection. The expression profiles of eight selected ITG genes, including ITGα1, ITGα5, ITGα8, ITGα11, ITGβ1, ITGβ2, ITGβ3, and ITGβ8, were analyzed in healthy tissues and after infection with Vibrio anguillarum, revealed their implications in immune response. The study provided a comprehensive characterization and expression analysis of ITG genes in half-smooth tongue sole, setting a solid foundation for further functional studies and promising potential in disease control.

Integrin Conformational Dynamics and Mechanotransduction

The function of the integrin family of receptors as central mediators of cell-extracellular matrix (ECM) and cell-cell adhesion requires a remarkable convergence of interactions and influences. Integrins must be anchored to the cytoskeleton and bound to extracellular ligands in order to provide firm adhesion, with force transmission across this linkage conferring tissue integrity. Integrin affinity to ligands is highly regulated by cell signaling pathways, altering affinity constants by 1000-fold or more, via a series of long-range conformational transitions. In this review, we first summarize basic, well-known features of integrin conformational states and then focus on new information concerning the impact of mechanical forces on these states and interstate transitions. We also discuss how these effects may impact mechansensitive cell functions and identify unanswered questions for future studies.

Plasmodium berghei-Released Factor, PbTIP, Modulates the Host Innate Immune Responses

The Plasmodium parasite has to cross various immunological barriers for successful infection. Parasites have evolved mechanisms to evade host immune responses, which hugely contributes to the successful infection and transmission by parasites. One way in which a parasite evades immune surveillance is by expressing molecular mimics of the host molecules in order to manipulate the host responses. In this study, we report a Plasmodium berghei hypothetical protein, PbTIP (PbANKA_124360.0), which is a Plasmodium homolog of the human T-cell immunomodulatory protein (TIP). The latter possesses immunomodulatory activities and suppressed the host immune responses in a mouse acute graft-versus-host disease (GvHD) model. The Plasmodium berghei protein, PbTIP, is expressed on the merozoite surface and exported to the host erythrocyte surface upon infection. It is shed in the blood circulation by the activity of an uncharacterized membrane protease(s). The shed PbTIP could be detected in the host serum during infection. Our results demonstrate that the shed PbTIP exhibits binding on the surface of macrophages and reduces their inflammatory cytokine response while upregulating the anti-inflammatory cytokines such as TGF-β and IL-10. Such manipulated immune responses are observed in the later stage of malaria infection. PbTIP induced Th2-type gene transcript changes in macrophages, hinting toward its potential to regulate the host immune responses against the parasite. Therefore, this study highlights the role of a Plasmodium-released protein, PbTIP, in immune evasion using macrophages, which may represent the critical strategy of the parasite to successfully survive and thrive in its host. This study also indicates the human malaria parasite TIP as a potential diagnostic molecule that could be exploited in lateral flow-based immunochromatographic tests for malaria disease diagnosis.

The N-terminal intrinsically disordered region mediates intracellular localization and self-oligomerization of ALS2

ALS2, a product of the causative gene for familial amyotrophic lateral sclerosis (ALS) type 2, plays a pivotal role in the regulation of endosome dynamics by activating small GTPase Rab5 via its intrinsic guanine nucleotide-exchange factor activity. Previously, we have reported that the N-terminal region of ALS2 has crucial roles in its endosomal localization and self-oligomerization, both of which are indispensable for the cellular function of ALS2. The N-terminus of ALS2 contains the regulator of chromosome condensation 1-like domain (RLD), which is predicted to form a seven-bladed β-propeller structure. Interestingly, the RLD is interrupted by the intrinsically disordered region (IDR), within which there are several amino acid residues which undergo phosphorylation. In this study, we sought to investigate as to whether and how the IDR as well as phosphorylation at either Ser483, Ser492 or Thr510 affect the intracellular localization and self-oligomerization of ALS2. All phospho- and dephospho-mimetic ALS2 mutants that were transiently expressed in HeLa cells were diffusely distributed throughout the cytosol with a partial localization to early endosomes. When expressed under Rac1-activating conditions, these mutants were localized to membrane ruffles as well as enlarged endosomes. Further, gel-filtration analysis revealed that these mutants primarily existed as a tetramer in cells. However, all these phenotypes were indistinguishable from those of wild-type ALS2. On the other hand, IDR-deleted ALS2 mutant was exclusively present in perinuclear aggregates colocalizing with the autophagy-related protein SQSTM1. Moreover, IDR-deleted ALS2 mutant formed an abnormally high molecular weight complex compared to wild-type ALS2. These results indicate that the IDR of ALS2 plays a crucial role not only in the regulation of intracellular localization but also in the self-oligomerization of ALS2 in cells, whereas phosphorylation of certain residues within the IDR exerts limited effects on such phenotypes.

Visualization of integrin molecules by fluorescence imaging and techniques

Integrin molecules are transmembrane αβ heterodimers involved in cell adhesion, trafficking, and signaling. Upon activation, integrins undergo dynamic conformational changes that regulate their affinity to ligands. The physiological functions and activation mechanisms of integrins have been heavily discussed in previous studies and reviews, but the fluorescence imaging techniques -which are powerful tools for biological studies- have not. Here we review the fluorescence labeling methods, imaging techniques, as well as Förster resonance energy transfer assays used to study integrin expression, localization, activation, and functions.

Integrins meet complement: The evolutionary tip of an iceberg orchestrating metabolism and immunity

Immunologists have recently realized that there is more to the classic innate immune sensor systems than just mere protection against invading pathogens. It is becoming increasingly clear that such sensors, including the inflammasomes, toll-like receptors, and the complement system, are heavily involved in the regulation of basic cell physiological processes and particularly those of metabolic nature. In fact, their "non-canonical" activities make sense as no system directing immune cell activity can perform such task without the need for energy. Further, many of these ancient immune sensors appeared early and concurrently during evolution, particularly during the developmental leap from the single-cell organisms to multicellularity, and therefore crosstalk heavily with each other. Here, we will review the current knowledge about the emerging cooperation between the major inter-cell communicators, integrins, and the cell-autonomous intracellularly and autocrine-active complement, the complosome, during the regulation of single-cell metabolism. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

An integrin-binding array platform identifies αvβ8 and α5β1 integrins on rat primary cortical neurons to support their survival and growth

BACKGROUND

Integrins are the major cell adhesion receptors expressed in almost all cell types connecting the extracellular matrix with cell cytoskeletons and transducing bi-directional signals across cell membranes. In the central nervous system (CNS), integrins are pivotal for CNS cell migration, differentiation, neurite outgrowth and synaptogenesis in both physiological and pathological conditions. Here we studied the effect of different integrin biding peptides for growth and development of primary cortical neurons in vitro.

NEW METHOD

Rat primary cortical neurons were cultured in an integrin-binding array platform, which contains immobilized varying short synthetic peptides that bind to 16 individual types of integrin on a 48-well cell culture plate. After cultured for 7 days, cells were fixed and processed for immunostaining with neuronal markers. The overall neuronal growth and neurite outgrowths were quantified.

RESULTS

We found that binding peptides for integrin α_vβ₈, α₅β₁ and α₃β₁ particularly the former two provided superior condition for neuronal growth, survival and maturation. Moreover, optimal neurite outgrowth was observed when neurons were cultured in 3-dimension using injectable hydrogel along with binding peptide for α_vβ₈ or α₅β₁ integrins.

COMPARISON WITH EXISTING METHOD

For primary neuronal culture, poly-D-lysine coating is conventional method to support cell attachment. Our study has demonstrated that selected integrin binding peptides provide greater support for the growth of cultured primary neurons.

CONCLUSION

These data suggest that integrin α_vβ₈ and α₅β₁ are conducive for survival, growth and maturation of primary cortical neurons. This information could be utilized in designing combinational biomaterial and cell-based therapy for neural regeneration following brain injury.

Diagnostic and prognostic values of integrin α subfamily mRNA expression in colon adenocarcinoma

The integrin α (ITGA) subfamily genes play a fundamental role in various cancers. However, the potential mechanism and application values of ITGA genes in colon adenocarcinoma (COAD) remain elusive. The present study investigated the significance of the expression of ITGA genes in COAD from the perspective of diagnosis and prognosis. A COAD RNA-sequencing dataset was obtained from The Cancer Genome Atlas. The present study investigated the biological function of the ITGA subfamily genes through bioinformatics analysis. Reverse transcription-quantitative polymerase chain reaction was applied to investigate the distribution of integrin α8 (ITGA8) expression in COAD tumors and adjacent normal tissues. Bioinformatics analysis indicated that ITGA genes were noticeably enriched in cell adhesion and the integrin-mediated signaling pathway, and co-expressed with each other. It was also revealed through observation that the majority of gene expression was significantly low in tumor tissues (P<0.05), and diagnostic receiver operating characteristic curves revealed that most of the genes could serve as significant diagnostic markers in COAD (P<0.05), especially ITGA8 which had a high diagnostic value with an area under curve (AUC) of 0.989 [95% confidence interval (CI) 0.980–0.997] in COAD (P<0.0001). In addition, ITGA8 expression was verified in clinical samples and it was revealed that it was higher in adjacent normal tissues (P=0.041) compared to COAD tissues, and the AUC was 0.704 (95% CI, 0.577–0.831; P<0.0085). Multivariate survival analysis indicated that integrin α (ITGA5) may be an independent prognostic indicator for COAD overall survival. Gene set enrichment analysis indicated that ITGA5 may participate in multiple biological processes and pathways. The present study revealed that ITGA genes were associated with the diagnosis and prognosis of COAD. The mRNA expression of ITGA8 may be a potential diagnosis biomarker and ITGA5 may serve as an independent prognosis indicator for COAD.

Integrins promote axonal regeneration after injury of the nervous system

Integrins are cell surface receptors that form the link between extracellular matrix molecules of the cell environment and internal cell signalling and the cytoskeleton. They are involved in several processes, e.g. adhesion and migration during development and repair. This review focuses on the role of integrins in axonal regeneration. Integrins participate in spontaneous axonal regeneration in the peripheral nervous system through binding to various ligands that either inhibit or enhance their activation and signalling. Integrin biology is more complex in the central nervous system. Integrins receptors are transported into growing axons during development, but selective polarised transport of integrins limits the regenerative response in adult neurons. Manipulation of integrins and related molecules to control their activation state and localisation within axons is a promising route towards stimulating effective regeneration in the central nervous system.

Granzyme B is elevated in autoimmune blistering diseases and cleaves key anchoring proteins of the dermal-epidermal junction

In healthy skin, epidermis and dermis are anchored together at the dermal-epidermal junction (DEJ), a specialized basement membrane pivotal for skin integrity and function. However, increased inflammation in the DEJ is associated with the disruption and separation of this junction and sub-epidermal blistering. Granzyme B (GzmB) is a serine protease secreted by immune cells. Dysregulated inflammation may lead to increased GzmB accumulation and proteolysis in the extracellular milieu. Although elevated GzmB is observed at the level of the DEJ in inflammatory and blistering skin conditions, the present study is the first to explore GzmB in the context of DEJ degradation in autoimmune sub-epidermal blistering. In the present study, GzmB induced separation of the DEJ in healthy human skin. Subsequently, α6/β4 integrin, collagen VII, and collagen XVII were identified as extracellular substrates for GzmB through western blot, and specific cleavage sites were identified by mass spectrometry. In human bullous pemphigoid, dermatitis herpetiformis, and epidermolysis bullosa acquisita, GzmB was elevated at the DEJ when compared to healthy samples, while α6/β4 integrin, collagen VII, and collagen XVII were reduced or absent in the area of blistering. In summary, our results suggest that regardless of the initial causation of sub-epidermal blistering, GzmB activity is a common final pathway that could be amenable to a single targeted treatment approach.

Structural proteins of the dermal-epidermal junction targeted by autoantibodies in pemphigoid diseases

The dermal-epidermal junction consists of a network of several interacting structural proteins that strengthen adhesion and mediate signalling events. This structural network consists of hemidesmosomal-anchoring filament complexes connecting the basal keratinocytes to the basement membrane. The anchoring filaments in turn interact with the anchoring fibrils to attach the basement membrane to the underlying dermis. Several of these structural proteins are recognized by autoantibodies in pemphigoid diseases, a heterogeneous group of clinically and immunopathologically diverse entities. Targeted proteins include the two intracellular plakins, plectin isoform 1a and BP230 (also called bullous pemphigoid antigen (BPAG) 1 isoform e (BPAG1e)). Plectin 1a and BP230 are connected to the intermediate filaments and to the cell surface receptor α6β4 integrin, which in turn is connected to laminin 332, a component of the anchoring filaments. Further essential adhesion proteins are BP180, a transmembrane protein, laminin γ1 and type VII collagen. Latter protein is the major constituent of the anchoring fibrils. Mutations in the corresponding genes of these adhesion molecules lead to inherited epidermolysis bullosa emphasizing the importance of these proteins for the integrity of the dermal-epidermal junction. This review will provide an overview on the structure and function of the proteins situated in the dermal-epidermal junction targeted by autoantibodies.

Structure-Function Relationships Underlying the Capacity of Bordetella Adenylate Cyclase Toxin to Disarm Host Phagocytes

Bordetellae, pathogenic to mammals, produce an immunomodulatory adenylate cyclase toxin-hemolysin (CyaA, ACT or AC-Hly) that enables them to overcome the innate immune defense of the host. CyaA subverts host phagocytic cells by an orchestrated action of its functional domains, where an extremely catalytically active adenylyl cyclase enzyme is delivered into phagocyte cytosol by a pore-forming repeat-in-toxin (RTX) cytolysin moiety. By targeting sentinel cells expressing the complement receptor 3, known as the CD11b/CD18 (α_Mβ₂) integrin, CyaA compromises the bactericidal functions of host phagocytes and supports infection of host airways by Bordetellae. Here, we review the state of knowledge on structural and functional aspects of CyaA toxin action, placing particular emphasis on signaling mechanisms by which the toxin-produced 3',5'-cyclic adenosine monophosphate (cAMP) subverts the physiology of phagocytic cells.

Bordetella adenylate cyclase toxin is a unique ligand of the integrin complement receptor 3

Integrins are heterodimeric cell surface adhesion and signaling receptors that are essential for metazoan existence. Some integrins contain an I-domain that is a major ligand binding site. The ligands preferentially engage the active forms of the integrins and trigger signaling cascades that alter numerous cell functions. Here we found that the adenylate cyclase toxin (CyaA), a key virulence factor of the whooping cough agent Bordetella pertussis, preferentially binds an inactive form of the integrin complement receptor 3 (CR3), using a site outside of its I-domain. CyaA binding did not trigger downstream signaling of CR3 in human monocytes and CyaA-catalyzed elevation of cAMP effectively blocked CR3 signaling initiated by a natural ligand. This unprecedented type of integrin-ligand interaction distinguishes CyaA from all other known ligands of the I-domain-containing integrins and provides a mechanistic insight into the previously observed central role of CyaA in the pathogenesis of B. pertussis.

Matrikines and the lungs

The extracellular matrix is a complex network of fibrous and nonfibrous molecules that not only provide structure to the lung but also interact with and regulate the behaviour of the cells which it surrounds. Recently it has been recognised that components of the extracellular matrix proteins are released, often through the action of endogenous proteases, and these fragments are termed matrikines. Matrikines have biological activities, independent of their role within the extracellular matrix structure, which may play important roles in the lung in health and disease pathology. Integrins are the primary cell surface receptors, characterised to date, which are used by the matrikines to exert their effects on cells. However, evidence is emerging for the need for co-factors and other receptors for the matrikines to exert their effects on cells. The potential for matrikines, and peptides derived from these extracellular matrix protein fragments, as therapeutic agents has recently been recognised. The natural role of these matrikines (including inhibitors of angiogenesis and possibly inflammation) make them ideal targets to mimic as therapies. A number of these peptides have been taken forward into clinical trials. The focus of this review will be to summarise our current understanding of the role, and potential for highly relevant actions, of matrikines in lung health and disease.

Integrin structure, activation, and interactions

Integrins are large, membrane-spanning, heterodimeric proteins that are essential for a metazoan existence. All members of the integrin family adopt a shape that resembles a large "head" on two "legs," with the head containing the sites for ligand binding and subunit association. Most of the receptor dimer is extracellular, but both subunits traverse the plasma membrane and terminate in short cytoplasmic domains. These domains initiate the assembly of large signaling complexes and thereby bridge the extracellular matrix to the intracellular cytoskeleton. To allow cells to sample and respond to a dynamic pericellular environment, integrins have evolved a highly responsive receptor activation mechanism that is regulated primarily by changes in tertiary and quaternary structure. This review summarizes recent progress in the structural and molecular functional studies of this important class of adhesion receptor.

Surface plasmon resonance biosensing in studies of the binding between β₂ integrin I domains and their ligands

Measurements on the kinetic aspects of binding between macromolecular species such as proteins have been greatly advanced by the application of surface plasmon resonance (SPR) biosensors. In studies of ligand binding by integrin I domains, technologies such as the BIAcore instruments have provided important insights into the role of conformational regulation. This chapter describes a protocol for studying the binding between the I domain from integrin α(X)β(2) and its ligand iC3b. Also included are topics on the interpretation of data. Integrin I domains appear to support heterogeneous interactions with ligands, which pose significant challenges in deriving valid information on the binding kinetics from the SPR measurements. Fortunately, new algorithms are available that may resolve even complex ligand-binding reactions; with the application to data on the binding between the α(X) I domain, a more consistent and unambiguous result is obtained compared to those obtained by classical approaches for analyzing SPR biosensor data.

Integrins

Integrins are cell adhesion receptors that are evolutionary old and that play important roles during developmental and pathological processes. The integrin family is composed of 24 αβ heterodimeric members that mediate the attachment of cells to the extracellular matrix (ECM) but that also take part in specialized cell-cell interactions. Only a subset of integrins (8 out of 24) recognizes the RGD sequence in the native ligands. In some ECM molecules, such as collagen and certain laminin isoforms, the RGD sequences are exposed upon denaturation or proteolytic cleavage, allowing cells to bind these ligands by using RGD-binding receptors. Proteolytic cleavage of ECM proteins might also generate fragments with novel biological activity such as endostatin, tumstatin, and endorepellin. Nine integrin chains contain an αI domain, including the collagen-binding integrins α1β1, α2β1, α10β1, and α11β1. The collagen-binding integrins recognize the triple-helical GFOGER sequence in the major collagens, but their ability to recognize these sequences in vivo is dependent on the fibrillar status and accessibility of the interactive domains in the fibrillar collagens. The current review summarizes some basic facts about the integrin family including a historical perspective, their structure, and their ligand-binding properties.

Comparison of degranulation of easily mobilizable intracellular granules by human phagocytes in healthy subjects and patients with infectious diseases

The aim of this study was to compare degranulation of easily mobilizable secretory vesicles (SVs) or secretory vesicle-like granules (SVLGs) in neutrophils, monocytes, and eosinophils of healthy controls (n = 60) and febrile patients with microbiologically confirmed or clinically diagnosed bacterial (n = 89) and viral (n = 46) infections. For this purpose, flow cytometric immunophenotyping of isolated phagocytes was performed using monoclonal antibodies against the phagocytosis receptors CR1 (CD35) and CR3 (CD11b) that are predominantly stored in the SVs of resting neutrophils. Similar to neutrophils, monocytes contain easily mobilizable SVLGs that constitute the main intracellular reservoir of CD35 and CD11b. In both neutrophils and monocytes, activation mechanisms leading to degranulation of SV and SVLG appeared dependent on both intra- and extracellular calcium levels. The kinetics of degranulation of SVLGs in control monocytes was significantly faster than that of SVs of control neutrophils. We conclude that phagocytes in patients with bacterial infections can be arranged in order of decreasing magnitude of SV or SVLG degranulation as follows (from left to right): neutrophils > monocytes " eosinophils. However, in viral infections, the corresponding degranulation order is monocytes > neutrophils approximately eosinophils.

An interrupted beta-propeller and protein disorder: structural bioinformatics insights into the N-terminus of alsin

Defects in the human ALS2 gene, which encodes the 1,657-amino-acid residue protein alsin, are linked to several related motor neuron diseases. We created a structural model for the N-terminal 690-residue region of alsin through comparative modelling based on regulator of chromosome condensation 1 (RCC1). We propose that this alsin region contains seven RCC1-like repeats in a seven-bladed beta-propeller structure. The propeller is formed by a double clasp arrangement containing two segments (residues 1-218 and residues 525-690). The 306-residue insert region, predicted to lie within blade 5 and to be largely disordered, is poorly conserved across species. Surface patches of evolutionary conservation probably indicate locations of binding sites. Both disease-causing missense mutations-Cys157Tyr and Gly540Glu-are buried in the propeller and likely to be structurally disruptive. This study aids design of experimental studies by highlighting the importance of construct length, will enhance interpretation of protein-protein interactions, and enable rational site-directed mutagenesis.

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.

Chondrocyte secreted CRTAC1: a glycosylated extracellular matrix molecule of human articular cartilage

Cartilage acidic protein 1 (CRTAC1), a novel human marker which allowed discrimination of human chondrocytes from osteoblasts and mesenchymal stem cells in culture was so far studied only on the RNA-level. We here describe its genomic organisation and detect a new brain expressed (CRTAC1-B) isoform resulting from alternate last exon usage which is highly conserved in vertebrates. In humans, we identify an exon sharing process with the neighbouring tail-to-tail orientated gene leading to CRTAC1-A. This isoform is produced by cultured human chondrocytes, localized in the extracellular matrix of articular cartilage and its secretion can be stimulated by BMP4. Of five putative O-glycosylation motifs in the last exon of CRTAC1-A, the most C-terminal one is modified according to exposure of serial C-terminal deletion mutants to the O-glycosylation inhibitor Benzyl-alpha-GalNAc. Both isoforms contain four FG-GAP repeat domains and an RGD integrin binding motif, suggesting cell-cell or cell-matrix interaction potential. In summary, CRTAC1 acquired an alternate last exon from the tail-to-tail oriented neighbouring gene in humans resulting in the glycosylated isoform CRTAC1-A which represents a new extracellular matrix molecule of articular cartilage.

Molecular cloning and characterization of cDNA encoding CD11b of cattle

CD18, the common beta subunit of beta2-integrins, associates with four distinct alpha chains to give rise to four different beta2-integrins: CD11a/CD18 (LFA-1), CD11b/CD18 (Mac-1), CD11c/CD18 (CR4), and CD11d/CD18. Previously, we and others showed that CD18 of LFA-1 serves as a receptor for Mannheimia haemolytica leukotoxin (Lkt). Level of expression of Mac-1 is higher than that of LFA-1 and other beta2-integrins on polymorphonuclear leukocytes (PMNs), which constitute the leukocyte subset most susceptible to Lkt. Hence, it is likely that CD18 of Mac-1 also mediates Lkt-induced cytolysis. Co-expression of CD11b and CD18 of cattle on Lkt-resistant cells is necessary to irrefutably demonstrate the role of Mac-1 in Lkt-induced cytolysis. This approach is hindered by lack of availability of complete sequence of cattle CD11b. Therefore, in this study, we cloned and sequenced the full length cDNA encoding cattle CD11b. The 3459 bp cDNA of cattle CD11b encodes a polypeptide of 1152 amino acids. The deduced amino acid sequence of CD11b of cattle exhibits 75% identity to that of humans and chimpanzees, 74% identity to that of dogs, and 70% identity to that of mice and rats. Availability of cattle CD11b cDNA should facilitate the elucidation of Lkt-receptor interactions in cattle and other species.

Molecular mechanics and dynamics of leukocyte recruitment during inflammation

Discovery of new genes and proteins directly supporting leukocyte adhesion is waning, whereas there is heightened interest in the cell mechanics and receptor dynamics that lead from transient tethering via selectins to affinity shifts and adhesion strengthening through integrins. New optical tools enable real-time imaging of leukocyte rolling and arrest in parallel plate flow channels (PPFCs), and detection of single-molecule force spectroscopy provides an inner view of the intercellular adhesive contact region. Leukocyte recruitment during acute inflammation is triggered by ligation of G protein-coupled chemotactic receptors (GPCRs) and clustering of selectins. This, in turn, activates beta(2)-integrin (CD18), which facilitates cell capture and arrest in shear flow. This review provides a conceptual model for the molecular events supporting leukocyte recruitment.

Macrophage--Mycobacterium tuberculosis interactions: role of complement receptor 3

Tuberculosis is the leading infectious disease in the world. Mycobacterium tuberculosis, the causal agent of this disease, invades macrophages and can replicate inside them. Because invasion of macrophages is a critical step for establishing a mycobacterial infection, there is much interest in understanding the mechanisms for M. tuberculosis entry into macrophages. Complement receptor 3 (CR3) is a heterodimeric surface receptor with multiple binding sites, which can mediate complement-opsonized as well as nonopsonic entrance of M. tuberculosis into macrophages. Here, we describe and discuss the role of CR3 in macrophage[bond]M. tuberculosis interactions. The actual information suggests that CR3 mediates a substantial amount of M. tuberculosis binding to macrophages, but CR3 is not related to the mechanisms that allow mycobacteria to survive and replicate intracellularly. Understanding the mechanisms of macrophage[bond]M. tuberculosis interaction will help developing more effective methods to prevent and treat tuberculosis in the future.

Targeting ICAM-1/LFA-1 interaction for controlling autoimmune diseases: designing peptide and small molecule inhibitors

This review describes the role of modulation of intracellular adhesion molecule-1 (ICAM-1)/leukocyte function-associated antigen-1 (LFA-1) interaction in controlling autoimmune diseases or inducing immunotolerance. ICAM-1/LFA-1 interaction is essential for T-cell activation as well as for migration of T-cells to target tissues. This interaction also functions, along with Signal-1, as a co-stimulatory signal (Signal-2) for T-cell activation, which is delivered by the T-cell receptors (TCR)-major histocompatibility complex (MHC)-peptide complex. Therefore, blocking ICAM-1/LFA-1 interaction can suppress T-cell activation in autoimmune diseases and organ transplantation. Many types of inhibitors (i.e. antibodies, peptides, small molecules) have been developed to block ICAM-1/LFA-1 interactions, and some of these molecules have reached clinical trials. Peptides derived from ICAM-1 and LFA-1 sequences have been shown to inhibit T-cell adhesion and activation. In addition, these inhibitors have been useful in elucidating the mechanism of ICAM-1/LFA-1 interaction. Besides binding to LFA-1, the ICAM-1 peptide can be internalized by LFA-1 receptors into the cytoplasmic domain of T-cells. Therefore, this ICAM-1 peptide can be utilized to selectively target toxic drugs to T-cells, thus avoiding harmful side effects. Finally, bi-functional inhibitory peptide (BPI), which is made by conjugating the antigenic peptide and an LFA-1 peptide, can alter the T-cell commitment from T-helper-1 (Th1) to T-helper-2 (Th2)-like cells, suggesting that this peptide may have a role in blocking the formation of the "immunological synapse."

Cell surface targeting of pregnancy-associated plasma protein A proteolytic activity. Reversible adhesion is mediated by two neighboring short consensus repeats

The activities of insulin-like growth factor (IGF)-I and -II are regulated by IGF-binding proteins (IGFBPs). Cleavage of IGFBP-4 by the metalloproteinase pregnancy-associated plasma protein-A (PAPP-A) causes release of bound IGF and has been established in several biological systems including the human reproductive system. Using flow cytometry, we first demonstrate that PAPP-A reversibly binds to the cell surface of several cell types analyzed. Heparin and heparan sulfate, but not dermatan or chondroitin sulfate, effectively compete for PAPP-A surface binding, and because incubation of cells with heparinase abrogated PAPP-A adhesion, binding is probably mediated by a cell surface heparan sulfate proteoglycan. Furthermore, the proteolytic activity of PAPP-A is preserved while bound to cells, suggesting that adhesion functions to target its activity to the vicinity of the IGF receptor, decreasing the probability that released IGF is captured by another IGFBP molecule before receptor binding. This mechanism potentially functions in both autocrine and paracrine regulation, as PAPP-A need not be synthesized in a cell to which it adheres. A truncated PAPP-A variant without the five short consensus repeats in the C-terminal third of the 1547-residue PAPP-A subunit, lacked surface binding. We also show that PAPP-A2, a recently discovered IGFBP-5 proteinase with homology to PAPP-A, does not bind cells. This finding allowed further mapping of the PAPP-A adhesion site to short consensus repeat modules 3 and 4 by the expression and analysis of nine PAPP-A/PAPP-A2 chimeras. Interestingly, the proteolytically inactive, disulfide-bound complex of PAPP-A and the proform of eosinophil major basic protein (proMBP), PAPP-A.proMBP, shows only weak surface binding, probably because the adhesion site of PAPP-A is occupied by heparan sulfate, known to be covalently bound to proMBP. This hypothesis was further substantiated by demonstrating that heparinase treatment of PAPP-A.proMBP restores surface binding. We finally propose a model in which IGF bioactivity is regulated by reversible cell surface binding of PAPP-A, which in turn is regulated by proMBP.

Role of the lectin domain of Mac-1/CR3 (CD11b/CD18) in regulating intercellular adhesion

Leukocyte diapedesis requires that Mac-1/CR3-dependent adhesion be regulated so that cells can move from one attachment site to another. The high affinity adhesion state of Mac-1/CR3 is generated when it forms a lectin-dependent complex with the receptor for urokinase plasminogen activator (uPAR; CD87). The extensively glycosylated uPAR binds to the same C-terminal lectin domain of CD11b that had previously been shown to prime Mac-1/CR3 for cytotoxic degranulation in response to beta-glucan. uPAR and beta-glucan compete for a lectin site that is near to the CBRM1/23 epitope (residues 943-1047) at the C-terminus of CD11b, and thus the lectin domain is critical to both the adhesion and cytotoxic functions of Mac-1/CR3. Adhesion is reversed when the uPA enzyme is captured by its receptor (uPAR), causing uPAR to bind to CD11b at a second site (residues 424-440) that is in between the N-terminal I-domain and the divalent cation binding region.

Integrin activation and structural rearrangement

Among adhesion receptor families, integrins are particularly important in biological processes that require rapid modulation of adhesion and de-adhesion. Activation on a timescale of < 1 s of beta2 integrins on leukocytes and beta3 integrins on platelets enables deposition of these cells at sites of inflammation or vessel wall injury. Recent crystal, nuclear magnetic resonance (NMR), and electron microscope (EM) structures of integrins and their domains lead to a unifying mechanism of activation for both integrins that contain and those that lack an inserted (I) domain. The I domain adopts two alternative conformations, termed open and closed. In striking similarity to signaling G-proteins, rearrangement of a Mg2+-binding site is linked to large conformational movements in distant backbone regions. Mutations that stabilize a particular conformation show that the open conformation has high affinity for ligand, whereas the closed conformation has low affinity. Movement of the C-terminal alpha-helix 10 A down the side of the domain in the open conformation is sufficient to increase affinity at the distal ligand-binding site 9,000-fold. This C-terminal "bell-rope" provides a mechanism for linkage to conformational movements in other domains. Recent structures and functional studies reveal interactions between beta-propeller, I, and I-like domains in the integrin headpiece, and a critical role for integrin epidermal growth factor (EGF) domains in the stalk region. The headpiece of the integrin faces down towards the membrane in the inactive conformation, and extends upward in a "switchblade"-like opening upon activation. These long-range structural rearrangements of the entire integrin molecule involving interdomain contacts appear closely linked to conformational changes within the I and I-like domains, which result in increased affinity and competence for ligand binding.

Conformational regulation of integrin structure and function

Integrins are a structurally elaborate family of heterodimers that mediate divalent cation-dependent cell adhesion in a wide range of biological contexts. The inserted (I) domain binds ligand in the subset of integrins in which it is present. Its structure has been determined in two alternative conformations, termed open and closed. In striking similarity to signaling G proteins, rearrangement of a Mg(2+)-binding site is linked to large conformational movements in distant backbone regions. Mutations have been used to stabilize either the closed or open structures. These show that the snapshots of the open conformation seen only in the presence of a ligand or a ligand mimetic represent a high-affinity, ligand-binding conformation, whereas those of the closed conformation correspond to a low-affinity conformation. The C-terminal alpha-helix moves 10 A down the side of the domain in the open conformation. Locking in the conformation of the preceding loop is sufficient to increase affinity for ligand 9000-fold. This C-terminal "bell-rope" provides a mechanism for linkage to conformational movements in other domains. The transition from the closed to open conformation has been implicated in fast (<1 s) regulation of integrin affinity in response to activation signals from inside the cell. Recent integrin structures and functional studies reveal interactions between beta-propeller, I, and I-like domains in the headpiece, and a critical role for integrin EGF domains in the stalk region. These studies suggest that the headpiece of the integrin faces down toward the membrane in the inactive conformation and extends upward in a "switchblade"-like opening motion upon activation. These long-range structural rearrangements of the entire integrin molecule involving multiple interdomain contacts appear closely linked to conformational changes in the I domain, which result in increased affinity and competence for ligand binding.

Differential regulation of a novel variant of the alpha(6) integrin, alpha(6p)

We have reported previously the existence of an M(r) 70,000 form of the alpha(6) integrin called alpha(6p) in a variety of human epithelial cell lines. Four different experimental conditions were used to examine the regulation of alpha(6) and alpha(6p) integrin. The production of the alpha(6) integrin was decreased by 45% using a protein translation inhibitor (2.25 microM puromycin), whereas production of the alpha(6p) variant was unaffected. The alpha(6p) variant was decreased 60% by actin depolymerization (10 microM cytochalasin D) corresponding to a decrease in its surface expression, whereas alpha(6) integrin production was unaffected. The alpha(6p) variant was resistant to endoglycosidase H treatment, whereas the alpha(6) integrin was both sensitive and resistant to endoglycosidase H treatment, indicating retention in the endoplasmic reticulum and processing through the Golgi apparatus. Additionally, digestion by endoglycosidase F demonstrated both alpha(6p) and alpha(6) integrin contained NH(2)-linked glycosylations and both shifted M(r) approximately 10,000 on enzymatic digestion. Finally, inhibition of serine/threonine phosphatases by either calyculin A (15 nM) or okadaic acid (62 microM) did not affect alpha(6p), whereas the production of alpha(6) integrin was decreased by 50%. These data suggest that the production of the alpha(6p) variant is distinct from alpha(6) integrin and may involve a post-translational processing event at the cell surface.

Alternative splice variants of alpha 7 beta 1 integrin selectively recognize different laminin isoforms

The integrin alpha(7)beta(1) occurs in several cytoplasmic (alpha(7A), alpha(7B)) and extracellular splice variants (alpha(7X1), alpha(7X2)), which are differentially expressed during development of skeletal and heart muscle. The extracellular variants result from the alternative splicing of exons X1 and X2, corresponding to a segment within the putative ligand binding domain. To study the specificity and affinity of the X1/X2 variants to different laminin isoforms, soluble alpha(7)beta(1) complexes were prepared by recombinant coexpression of the extracellular domains of the alpha- and beta-subunits. The binding of these complexes to purified ligands was measured by solid phase binding assays. Surprisingly, the alternative splice variants revealed different and specific affinities to different laminin isoforms. While the alpha(7X2) variant bound much more strongly to laminin-1 than the alpha(7X1) variant, the latter showed a high affinity binding to laminins-8 and -10/11. Laminin-2, the major laminin isoform in skeletal muscle, was recognized by both variants, whereas none of the two variants were able to interact with laminin-5. A specific blocking antibody inhibited the binding of both variants to all laminins tested, indicating the involvement of common epitopes in alpha(7X1)beta(1) and alpha(7X2)beta(1). Because laminin-8 and -10/11 as well as alpha(7X1) are expressed in developing skeletal and cardiac muscle, these findings suggest that alpha(7X1)beta(1) may represent a physiological receptor with novel specificities for laminin-8 and -10.

Insights into integrin-ligand binding and activation from the first crystal structure

Integrin receptors transduce bidirectional signals between extracellular adhesion molecules and intracellular cytoskeletal and signalling molecules. The structural basis of integrin signalling is unknown, but the recent publication of the first crystal structure of the extracellular domain of integrin alphaVbeta3 has provided a number of insights. In this review, previous structure-function analyses of integrins that have employed biochemical and molecular biological approaches are placed in the context of the crystal structure, and novel routes to the development of integrin antagonists are discussed.

Amino acid residues in the alpha IIb subunit that are critical for ligand binding to integrin alpha IIbbeta 3 are clustered in the beta-propeller model

Several distinct regions of the integrin alpha(IIb) subunit have been implicated in ligand binding. To localize the ligand binding sites in alpha(IIb), we swapped all 27 predicted loops with the corresponding sequences of alpha(4) or alpha(5). 19 of the 27 swapping mutations had no effect on binding to both fibrinogen and ligand-mimetic antibodies (e.g. LJ-CP3), suggesting that these regions do not contain major ligand binding sites. In contrast, swapping the remaining 8 predicted loops completely blocked ligand binding. Ala scanning mutagenesis of these critical predicted loops identified more than 30 discontinuous residues in repeats 2-4 and at the boundary between repeats 4 and 5 as critical for ligand binding. Interestingly, these residues are clustered in the predicted beta-propeller model, consistent with this model. Most of the critical residues are located at the edge of the upper face of the propeller, and several critical residues are located on the side of the propeller domain. None of the predicted loops in repeats 1, 6, and 7, and none of the four putative Ca(2+)-binding predicted loops on the lower surface of the beta-propeller were important for ligand binding. The results map an important ligand binding interface at the edge of the top and on the side of the beta-propeller toroid, centering on repeat 3.

Generation of a minimal alpha5beta1 integrin-Fc fragment

The tertiary structure of the integrin heterodimer is currently unknown, although several predictive models have been generated. Detailed structural studies of integrins have been consistently hampered for several reasons, including the small amounts of purified protein available, the large size and conformational flexibility of integrins, and the presence of transmembrane domains and N-linked glycosylation sites in both receptor subunits. As a first step toward obtaining crystals of an integrin receptor, we have expressed a minimized dimer. By using the Fc dimerization and mammalian cell expression system designed and optimized by Stephens et al. (Stephens, P. E., Ortlepp, S., Perkins, V. C., Robinson, M. K., and Kirby, H. (2000) Cell. Adhes. Commun. 7, 377-390), a series of recombinant soluble human alpha(5)beta(1) integrin truncations have been expressed as Fc fusion proteins. These proteins were examined for their ligand-binding properties and for their expression of anti-integrin antibody epitopes. The shortest functional alpha(5)-subunit truncation contained the N-terminal 613 residues, whereas the shortest beta(1)-subunit was a fragment containing residues 121-455. Each of these minimally truncated integrins displayed the antibody binding characteristics of alpha(5)beta(1) purified from human placenta and bound ligand with the same apparent affinity as the native receptor.

Critical amino acid residues of the alpha4 subunit for alpha4beta7 integrin function

A characteristic feature of integrin-ligand interactions is the requirement for divalent cations. Putative cation binding sites have been identified in the alpha and beta subunit of the alpha4 integrins, alpha4beta1 and alpha4beta7, and within their ligands which display the tripeptide LDV in fibronectin and homologous motifs in VCAM-1 and MAdCAM-1. The extracellular domain of the murine and human alpha4-subunit contains three conserved LDV motifs, designated LDV-1 to -3. Using site directed mutagenesis and transfection studies, we now examined the functional relevance of the LDV motifs for alpha4beta7 integrins. We present evidence that LDV-1 mutants (D489N) behave like alpha4 wt cells, but LDV-3 mutants (D811N) are impaired in alpha4beta7 integrin-triggered homotypic cell aggregation and in adhesion and spreading on alpha4 specific ligands. Further characterization of LDV-3 mutants revealed a defect in mAb-induced alpha4beta7-cell surface cluster formation. Mutation of the LDV-2 motif (D698N) caused loss of alpha4beta7 integrin cell surface expression. Our results indicate: (i) that LDV-3, located proximal to the cell membrane, is important for alpha4beta7 integrin-triggered functions and for lateral clustering and (ii) that LDV-2 affects alpha4beta7 heterodimer stability.

Identification of a novel structural variant of the alpha 6 integrin

The alpha(6) integrin is a 140-kDa (nonreduced) laminin receptor. We have identified a novel 70-kDa (nonreduced) form of the alpha(6) integrin called alpha(6)p for the latin word parvus, meaning small. The variant was immunoprecipitated from human cells using four different alpha(6)-specific monoclonal antibodies but not with alpha(3) or alpha(5) antibodies. The alpha(6)p integrin contained identical amino acid sequences within exons 13--25, corresponding to the extracellular "stalk region" and the cytoplasmic tail of the alpha(6) integrin. The light chains of alpha(6) and alpha(6)p were identical as judged by alpha(6)A-specific antibodies and electrophoretic properties. The alpha(6)p variant paired with either beta(1) or beta(4) subunits and was retained on the cell surface three times longer than alpha(6). Reverse transcription/polymerase chain reaction analysis revealed a single polymerase chain reaction product. The alpha(6)p variant was found in human prostate (DU145H, LnCaP, PC3) and colon (SW480) cancer cell lines but not in normal prostate (PrEC), breast cancer (MCF-7), or lung cancer (H69) cell lines or a variant of a prostate carcinoma cell line (PC3-N). Protein levels of alpha(6)p increased 3-fold during calcium-induced terminal differentiation in a normal mouse keratinocyte model system. A novel form of the alpha(6) integrin exists on cell surfaces that contains a dramatically altered extracellular domain.

Relating structure to function in the beta-propeller domain of dipeptidyl peptidase IV. Point mutations that influence adenosine deaminase binding, antibody binding and enzyme activity

Point mutations in human CD26/DP IV were analysed for adenosine deaminase (ADA) binding, monoclonal antibody (mAb) binding and DP IV enzyme activity. Point mutations at either Leu294 or Val341 ablated ADA binding. Binding by mAbs that inhibit ADA binding was found to involve both Leu340 to Arg343 and Thr440/Lys441. Glu205 and Glu206 were found to be essential for enzyme activity. All residues of interest were mapped onto a model of the beta-propeller domain of DP IV. These data led us to suggest that in DP IV and related peptidases ligand and antibody binding sites are non-linear and that enzyme activity depends on charged sidechains that surround the entrance to the central tunnel of the beta-propeller.

Genomic organization, amplification, fine mapping, and intragenic polymorphisms of the human hemidesmosomal tetraspanin CD151 gene

CD151 is a member of the tetraspanin family that is involved in cellular processes including cell adhesion. The protein is expressed in a variety of tissues including vascular endothelium and epidermis, and has been shown to be a component of hemidesmosomes. Mutations in genes encoding other hemidesmosomal proteins give rise to a range of human disorders, characterized by fragility of the skin and/or mucous membranes. It is, therefore, plausible that inherited or acquired mutations in the gene encoding CD151 may be fundamental to the integrity and maturation of basal cell keratinocytes. To aid mutation analysis, we have characterized the intron-exon organization of the CD151 gene which comprises 8 exons spanning approximately 4.3 kb, and have developed a comprehensive PCR-based mutation detection strategy. In addition, to aid linkage analysis of CD151 in genetic disease we have fine-mapped the gene by radiation-hybrid methodology to 11p15.5, and detected a number of intragenic polymorphisms.

Cloning and characterization of integrin alpha subunits from the solitary ascidian, Halocynthia roretzi

Recent molecular and biochemical analysis has revealed the presence of an opsonic complement system in the solitary ascidian, Halocynthia roretzi, composed of at least C3, two mannan binding protein-associated serine proteases, and factor B. To elucidate further the structure and function of this apparently primitive complement system in the urochordates, we looked for the ascidian complement receptor type 3 (CR3), or type 4 (CR4), which are members of the leukocyte integrin family in mammals. Using degenerate primers, we isolated two integrin alpha subunits (alpha(Hr1) and alpha(Hr2)) from the hemocyte mRNA of H. roretzi, by RT-PCR, and the entire coding sequence of alpha(Hr1) was determined from cDNA clones. alpha(Hr1) contains an I domain, the inserted domain characteristic of a subset of mammalian alpha subunits, including the leukocyte integrin family. A phylogenetic tree constructed for the alpha subunits also supports the ancestral position of alpha(Hr1) in the monophyletic cluster of I domain-containing alpha integrins. The alpha(Hr1) gene shows hemocyte-specific expression on Northern blot analysis. Western blot analysis and immunocytochemical staining of the hemocytes of H. roretzi using anti-alpha(Hr1) Ab showed that alpha(Hr1) subunits exist on the surface of a subpopulation of phagocytic hemocytes. Furthermore, anti-alpha(Hr1) Ab inhibited C3-dependent phagocytosis, but not basic phagocytosis, of yeast cells by ascidian hemocytes. These observations strongly suggest that alpha(Hr1) constitutes an integrin molecule on the hemocytes of H. roretzi that functions as an ancestral form of CR3 and CR4 and mediates phagocytosis in the primitive complement system of the ascidian.

The regulation of integrin function by Ca(2+)

Integrins are metalloproteins whose receptor function is dependent on the interplay between Mg(2+) and Ca(2+). Although the specificity of the putative divalent cation binding sites has been poorly understood, some issues are becoming clearer and this review will focus on the more recent information. The MIDAS motif is a unique Mg(2+)/Mn(2+) binding site located in the integrin alpha subunit I domain. Divalent cation bound at this site has a structural role in coordinating the binding of ligand to the I domain containing integrins. The I-like domain of the integrin beta subunit also has a MIDAS-like motif but much less is known about its cation binding preferences. The N-terminal region of the integrin alpha subunit has been modelled as a beta-propeller, containing three or four 'EF hand' type divalent cation binding motifs for which the function is ill defined. It seems certain that most integrins have a high affinity Ca(2+) site which is critical for alphabeta heterodimer formation, but the location of this site is unknown. Finally intracellular Ca(2+) fluxes activate the Ca(2+) requiring enzyme, calpain, which regulates cluster formation of leucocyte integrins.

Micromolar Ca2+ concentrations are essential for Mg2+-dependent binding of collagen by the integrin alpha 2beta 1 in human platelets

Integrin receptor alpha(2)beta(1) requires micromolar Ca(2+) to bind to collagen and to the peptide GPC(GPP)(5)GFOGER(GPP)(5)GPC (denoted GFOGER-GPP, where O represents hydroxyproline), which contains the minimum recognition sequence for the collagen-binding alpha(2) I-domain (Knight, C. G., Morton, L. F., Peachey, A. R., Tuckwell, D. S., Farndale, R. W., and Barnes, M. J. (2000) J. Biol. Chem. 275, 35-40). Platelet adhesion to these ligands is completely dependent on alpha(2)beta(1) in the presence of 2 mm Mg(2+). However, we show here that this interaction was abolished in the presence of 25 microm EGTA. Adhesion of Glanzmann's thrombasthenic platelets, which lack the fibrinogen receptor alpha(IIb)beta(3), was also inhibited by micromolar EGTA. Mg(2+)-dependent adhesion of platelets was restored by the addition of 10 microm Ca(2+), but millimolar Ca(2+) was inhibitory. Binding of isolated alpha(2)beta(1) to GFOGER-GPP was 70% inhibited by 50 microm EGTA but, as with intact platelets, was fully restored by the addition of micromolar Ca(2+). 2 mm Ca(2+) did not inhibit binding of isolated alpha(2)beta(1) to collagen or to GFOGER-GPP. Binding of recombinant alpha(2) I-domain was not inhibited by EGTA, nor did millimolar Ca(2+) inhibit binding. Our data suggest that high affinity Ca(2+) binding to alpha(2)beta(1), outside the I-domain, is essential for adhesion to collagen. This is the first demonstration of a Ca(2+) requirement in alpha(2)beta(1) function.

Folding and function of I domain-deleted Mac-1 and lymphocyte function-associated antigen-1

In those integrins that contain it, the I domain is a major ligand recognition site. The I domain is inserted between beta-sheets 2 and 3 of the predicted beta-propeller domain of the integrin alpha subunit. We deleted the I domain from the integrin alpha(M) and alpha(L) subunits to give I-less Mac-1 and lymphocyte function-associated antigen-1 (LFA-1), respectively. The I-less alpha(M) and alpha(L) subunits were expressed in association with the wild-type beta(2) subunit on the surface of transfected cells and bound to all the monoclonal antibodies mapped to the putative beta-propeller and C-terminal regions of the alpha(M) and alpha(L) subunits, suggesting that the folding of these domains is independent of the I domain. I-less Mac-1 bound to the ligands iC3b and factor X, but this binding was reduced compared with wild-type Mac-1. In contrast, I-less Mac-1 did not bind to fibrinogen or denatured bovine serum albumin. Binding to iC3b and factor X by I-less Mac-1 was inhibited by the function-blocking antibody CBRM1/32, which binds to the beta-propeller domain of the alpha(M) subunit. I-less LFA-1 did not bind its ligands intercellular adhesion molecule-1 and -3. Thus, the I domain is not essential for the folding, heterodimer formation, and surface expression of Mac-1 and LFA-1 and is required for binding to some ligands, but not others.

The top of the inserted-like domain of the integrin lymphocyte function-associated antigen-1 beta subunit contacts the alpha subunit beta -propeller domain near beta-sheet 3

We find that monoclonal antibody YTA-1 recognizes an epitope formed by a combination of the integrin alpha(L) and beta(2) subunits of LFA-1. Using human/mouse chimeras of the alpha(L) and beta(2) subunits, we determined that YTA-1 binds to the predicted inserted (I)-like domain of the beta(2) subunit and the predicted beta-propeller domain of the alpha(L) subunit. Substitution into mouse LFA-1 of human residues Ser(302) and Arg(303) of the beta(2) subunit and Pro(78), Thr(79), Asp(80), Ile(365), and Asn(367) of the alpha(L) subunit is sufficient to completely reconstitute YTA-1 reactivity. Antibodies that bind to epitopes that are nearby in models of the I-like and beta-propeller domains compete with YTA-1 monoclonal antibody for binding. The predicted beta-propeller domain of integrin alpha subunits contains seven beta-sheets arranged like blades of a propeller around a pseudosymmetry axis. The antigenic residues cluster on the bottom of this domain in the 1-2 loop of blade 2, and on the side of the domain in beta-strand 4 of blade 3. The I domain is inserted between these blades on the top of the beta-propeller domain. The antigenic residues in the beta subunit localize to the top of the I-like domain near the putative Mg(2+) ion binding site. Thus, the I-like domain contacts the bottom or side of the beta-propeller domain near beta-sheets 2 and 3. YTA-1 preferentially reacts with activated LFA-1 and is a function-blocking antibody, suggesting that conformational movements occur near the interface it defines between the LFA-1 alpha and beta subunits.

Molecular basis of ligand recognition by integrin alpha 5beta 1. I. Specificity of ligand binding is determined by amino acid sequences in the second and third NH2-terminal repeats of the alpha subunit

The NH(2)-terminal portion (putative ligand-binding domain) of alpha subunits contains 7 homologous repeats, the last 3 or 4 of which possess divalent cation binding sequences. These repeats are predicted to form a seven-bladed beta-propeller structure. To map ligand recognition sites on the alpha(5) subunit we have taken the approach of constructing and expressing alpha(V)/alpha(5) chimeras. Although the NH(2)-terminal repeats of alpha(5) and alpha(V) are >50% identical at the amino acid level, alpha(5)beta(1) and alpha(V)beta(1) show marked differences in their ligand binding specificities. Thus: (i) although both integrins recognize the Arg-Gly-Asp (RGD) sequence in fibronectin, the interaction of alpha(5)beta(1) but not of alpha(V)beta(1) with fibronectin is strongly dependent on the "synergy" sequence Pro-His-Ser-Arg-Asn; (ii) alpha(5)beta(1) binds preferentially to RGD peptides in which RGD is followed by Gly-Trp (GW) whereas alpha(V)beta(1) has a broader specificity; (iii) only alpha(5)beta(1) recognizes peptides containing the sequence Arg-Arg-Glu-Thr-Ala-Trp-Ala (RRETAWA). Therefore, amino acid residues involved in ligand recognition by alpha(5)beta(1) can potentially be identified in gain-of-function experiments by their ability to switch the ligand binding properties of alpha(V)beta(1) to those of alpha(5)beta(1). By introducing appropriate restriction enzyme sites, or using site-directed mutagenesis, parts of the NH(2)-terminal repeats of alpha(V) were replaced with the corresponding regions of the alpha(5) subunit. Chimeric subunits were expressed on the surface of Chinese hamster ovary-B2 cells (which lack endogenous alpha(5)) as heterodimers with hamster beta(1). Stable cell lines were generated and tested for their ability to attach to alpha(5)beta(1)-selective ligands. Our results demonstrate that: (a) the first three NH(2)-terminal repeats contain the amino acid sequences that determine ligand binding specificity and the same repeats include the epitopes of function blocking anti-alpha subunit mAbs; (b) the divalent cation-binding sites (in repeats 4-7) do not confer alpha(5)beta(1)- or alpha(V)beta(1)-specific ligand recognition; (c) amino acid residues Ala(107)-Tyr(226) of alpha(5) (corresponding approximately to repeats 2 and 3) are sufficient to change all the ligand binding properties of alpha(V)beta(1) to those of alpha(5)beta(1); (d) swapping a small part of a predicted loop region of alpha(V) with the corresponding region of alpha(5) (Asp(154)-Ala(159)) is sufficient to confer selectivity for RGDGW and the ability to recognize RRETAWA.

A naturally occurring mutation near the amino terminus of αIIb defines a new region involved in ligand binding to αIIbβ3

Decreased expression of functional IIbβ3 complexes on the platelet surface produces Glanzmann thrombasthenia. We have identified mutations of IIbP145 in 3 ethnically distinct families affected by Glanzmann thrombasthenia. Affected Mennonite and Dutch patients were homozygous and doubly heterozygous, respectively, for a P145A substitution, whereas a Chinese patient was doubly heterozygous for a P145L substitution. The mutations affect expression levels of surface IIbβ3 receptors on their platelets, which was confirmed by co-transfection of IIbP145A and β3 cDNA constructs in COS-1 cells. Each mutation also impaired the ability of IIbβ3 on affected platelets to interact with ligands. Moreover, when IIbP145A and β3 were stably coexpressed in Chinese hamster ovary cells, IIbβ3 was readily detected on the cell surface, but the cells were unable to adhere to immobilized fibrinogen or to bind soluble fluorescein isothiocyanate–fibrinogen after IIbβ3 activation by the activating monoclonal antibody PT25-2. Nonetheless, incubating affected platelets with the peptide LSARLAF, which binds to IIb, induced PF4 secretion, indicating that the mutant IIbβ3 retained the ability to mediate outside-in signaling. These studies indicate that mutations involving IIbP145 impair surface expression of IIbβ3 and that the IIbP145A mutation abrogates ligand binding to the activated integrin. A comparative analysis of other IIb mutations with a similar phenotype suggests that these mutations may cluster into a single region on the surface of the IIb and may define a domain influencing ligand binding. (Blood. 2000;95:180188)