Homology modelling of integrin EF-hands. Evidence for widespread use of a conserved cation-binding site

Molecular Modeling Insights into the Structure and Behavior of Integrins: A Review

Integrins are heterodimeric glycoproteins crucial to the physiology and pathology of many biological functions. As adhesion molecules, they mediate immune cell trafficking, migration, and immunological synapse formation during inflammation and cancer. The recognition of the vital roles of integrins in various diseases revealed their therapeutic potential. Despite the great effort in the last thirty years, up to now, only seven integrin-based drugs have entered the market. Recent progress in deciphering integrin functions, signaling, and interactions with ligands, along with advancement in rational drug design strategies, provide an opportunity to exploit their therapeutic potential and discover novel agents. This review will discuss the molecular modeling methods used in determining integrins' dynamic properties and in providing information toward understanding their properties and function at the atomic level. Then, we will survey the relevant contributions and the current understanding of integrin structure, activation, the binding of essential ligands, and the role of molecular modeling methods in the rational design of antagonists. We will emphasize the role played by molecular modeling methods in progress in these areas and the designing of integrin antagonists.

A Rice Ca2+ Binding Protein Is Required for Tapetum Function and Pollen Formation

In flowering plants, successful male reproduction requires the sophisticated interaction between somatic anther wall layers and reproductive cells. Timely degradation of the innermost tissue of the anther wall layer, the tapetal layer, is critical for pollen development. Ca²⁺ is a well-known stimulus for plant development, but whether it plays a role in affecting male reproduction remains elusive. Here we report a role of Defective in Exine Formation 1 (OsDEX1) in rice (Oryza sativa), a Ca²⁺ binding protein, in regulating rice tapetal cell degradation and pollen formation. In osdex1 anthers, tapetal cell degeneration is delayed and degradation of the callose wall surrounding the microspores is compromised, leading to aborted pollen formation and complete male sterility. OsDEX1 is expressed in tapetal cells and microspores during early anther development. Recombinant OsDEX1 is able to bind Ca²⁺ and regulate Ca²⁺ homeostasis in vitro, and osdex1 exhibited disturbed Ca²⁺ homeostasis in tapetal cells. Phylogenetic analysis suggested that OsDEX1 may have a conserved function in binding Ca²⁺ in flowering plants, and genetic complementation of pollen wall defects of an Arabidopsis (Arabidopsis thaliana) dex1 mutant confirmed its evolutionary conservation in pollen development. Collectively, these findings suggest that OsDEX1 plays a fundamental role in the development of tapetal cells and pollen formation, possibly via modulating the Ca²⁺ homeostasis during pollen development.

Integrin-α FG-GAP repeat-containing protein 2 is critical for normal B cell differentiation and controls disease development in a lupus model

The phenylalanyl-glycyl-glycyl-alanyl-prolyl (FG-GAP) domain plays an important role in protein-protein interactions, including interaction of integrins with their ligands. Integrin-α FG-GAP repeat-containing protein 2 (Itfg2) is a highly conserved protein in vertebrates that carries two FG-GAP domains, but its role in mammalian physiology is unknown. In this article, we show that Itfg2 is an intracellular protein and it plays a critical role in B cell differentiation and development of autoimmunity. Itfg2-deficient mice displayed a phenotype consistent with retention of B cells in the spleen and had a lower concentration of IgG in the blood when compared with wild-type littermates. Itfg2-deficient splenocytes also showed a defect in cell migration in vitro. After immunization with a thymus-dependent Ag, the absence of Itfg2 caused a shift in B cell maturation from the germinal centers to the extrafollicular regions of the spleen and blocked deposition of Ag-specific plasma cells in the bone marrow. In support of hematopoietic cell intrinsic activity of Itfg2, bone marrow transplantation of Itfg2-deficient cells was sufficient to impair germinal center development in wild-type mice. Furthermore, Itfg2 deficiency exacerbated development of autoimmune disease in MRL/lpr lupus-prone mice. These results identify Itfg2 as a novel contributor to B cell differentiation and a negative regulator of the autoimmune response during lupus.

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.

Divalent cations regulate the folding and activation status of integrins during their intracellular trafficking

Integrins are divalent cation-dependent, αβ heterodimeric adhesion receptors that control many fundamental aspects of cell behaviour by bi-directional signalling between the extracellular matrix and intracellular cytoskeleton. The activation state of cell surface integrins is tightly regulated by divalent cation occupancy of the ligand-binding pocket and by interaction with cytoplasmic adaptor proteins, such as talin. These agents elicit gross conformational changes across the entire molecule, which specify the activation state. Much less is known about the activation state of newly synthesised integrins or the role of cations during the early folding and trafficking of integrins. Here we use a number of well-characterised, conformation-specific antibodies to demonstrate that β1-integrins adopt the bent, inactive conformation after assembly with α-integrins in the endoplasmic reticulum. Folding and assembly are totally dependent on the binding of Ca(2+) ions. In addition, Ca(2+) binding prevents integrin activation before its arrival at the cell surface. Activation at the cell surface occurs only following displacement of Ca(2+) with Mg(2+) or Mn(2+). These results demonstrate the essential roles played by divalent cations to facilitate folding of the β-integrin subunit, to prevent inappropriate intracellular integrin signalling, and to activate ligand binding and signalling at the cell surface.

CRTAC1 homolog proteins are conserved from cyanobacteria to man and secreted by the teleost fish pituitary gland

Cartilage acidic protein 1 (CRTAC1) gene expression is used as a marker for chondrocyte differentiation in stem cell-based tissue engineering. It is also transcribed outside the skeleton where at least two different transcripts are expressed in lung and brain. In the pituitary gland of the teleost fish sea bream Sparus auratus, we have found a transcript with a high degree of sequence identity to CRTAC1 family members but lacking the EGF-like calcium-binding domain encoding sequence of CRTAC1 and designated it as CRTAC2. Database searches revealed many previously unidentified members of the CRTAC1 and CRTAC2 in phylogenetically distant organisms, such as cyanobacteria, bryophyta, lancelets, and diverse representatives of vertebrates. Phylogenetic analyses showed that the genes encoding CRTAC1 and CRTAC2 proteins coexist in teleost fish genomes. Structural prediction analysis identified the N-terminal region of the CRTAC1/CRTAC2 family members as a potential seven-bladed beta-propeller structure, closely related to those of integrin alpha chains and glycosylphosphatidylinositol-specific phospholipase D1 protein families. This relationship is confirmed by phylogenetic analysis with the N-terminal domain of sea bream CRTAC2 as the most divergent sequence. Because teleost fishes are the only phylogenetic group where both CRTAC1 and CRTAC2 genes are present, they occupy a pivotal position in studies of the mechanisms governing the specific expression patterns of each gene/protein subfamily. This will be essential to elucidate their respective biological roles.

Unexpected diversity of cnidarian integrins: expression during coral gastrulation

BACKGROUND

Adhesion mediated through the integrin family of cell surface receptors is central to early development throughout the Metazoa, playing key roles in cell-extra cellular matrix adhesion and modulation of cadherin activity during the convergence and extension movements of gastrulation. It has been suggested that Caenorhabditis elegans, which has a single beta and two alpha integrins, might reflect the ancestral integrin complement. Investigation of the integrin repertoire of anthozoan cnidarians such as the coral Acropora millepora is required to test this hypothesis and may provide insights into the original roles of these molecules.

RESULTS

Two novel integrins were identified in Acropora. AmItgalpha1 shows features characteristic of alpha integrins lacking an I-domain, but phylogenetic analysis gives no clear indication of its likely binding specificity. AmItgbeta2 lacks consensus cysteine residues at positions 8 and 9, but is otherwise a typical beta integrin. In situ hybridization revealed that AmItgalpha1, AmItgbeta1, and AmItgbeta2 are expressed in the presumptive endoderm during gastrulation. A second anthozoan, the sea anemone Nematostella vectensis, has at least four beta integrins, two resembling AmItgbeta1 and two like AmItgbeta2, and at least three alpha integrins, based on its genomic sequence.

CONCLUSION

In two respects, the cnidarian data do not fit expectations. First, the cnidarian integrin repertoire is more complex than predicted: at least two betas in Acropora, and at least three alphas and four betas in Nematostella. Second, whereas the bilaterian alphas resolve into well-supported groups corresponding to those specific for RGD-containing or laminin-type ligands, the known cnidarian alphas are distinct from these. During early development in Acropora, the expression patterns of the three known integrins parallel those of amphibian and echinoderm integrins.

Calcium binding by the essential virulence factor BAD-1 of Blastomyces dermatitidis

BAD-1 (Blastomyces adhesin 1), a 120-kDa protein of Blastomyces dermatitidis, functions as an adhesin, immune modulator, and essential virulence factor. Structurally, BAD-1 is composed of a short N-terminal region, a core of 30 tandem repeats critical for virulence, and a C-terminal epidermal growth factor domain that binds the protein to yeast cell surface chitin. Each of the 30 acidic residue-rich tandem repeats contains a sequence that resembles the calcium-binding loop of the EF-hand domain found in many calcium-binding proteins. Here, we investigated the binding of calcium by BAD-1 and its biological significance. Yeast washed with double distilled H2O released surface-bound BAD-1, but EGTA washes were an order of magnitude more efficient, suggesting an interaction between BAD-1 and calcium. Immobilized BAD-1 was stained with ruthenium red dye, an indicator of calcium-binding proteins. In equilibrium dialysis, BAD-1 bound 45Ca2+ with an affinity of 0.41 x 10(-5) m and a capacity of 27 calcium/mol. Mass spectrometry confirmed this capacity. Elevated [Ca2+] diminished BAD-1 solubility. Upon deletion of its C-terminal epidermal growth factor-like domain, BAD-1 resisted aggregation by elevated [Ca2+] but retained its affinity and capacity for calcium. Removing 20 copies of the tandem repeat, however, sharply reduced the capacity of BAD-1 for calcium. Growth of the bad-1 null yeast was inhibited by 5 mm EGTA, and re-expression of BAD-1 in trans or the addition of exogenous purified BAD-1 restored growth. Thus, BAD-1 is a high capacity calcium-binding protein. This property contributes to the structure and function of BAD-1, as well as to B. dermatitidis acquisition of calcium from the environment.

Mutations in and near the second calcium-binding domain of integrin alphaIIb affect the structure and function of integrin alphaIIbbeta3

Calcium-binding domains in the alpha-subunit of integrins contain a central loop structure. To examine the importance of the loop structure, a series of alphaIIb mutants containing changes to the calcium-liganding amino acids have been constructed. Significantly, none of the mutant alphaIIbbeta3 complexes was detected on the surface of transfected cells, but mutant pro-alphaIIb was detected in cell lysates in complex with beta3. To study the importance of the regions flanking the second calcium-binding domain for ligand-binding and ligand-binding specificity, three alphaIIb/alpha5 chimaeras containing alpha5 sequences flanking or flanking and including the second calcium-binding domain were constructed. The chimaera containing both alpha5-flanking regions was not expressed on the cell surface, but FR1 and FR2, substituting either the first or second flanking region, were expressed. FR1beta3-transfected cells lost the ability to adhere to fibrinogen and to support aggregation and had minimal fibrinogen-binding ability. The heterodimer complex was less stable than the wild-type. FR2beta3-transfected cells adhered to fibrinogen and bound soluble fibrinogen with higher affinity when compared with wild-type. In addition, the heterodimer complex was more stable than wild-type. These results indicate that the conformation of the second calcium-binding domain is critical for maturation of the alphaIIbbeta3 complex and expression on the cell surface and that the surrounding sequences are critical for alphaIIbbeta3 function.

A C-terminal EGF-like domain governs BAD1 localization to the yeast surface and fungal adherence to phagocytes, but is dispensable in immune modulation and pathogenicity of Blastomyces dermatitidis

BAD1, an adhesin and immune modulator of Blastomyces dermatitidis, is an essential virulence factor that is released extracellularly before association with the yeast surface. Here, deletion of the C-terminal EGF-like domain profoundly affected BAD1 function, leading to non-association with yeast, extracellular accumulation and impaired yeast adherence to macrophages. In equilibrium binding assays, DeltaC-term BAD1, lacking an EGF-like domain, bound poorly to BAD1 null yeast, yielding a low affinity (Kd, 3 x 10(-7) M versus 5 x 10(-8) M) and Bmax (1.9 x 10(5) versus 7.9 x 10(5)) compared with BAD1. Similar protein binding profiles were observed using chitin particles, reinforcing the notion that chitin fibrils are a receptor for BAD1, and that the EGF-like domain is critical for BAD1 interactions with chitin on yeast. DeltaC-term strains bound poorly to macrophages, compared with parental or BAD1-reconstituted null strains. However, DeltaC-term strains and the purified protein itself sharply suppressed tumour necrosis factor (TNF)-alpha release by phagocytes in vitro and in lung in vivo, and the strains retained pathogenicity in a murine model of blastomycosis. Our results illustrate the previously undefined role of the EGF-like domain for BAD1 localization to yeast surfaces during cell wall biogenesis. They also demonstrate that the requirements for host cell binding and immune modulation by BAD1 can be dissociated from one another, and that the former is unexpectedly dispensable in the requisite role of BAD1 in pathogenesis.

Two novel mutations in the alpha IIb calcium-binding domains identify hydrophobic regions essential for alpha IIbbeta 3 biogenesis

The recently published crystal structure of the external domains of alphaVbeta3 confirms the prediction that the aminoterminal portion of alphaV, which shares 40% homology with alphaIIb, folds into a beta-propeller structure and that the 4 calcium-binding domains are positioned on the bottom of the propeller. To gain insight into the role of the calcium-binding domains in alphaIIb biogenesis, we characterized mutations in the second and third calcium-binding domains of alphaIIb in 2 patients with Glanzmann thrombasthenia. One patient inherited a Val298Phe mutation in the second domain, and the other patient inherited an Ile374Thr mutation in the third domain. Mammalian cell expression studies were performed with normal and mutant alphaIIb and beta3 cDNA constructs. By flow cytometry, expression of alphaIIb Val298Phe/beta3 in transfected cells was 28% of control, and expression of alphaIIbIle374Thr/beta3 was 11% of control. Pulse-chase analyses showed that both mutant pro-alphaIIb subunits are retained in the endoplasmic reticulum and degraded. Mutagenesis studies of the Val298 and Ile374 residues showed that these highly conserved, branch-chained hydrophobic residues are essential at these positions and that biogenesis and expression of alphaIIbbeta3 is dramatically affected by structural variations in these regions of the calcium-binding domains. Energy calculations derived from a new model of the alphaIIb beta-propeller indicate that these mutations interfere with calcium binding. These data suggest that the alphaIIb calcium-binding domains play a key structural role in the beta-propeller, and that the structural integrity of the calcium-binding domains is critical for integrin biogenesis.

Effect of divalent cations on the affinity and selectivity of alpha4 integrins towards the integrin ligands vascular cell adhesion molecule-1 and mucosal addressin cell adhesion molecule-1: Ca2+ activation of integrin alpha4beta1 confers a distinct ligand specificity

A microtiter plate assay measuring the binding of cells expressing integrins alpha4beta1 or alpha4beta7 to VCAM-1 and MAdCAM-1, expressed as Ig fusion proteins, was used to explore the interplay between the variables of integrin beta-chain, identity and density of ligand, and identity and concentration of activating cations. Both Mn2+ and Mg2+ supported binding of either integrin to either ligand. Ca2+ supported only the binding of alpha4beta1 to VCAM-Ig. Cation concentrations required for half-maximal binding (EC50) ranged from 0.8-280 microM for Mn2+ and 0.8-30 mM for Mg2+, being thus 2-3 logs lower for Mn2+ compared to Mg2+ independent of ligand. EC50 values for binding of alpha4beta1 to VCAM-Ig were 30-45-fold lower compared to MAdCAM-Ig, while alpha4beta7 showed an opposite 3-15-fold selectivity for MAdCAM-Ig over VCAM-Ig. The density of ligand required for adhesion via alpha4beta1 was markedly lower with Mn2+ versus Mg2+, and with VCAM-Ig versus MAdCAM-Ig. These results were interpreted in terms of a coupled equilibrium model, in which binding of activating metal ions and of integrin ligands each stabilizes activated integrin. We conclude that Mn2+ and Mg2+ bind to common regulatory sites with different affinities, producing similar activated states of the integrin. The resulting activated alpha4beta1 binds more strongly to VCAM-Ig versus MAdCAM-Ig by 30-45-fold, while similarly activated alpha4beta7 binds more strongly to MAdCAM-Ig versus VCAM-Ig by 3-15-fold. Inhibition studies showed that Ca2+ also binds to regulatory sites on both integrins. However, the Ca2+-activated state of alpha4beta1 is distinct from that achieved by Mn2+ and Mg2+, possessing increased selectivity for binding to VCAM-1 versus MAdCAM-1.

Platelet integrin alphaIIbbeta3-ligand interactions: what can we learn from the structure?

Upon vascular injury, platelets initiate interaction with exposed subendothelial matrices through various receptors such as glycoprotein (GP) Ib/IX/V complex, alpha2beta1 integrin, and GPVI/FcRgamma. Although these interactions cannot sustain stable platelet thrombus formation by themselves, they ultimately lead to the activation of alphaIIbbeta3 integrin (GPIIb-IIIa complex [GPIIb-IIIa]), the most abundant receptor in platelets. The alphaIIbbeta3 integrin plays a central role in primary hemostasis by serving as a receptor for fibrinogen and von Willebrand factor (vWf). It establishes a stable interaction with vWf bound to the extracellular matrices and uses fibrinogen as a bridging molecule in platelet aggregate formation. The alphaIIbbeta3 integrin also plays an important role in the pathogenesis of thrombosis. Over the past decades, a tremendous amount of effort has been made to elucidate the ligand-binding mechanisms of alphaIIbbeta3, in part because of its clinical significance. Most of the studies have relied on biochemical analyses of purified alphaIIbbeta3 or recombinant proteins generated in vitro. With the lack of actual 3-dimensional structure, molecular modeling has provided a useful framework for interpreting such experimental data on structure-function correlation of integrin molecules. However, it has also generated disagreement between different models. The aim of this minireview is to summarize the past efforts as well as the recent accomplishments in elucidating the structure/function of alphaIIbbeta3. Finally, we will try to explain all those experimental data using the recently published crystal structure of the extracellular domains of the alphaVbeta3 heterodimeric complex.

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.

Biophysical and molecular properties of annexin-formed channels

The annexins are water soluble proteins possessing a hydrophilic surface, which belong to a family of proteins which (a) bind ('annex') both calcium and phospholipids, and (b) form voltage-dependent calcium channels within planar lipid bilayers. Annexins types are diverse (94 annexins in 45 species) and they belong to an enormous multigene family that ranges throughout all eukaryotic kingdoms. Although the structure of these proteins is now well known their functional and physiological roles remain largely unknown and circumstantial. Various experimental approaches provided evidence that annexins function as Ca(2+) channels that could act as regulators of membrane fusion. The identity of annexins is derived from the conserved 34 kDa C-terminal domain which comprises four repeats - except for annexin VI, with eight repeats - of a sequence of approximately seventy amino acids, which holds the area known as the 'endonexin fold', with its identifying GXGTDE. Annexins have been placed into three subgroups of (1) tetrad core and short amino terminal, (2) tetrad core and long amino terminal, and (3) octad core and short amino terminal. The repeats are highly conserved, each forming a compact alpha-helical domain comprising five alpha-helices wound in a right-handed superhelix. Four domains are formed, arranged in a nearly flat and cyclical array, with domains I and IV, and II and III respectively forming two tightly organised modules with almost twofold symmetry. A hydrophilic pore lies at the centre of the molecule, forming a prominent ion channel coated with charged and highly conserved residues. The annexin molecule is slightly curved, with both a convex and a concave face. The cation/anion permeability ratios and the selectivity sequence of the ion channels formed by several annexins confirm the selectivity of the annexins for Ca(2+) over other divalent cations, and reveals the importance of structural sites, e.g. amino acid positions 17, 78, 95 and 112 for the identification of the ion channel's position, function and regulation. Some are sensitive to low doses of the phenothiazine drugs, trifluoperazine (an anti-schizophrenia drug) and promethazine (anti nausea drug) La(3+) and Cd(2+), (blockers of voltage-gated Ca(2+) channels) nifedipine (an inhibitor of non-activating Ca(2+) channels). There are two main competing models used to explain in vitro ion channel activity of annexins: one involves changes in the conductance of ion via electrostatic disturbance of the membrane surface; the other involves a much more extensive alteration in protein structure and a correspondingly deeper penetration into the membrane.

Attenuation of very late antigen-5-mediated adhesion of bone marrow-derived mast cells to fibronectin by peptides with inverted hydropathy to EF-hands

Release of allergic mediators from mast cells is enhanced by very late Ag (VLA)-5-mediated interaction of these cells with fibronectin. In this report, we show that VLA-5-mediated adhesion of bone marrow-derived mast cells to fibronectin can be induced by two different pathways: first, FcepsilonRI clustering, which depends on calmodulin activation and extracellular Ca(2+), and, second, by Mn(2+) stimulation, which is independent of calmodulin activation and antagonized by Ca(2+). Previous studies have shown the presence of several cation-binding domains in VLA-5 that are homologous to the calcium-binding EF-hands of calmodulin. To show a role for EF-hands of different proteins in VLA-5-mediated adhesion, we used calcium-like peptides (CALP), CALP1 and CALP2, designed to bind to EF-hands based on inverted hydropathy. CALP1 and, more potently, CALP2 inhibited FcepsilonRI-induced adhesion to fibronectin via different mechanisms. The target for the effects of CALP1 and 2 on FcepsilonRI-induced adhesion and degranulation was intracellular and likely involved calmodulin. Interestingly only CALP2 was able to inhibit Mn(2+)-induced calmodulin-independent adhesion by interfering with an extracellular target, which is probably VLA-5. We conclude that CALP1 and 2 can inhibit VLA-5-mediated adhesion of mast cells to fibronectin through binding to EF-hands of multiple proteins, and that these peptides can be used as lead compounds for the development of future therapy against allergy.

The alphaBbetaC integrin is expressed on the surface of the sea urchin egg and removed at fertilization

Integrins are expressed on the surface of some vertebrate eggs where they are thought to have a role in fertilization. The objective of this study is to determine if integrins are expressed on sea urchin eggs. The alphaB and betaC subunits were cloned using the homology polymerase chain reaction. Monoclonal and polyclonal antibodies were developed against bacterially expressed fragments of the extracellular domains of the betaC subunit and the alphaB subunit. As well, a monoclonal antibody was developed against a synthesized peptide corresponding to part of the cytoplasmic domain of betaC. Analysis of biotinylated egg cortex extracts immunoprecipitated with either anti-betaC or anti-alphaB yields bands of 130 and 225 kDa. Immunoblots confirm that betaC is part of the complex immunoprecipitated with anti-alphaB. Confocal immunofluorescence and immunogold electron microscopy show that betaC is present on the surface of the unfertilized egg at the tips of microvilli and in cortical granules. During the cortical reaction, immunoreactivity with antibodies to the extracellular domains of betaC and alphaB disappears from the egg surface, and microvillar casts on the fertilization envelope become immunoreactive. With antibodies to the cytoplasmic domain of betaC, immunoreactivity is lost from the surface of the egg, but the fertilization envelope does not immediately become immunoreactive. In immunoblots of egg cortex there are immunoreactive bands of the predicted sizes for alphaB and betaC. However, in fertilization envelopes, a second band that is slightly lower in molecular weight is also present. Eggs fertilized in the presence of soybean trypsin inhibitor have elongated microvilli that remain bound to the elevating fertilization envelope and immunoreactive to anti-betaC antibodies. Eggs fertilized in the presence of an ovoperoxidase inhibitor, 3-amino-1,2,4-triazole, have a patchy distribution of betaC immunoreactivity in fertilization envelopes. Together, these data suggest that alphaBbetaC integrins are expressed on the surface of unfertilized eggs and, during the cortical reaction, the extracellular domains are cleaved by proteases and cross-linked into the fertilization envelope by ovoperoxidase. The alphaBbetaC integrin receptors may have several potential functions prior to their removal at fertilization, including attachment of the vitelline envelope to the egg surface and anchoring the cortical cytoskeleton.

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.

Molecular basis of ligand recognition by integrin alpha5beta 1. II. Specificity of arg-gly-Asp binding is determined by Trp157 OF THE alpha subunit

Different beta(1) integrins bind Arg-Gly-Asp (RGD) peptides with differing specificities, suggesting a role for residues in the alpha subunit in determining ligand specificity. Integrin alpha(5)beta(1) has been shown to bind with high affinity to peptides containing an Arg-Gly-Asp-Gly-Trp (RGDGW) sequence but with relatively low affinity to other RGD peptides. The residues within the ligand-binding pocket that determine this specificity are currently unknown. A cyclic peptide containing the RGDGW sequence was found to strongly perturb the binding of the anti-alpha(5) monoclonal antibody (mAb) 16 to alpha(5)beta(1). In contrast, RGD peptides lacking the tryptophan residue acted as weak inhibitors of mAb 16 binding. The epitope of mAb 16 has previously been localized to a region of the alpha(5) subunit that contains Ser(156)-Trp(157). Mutation of Trp(157) (but not of Ser(156) or surrounding residues) to alanine blocked recognition of mAb 16 and perturbed the high affinity binding of RGDGW-containing peptides to alpha(5)beta(1). The same mutation also abrogated recognition of the alpha(5)beta(1)-specific ligand peptide Arg-Arg-Glu-Thr-Ala-Trp-Ala (RRETAWA). Based on these findings, we propose that Trp(157) of alpha(5) participates in a hydrophobic interaction with the tryptophan residue in RGDGW, and that this interaction determines the specificity of alpha(5)beta(1) for RGDGW-containing peptides. Since the RGD sequence is recognized predominantly by amino acid residues on the beta(1) subunit, our results suggest that Trp(157) of alpha(5) must lie very close to these residues. Our findings therefore provide new insights into the structure of the ligand-binding pocket of alpha(5)beta(1).

Overexpression, purification, and characterization of Bacillus subtilis N-acetylmuramoyl-L-alanine amidase CwlC

N-acetylmuramoyl-L-alanine amidase CwlC of Bacillus subtilis was overproduced in Escherichia coli and purified 21-fold. The amidase hydrolyzed type A cell walls such as B. subtilis. The amidase bound slightly to the Microbacterium lacticum cell wall (type B), but did not entirely hydrolyze it. The presence of calcium or magnesium ion increased the resistance of the amidase to heat denaturation.

Expression and function of calcium binding domain chimeras of the integrins alpha(IIb) and alpha(5)

To further identify amino acid domains involved in the ligand binding specificity of alpha(IIb)beta(3), chimeras of the conserved calcium binding domains of alpha(IIb) and the alpha subunit of the fibronectin receptor alpha(5)beta(1) were constructed. Chimeras that replaced all four calcium binding domains, replaced all but the second calcium binding domain of alpha(IIb) with those of alpha(5), or deleted all four calcium binding domains were synthesized but not expressed on the cell surface. Additional chimeras exchanged subsets or all of the variant amino acids in the second calcium binding domain, a region implicated in ligand binding. Cell surface expression of each second calcium binding domain mutant complexed with beta(3) was observed. Each second calcium binding domain mutant was able to 1) bind to immobilized fibrinogen, 2) form fibrinogen-dependent aggregates after treatment with dithiothreitol, and 3) bind the activation-dependent antibody PAC1 after LIBS 6 treatment. Soluble fibrinogen binding studies suggested that there were only small changes in either the K(d) or B(max) of any mutant. We conclude that chimeras of alpha(IIb) containing the second calcium binding domain sequences of alpha(5) are capable of complexing with beta(3), that the complexes are expressed on the cell surface, and that mutant complexes are capable of binding both immobilized and soluble fibrinogen, suggesting that the second calcium binding domain does not determine ligand binding specificity.

A minimized human integrin alpha(5)beta(1) that retains ligand recognition

Two isolated recombinant fragments from human integrin alpha(5)beta(1) encompassing the FG-GAP repeats III to VII of alpha(5) and the insertion-type domain from beta(1), respectively, are structurally well defined in solution, based on CD evidence. Divalent cation binding induces a conformational adaptation that is achieved by Ca(2+) or Mg(2+) (or Mn(2+)) with alpha(5) and only by Mg(2+) (or Mn(2+)) with beta(1). Mn(2+) bound to beta(1) is highly hydrated ( approximately 3 water molecules), based on water NMR relaxation, in agreement with a metal ion-dependent adhesion site-type metal coordination. Each fragment saturated with Mg(2+) (or Mn(2+)) binds a recombinant fibronectin ligand in an RGD-dependent manner. A conformational rearrangement is induced on the fibronectin ligand upon binding to the alpha(5), but not to the beta(1) fragment, based on CD. Ligand binding results in metal ion displacement from beta(1). Both alpha(5) and beta(1) fragments form a stable heterodimer (alpha(5)beta(1) mini-integrin) that retains ligand recognition to form a 1:1:1 ternary complex, in the presence of Mg(2+), and induces a specific conformational adaptation of the fibronectin ligand. A two-site model for RGD binding to both alpha and beta integrin components is inferred from our data using low molecular weight RGD mimetics.

Integrin-like proteins in Candida spp. and other microorganisms

The vertebrate integrins provide a paradigm for cell surface proteins involved in adhesion and morphogenesis. However, homologs of integrins have been found in more primitive organisms. This review will discuss the evidence for surface proteins in Candida albicans and Candida tropicalis that contain motifs reminiscent of integrins and will analyze the contributions of one of these proteins, Int1p, to adhesion, morphogenesis, and virulence. Other microorganisms thought to express integrin-like proteins will also be addressed.

Fine mapping of inhibitory anti-alpha5 monoclonal antibody epitopes that differentially affect integrin-ligand binding

The high-affinity interaction of integrin alpha5beta1 with the central cell-binding domain of fibronectin requires both the Arg-Gly-Asp (RGD) sequence (in the tenth type III repeat) and a second site Pro-His-Ser-Arg-Asn (PHSRN) in the adjacent ninth type III repeat, which synergizes with RGD. Arg-Arg-Glu-Thr-Ala-Trp-Ala (RRETAWA) is a novel peptidic ligand for alpha5beta1, identified by phage display, which blocks alpha5beta1-mediated cell adhesion to fibronectin. A key question is the location of the binding sites for these ligand sequences within the integrin. In this study we have identified residues that form part of the epitopes of three inhibitory anti-alpha5 monoclonal antibodies (mAbs): 16, P1D6 and SNAKA52. These mAbs have distinct functional properties. mAb 16 blocks the recognition of RGD and RRETAWA, whereas P1D6 blocks binding to the synergy sequence. The binding of SNAKA52 is inhibited by anti-beta1 mAbs, indicating that its epitope is close to the interface between the alpha and beta subunits. Residues in human alpha5 were replaced with the corresponding residues in mouse alpha5 by site-directed mutagenesis; wild-type or mutant human alpha5 was expressed on the surface of alpha5-deficient Chinese hamster ovary cells. mAb binding was assessed by flow cytometry and by adhesion to the central cell-binding domain of fibronectin or RRETAWA by cell attachment assay. All three epitopes were located to different putative loops in the N-terminal domain of alpha5. As expected, disruption of these epitopes had no effect on ligand recognition by alpha5beta1. The locations of these epitopes are consistent with the beta-propeller model for integrin alpha-subunit structure and allow us to propose a topological image of the integrin-ligand complex.

Inhibition of the plasma glycosylphosphatidylinositol-specific phospholipase D by synthetic analogs of lipid A and phosphatidic acid

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD), a plasma enzyme with extensive sequence similarity to integrin alpha subunits, is inhibited by micromolar concentrations of lipid A, phosphatidic acid (PA) and lysophosphatidic acid (M. G. Low and K.-S. Huang, J. Biol. Chem. 268, 8480-8490, 1993). In this study we have explored the mechanism of inhibition using synthetic analogs of lipid A, and PA. Monosaccharide analogs of lipid A, which varied in the number and position of the phosphate groups, the type of acyl group, and its linkage to the glucosamine ring, were tested for their ability to inhibit GPI-PLD. A compound (SDZ 880.431) containing 3-aza-glucosamine 1,4-diphosphate as the polar headgroup was identified which had a potency (IC(50) approximately 1 microM) similar to natural lipid A preparations. Removal of either phosphate residue increased the IC(50) markedly. Analogs of PA such as (7-nitro-2-1,3-benzoxadiazo-4-yl)amino-PA, ceramide 1-phosphate, and hexadecyl phosphate had approximately IC(50) values ranging from 1 to 5 microM, indicating that considerable variation in the structure of the hydrophobic groups was permissible. Inhibition of GPI-PLD by long-chain PA could not be blocked by high concentrations of glycerol 1-phosphate or dibutyryl PA. These results indicate that the hydrophobic groups do not have a passive role in inhibition but are directly involved in the binding interaction with GPI-PLD. We propose that this diverse group of inhibitors all bind to a common site on GPI-PLD, the central hydrophobic cavity predicted by the beta-propeller model for integrin alpha subunits and GPI-PLD.

Increased expression of integrin and receptor tyrosine kinase genes during autograft fusion in the sponge Geodia cydonium

Recently cDNAs coding for cell surface molecules have been isolated from sponges. The molecules for alpha-integrin, galectin, and receptor tyrosine kinase (RTK), obtained from the marine sponge, Geodia cydonium, have been described earlier. In the present study also the cDNA for one putative beta-integrin has been identified from G. cydonium. The deduced aa sequence comprises the characteristic signatures, found in other metazoan beta-integrin molecules; the estimated size is 95,215 Da. To obtain first insights into the molecular events which proceed during autograft fusion, the expressions of these genes were determined on transcriptional and translational level. The cDNAs as well as antibodies raised against the recombinant sponge proteins alpha-integrin, RTK and galectin were used and Northern blot experiments and immunocytochemical analyses have been performed. The results show that transcription of the two subunits of an integrin receptor as well as of the RTK are strongly upregulated after grafting; levels of > 10-fold have been determined in the fusion zone of the grafts after a 10 days incubation. Immunofluorescence studies of sections through the fusion zone support these data. In contrast the transcription of the gene encoding galectin is drastically downregulated after grafting. In a parallel series of experiments the level of the heat-shock protein-70 was determined and it was found that it remained unchanged after grafting. We conclude that integrin subunits and the RTK molecule are involved in self-self recognition of sponge.

How LFA-1 binds to different ligands

The integrin leukocyte function-associated antigen 1 (LFA-1) is pivotal for cell adhesion and signalling within the immune system. Here, Minke Binnerts and Yvette van Kooyk discuss evidence from mutational and crystallographic studies showing that binding of LFA-1 to its ligands intercellular adhesion molecule 1 (ICAM-1), -2 and -3 might be structurally distinct.

A New Member of the Ig Superfamily and a V-ATPase G Subunit Are Among the Predicted Products of Novel Genes Close to the TNF Locus in the Human MHC

It is becoming increasingly apparent that many of the genes in the class III region of the human MHC encode proteins involved in the immune and inflammatory responses. Furthermore, genetic studies have indicated that genes within the class III region, particularly the telomeric segment containing the TNF gene, could contribute to susceptibility to diseases of immune-related etiology. We have sequenced an 82-kb segment of DNA around the TNF gene to identify candidate disease susceptibility genes in this region. The 10 known genes in this region have been precisely positioned with the order allograft inflammatory factor 1, G1, 1C7, leukocyte-specific transcript 1 (B144), lymphotoxin B, TNF, lymphotoxin A, NB6, IKBL, BAT1 (centromere to telomere), and their genomic structures have been defined. Comparison of the G1 genomic region with previously described cDNA and genomic sequences, together with the results of reverse transcriptase-PCR, indicates that three alternative transcripts, G1, allograft inflammatory factor 1, and IFN-γ-responsive transcript, are all derived from this gene. The completion of the sequence of 1C7 (D6S2570) has revealed that this gene encodes a putative novel member of the Ig superfamily. A number of alternatively spliced transcripts of 1C7 were identified by reverse transcriptase-PCR, all of which are expressed in immune-related cell lines. Alternative splicing within the Ig domain-encoding region was seen to result in possible set switching between an IgV domain and an IgC2 domain. Lastly, a previously unidentified gene, homologous to a number of V-ATPase G subunits, has been located 1 kb telomeric of IKBL.

A cell-free system to study regulation of focal adhesions and of the connected actin cytoskeleton

Assembly and modulation of focal adhesions during dynamic adhesive processes are poorly understood. We describe here the use of ventral plasma membranes from adherent fibroblasts to explore mechanisms regulating integrin distribution and function in a system that preserves the integration of these receptors into the plasma membrane. We find that partial disruption of the cellular organization responsible for the maintenance of organized adhesive sites allows modulation of integrin distribution by divalent cations. High Ca2+ concentrations induce quasi-reversible diffusion of beta1 integrins out of focal adhesions, whereas low Ca2+ concentrations induce irreversible recruitment of beta1 receptors along extracellular matrix fibrils, as shown by immunofluorescence and electron microscopy. Both effects are independent from the presence of actin stress fibers in this system. Experiments with cells expressing truncated beta1 receptors show that the cytoplasmic portion of beta1 is required for low Ca2+-induced recruitment of the receptors to matrix fibrils. Analysis with function-modulating antibodies indicates that divalent cation-mediated receptor distribution within the membrane correlates with changes in the functional state of the receptors. Moreover, reconstitution experiments show that purified alpha-actinin colocalizes and redistributes with beta1 receptors on ventral plasma membranes depleted of actin, implicating binding of alpha-actinin to the receptors. Finally, we found that recruitment of exogenous actin is specifically restricted to focal adhesions under conditions in which new actin polymerization is inhibited. Our data show that the described system can be exploited to investigate the mechanisms of integrin function in an experimental setup that permits receptor redistribution. The possibility to uncouple, under cell-free conditions, events involved in focal adhesion and actin cytoskeleton assembly should facilitate the comprehension of the underlying molecular mechanisms.

Covalent immobilization of recombinant human alphavbeta3 integrin on a solid support with retention of functionality

Alphavbeta3 is the major receptor mediating the attachment of osteoclasts to bone surface and plays a critical role in bone resorption and remodeling. Interfering with alphavbeta3 binding inhibits osteoclast-mediated bone resorption, and thus demonstrates the potential utility of alphavbeta3 antagonists for therapy of osteoporosis. This report describes the generation of an alphavbeta3 affinity column which was created to enable screening of collections of large numbers of ligands, e.g., combinatorial libraries (previously prepared by us), to sort and identify ligands with the highest affinity for alphavbeta3. We demonstrate that covalent immobilization of the heterodimeric alphavbeta3 receptor can be achieved with retention of characteristic ligand binding properties. Human alphavbeta3 was isolated from human embryonic kidney cells (HEK 293) that stably express high levels of the recombinant receptor and then affinity purified to homogeneity. Purified alphavbeta3 receptor was linked covalently to activated CH-Sepharose 4B beads. Quantification of immobilized functional receptor was determined by Scatchard analysis. The immobilized functional receptor maintains binding properties similar to the membrane-embedded and soluble receptor. The immobilized receptor also can be used to select the highest affinity ligand from among a mixture of peptides which differ in their binding affinity, structure, and hydrophobicity, both when the peptides are loaded in equimolar concentrations in a mixture and when the concentration of the highest affinity ligand is reduced 10-fold. Liquid chromatography-mass spectrometry was utilized to confirm selective ligand binding and to demonstrate that preferential binding was not due to nonspecific hydrophobic interactions with immobilized alphavbeta3 receptor or the affinity column. This approach may be of general use for affinity-based screening of ligands for other integrins and should enable practical screening of combinatorial libraries containing large numbers of potential ligands for the human alphavbeta3 integrin receptor, including linear peptides, cyclic peptides, and peptidomimetics.

Studies of the role of the integrin EF-hand, Ca2+-binding sites in glycosylphosphatidylinositol-specific phospholipase D: reduced expression following mutagenesis of residues predicted to bind Ca2+

Previous studies of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) have demonstrated that GPI-PLD can bind Ca2+ ions with high specificity (J.-Y. Li, K. Hollfelder, K.-S. Huang, and M. G. Low, J. Biol. Chem. 269, 28063-28971, 1994). In this study the functional role of the bound Ca2+ ions was evaluated. The enzymatic activity of purified GPI-PLD, which was depleted of divalent cations by pretreatment with EDTA, EGTA, or 1, 10-phenanthroline, could be completely restored with Zn2+ (and partially with Co2+), which indicates that Ca2+ can be removed from the protein without affecting its enzymatic activity. This result suggested that Ca2+ bound to GPI-PLD has a structural or regulatory role but is not required for GPI hydrolysis. To evaluate these possibilities we transfected COS cells with GPI-PLD mutants in which the predicted Ca2+-binding sites were either deleted completely or altered by single-residue substitution. All of the mutations showed substantial reductions in the amount of GPI-PLD secreted into the medium (0-6% of wild type). The data indicate that bound Ca2+ plays an important role in the initial folding, intracellular transport, or secretion of GPI-PLD even though it has no discernible role in the mature, secreted protein.

Differential effects of integrin alpha chain mutations on invasin and natural ligand interaction

To determine if recognition of the Yersinia pseudotuberculosis invasin protein and natural substrates requires identical integrin residues, a region of the human alpha3 integrin chain predicted to be involved in substrate adhesion was targeted for mutation. One point mutation located in a region of the third N-terminal repeat of the alpha3 chain, alpha3-W220A, failed to promote adhesion to the natural alpha3 beta1 substrate epiligrin but maintained near wild type levels of adhesion to invasin. A second nearby mutation, alpha3-Y218A, which showed no detectable adhesion to epiligrin, was only partially attenuated for invasin binding as well as invasin-mediated bacterial uptake. A third substitution, alpha3-D154A, predicted to be in the second N-terminal repeat not known to be implicated in cell adhesion, was competent for invasin-promoted adhesion events and appeared to encode a receptor of increased activity, as it had a higher efficiency than wild type receptor for adhesion to epiligrin. Cell lines expressing this derivative were not recognized by a function blocking anti-alpha3 antibody, indicating that the second and third repeats of the alpha3 chain are either closely linked in space or the second repeat can modulate activity of the third. Differential effects on substrate adhesion do not appear to be associated with all integrin alpha chain mutations, as alpha4 chain mutations affecting the divalent cation binding domains depressed adhesion to invasin to a significant extent.

Identification of amino acid residues that form part of the ligand-binding pocket of integrin alpha5 beta1

Arg-Arg-Glu-Thr-Ala-Trp-Ala (RRETAWA) is a novel ligand peptide for integrin alpha5 beta1, which blocks alpha5 beta1-mediated cell adhesion to fibronectin (Koivunen, E., Wang, B., and Ruoslahti, E. (1994) J. Cell Biol. 124, 373-380). Here we have localized the binding site for RRETAWA on alpha5 beta1 using inhibitory monoclonal antibodies (mAbs) and site-directed mutagenesis. A cyclic peptide containing this sequence (*CRRETAWAC*) had little effect on the binding of most anti-alpha5 and anti-beta1 mAbs to alpha5 beta1 but completely blocked binding of the anti-alpha5 mAb 16 in a directly competitive manner. Hence, the binding site of RRETAWA appears to closely overlap with the epitope of mAb 16. *CRRETAWAC* also acted as a direct competitive inhibitor of the binding of Arg-Gly-Asp (RGD)-containing fibronectin fragments to alpha5 beta1, suggesting that the binding site for RRETAWA is also closely overlapping with that for RGD. However, differences between the binding sites of RRETAWA and RGD were apparent in that (i) RGD peptides allosterically inhibited the binding of mAb 16 to alpha5 beta1, and (ii) several mAbs that perturbed binding of alpha5 beta1 to RGD had little effect on binding of alpha5 beta1 to RRETAWA. A double mutation in alpha5 (S156G/W157S) blocked the interaction of both RRETAWA and mAb 16 with alpha5 beta1 but had no effect on fibronectin binding or on the binding of other anti-alpha5 mAbs. Ser156-Trp157 is located near the apex of a putative loop region on the upper surface of a predicted beta-propeller structure formed by the NH2-terminal repeats of alpha5. Our findings suggest that this sequence forms part of the ligand-binding pocket of alpha5 beta1. Furthermore, as Ser156-Trp157 is unique to the alpha5 subunit, it may be responsible for the specific recognition of RRETAWA by alpha5 beta1.

Expression of the H52 epitope on the beta2 subunit is dependent on its interaction with the alpha subunits of the leukocyte integrins LFA-1, Mac-1 and p150,95 and the presence of Ca2+

Integrin-mediated adhesion is a divalent cation-dependent process. Whether divalent cations directly participate in ligand binding or exert their effects indirectly by affecting the overall structure of the integrin heterodimers is not known. In this study we describe the epitope of the mAb H52 which has been mapped to a predicted disulfide-bonded loop (C386 and C400) in the beta2 integrin subunit. In the presence of Ca2+ and Mg2+, the H52 epitope is expressed on the monomeric beta2 subunit, the LFA-1 and Mac-1 heterodimers but not on p150,95, thus implying that this epitope is masked in p150,95. However, expression of the H52 epitope on Mac-1, but not on LFA-1, or the monomeric beta2 subunit, is dependent on the presence of Ca2+, thus suggesting that the chelation of Ca2+ causes a conformational change in Mac-1 which results in the loss of the epitope. These results suggest that expression of the H52 epitope on the beta2 subunit is dependent on its interaction with the different alpha subunits. Since the epitope itself is not required for heterodimer formation nor for ligand binding, occupancy of a Ca2+ binding site(s) must therefore affect the alphabeta subunit interactions, and thus the overall conformation of Mac-1.

The cation-binding domain from the alpha subunit of integrin alpha5 beta1 is a minimal domain for fibronectin recognition

The cation-binding domain from the alpha subunit of human integrin alpha5beta1 was produced as a recombinant protein, alpha5-(229-448). This protein displays a well defined fold with a content of 30-35% alpha-helix and 20-25% beta-strand, based on circular dichroism. The binding of Ca2+ or Mg2+ to alpha5-(229-448) results in a biphasic conformational rearrangement consistent with the occurrence of two classes of cation-binding sites differing by their affinities. The two classes of sites are located in two conformationally independent lobes, as established by a parallel study of two recombinant half-domains (N- and C-terminal) that also adopt stable folds. Upon saturation with divalent cations, alpha5-(229-448) binds an Arg-Gly-Asp (RGD)-containing fibronectin ligand to form a 1:1 complex. Complex formation is associated with a specific conformational adaptation of the ligand, suggesting an induced fit mechanism. In contrast, neither of the half-domains is competent for ligand binding. The alpha5-(229-448)-fibronectin complex is dissociated in the presence of an RGD peptide, as well as of a simple carboxylic acid, suggesting that the RGD aspartyl carboxylate is an essential element that directly interacts with the alpha5 cation-binding domain.

Isolation, cloning, and sequence analysis of the integrin subunit alpha10, a beta1-associated collagen binding integrin expressed on chondrocytes

We have found that chondrocytes express a novel collagen type II-binding integrin, a new member of the beta1-integrin family. The integrin alpha subunit, which has a Mr of 160 kDa reduced, was isolated from bovine chondrocytes by collagen type II affinity purification. The human homologue was obtained by screening a human chondrocyte library with a bovine cDNA probe. Cloning and cDNA sequence analysis of the human integrin alpha subunit designated alpha10 show that it shares the general structure of other integrin alpha subunits. The predicted amino acid sequence consists of a 1167-amino acid mature protein, including a signal peptide (22 amino acids), a long extracellular domain (1098 amino acids), a transmembrane domain (25 amino acids), and a short cytoplasmic domain (22 amino acids). The extracellular part contains a 7-fold repeated sequence, an I-domain (199 amino acids) and three putative divalent cation-binding sites. The deduced amino acid sequence of alpha10 is 35% identical to the integrin subunit alpha2 and 37% identical to the integrin subunit alpha1. Northern blot analysis shows a single mRNA of 5.4 kilobases in chondrocytes. A peptide antibody against the predicted sequence of the cytoplasmic domain of alpha10 immunoprecipitated two proteins with masses of 125 and 160 kDa from chondrocyte lysates under reducing conditions. The peptide antibody specifically stained chondrocytes in tissue sections of human articular cartilage, showing that alpha10 beta1 is expressed in cartilage tissue.

Regulation of integrin function: evidence that bivalent-cation-induced conformational changes lead to the unmasking of ligand-binding sites within integrin alpha5 beta1

The molecular mechanisms that regulate integrin-ligand binding are unknown; however, bivalent cations are essential for integrin activity. According to recent models of integrin tertiary structure, sites involved in ligand recognition are located on the upper face of the seven-bladed beta-propeller formed by the N-terminal repeats of the alpha subunit and on the von Willebrand factor A-domain-like region of the beta subunit. The epitopes of function-altering monoclonal antibodies (mAbs) cluster in these regions of the alpha and beta subunits; hence these mAbs can be used as probes to detect changes in the exposure or shape of the ligand-binding sites. Bivalent cations were found to alter the apparent affinity of binding of the inhibitory anti-alpha5 mAbs JBS5 and 16, the inhibitory anti-beta1 mAb 13, and the stimulatory anti-beta1 mAb 12G10 to alpha5 beta1. Analysis of the binding of these mAbs to alpha5beta1 over a range of Mn2+, Mg2+ or Ca2+ concentrations demonstrated that there was a concordance between the ability of cations to elicit conformational changes and the ligand-binding potential of alpha5 beta1. Competitive ELISA experiments provided evidence that the domains of the alpha5 and beta1 subunits recognized by mAbs JBS5/16 and 13/12G10 are spatially close, and that the distance between these two domains is increased when alpha5 beta1 is occupied by bivalent cations. Taken together, our findings suggest that bivalent cations induce a conformational relaxation in the integrin that results in exposure of ligand-binding sites, and that these sites lie near an interface between the alpha subunit beta-propeller and the beta subunit putative A-domain.

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

Integrins are large, heterodimeric surface molecules of wide importance in cell adhesion. The N-terminal half of all integrin alpha-subunits contains seven weak sequence repeats of approximately 60 amino acids that are important in ligand binding and have been predicted to fold cooperatively into a single beta-propeller domain with seven beta-sheets. We provide evidence supporting this model with a mouse mAb to human Mac-1 (alphaM beta2, CD11b/CD18). This antibody, CBRM1/20, binds to amino acid residues that are in different repeats and are 94 residues apart in the primary structure in the loop between strands 1 and 2 of beta-sheet 5 and in the loop between strands 3 and 4 of beta-sheet 6. The 1-2 loops of beta-sheets 5-7 in integrins have EF hand-like Ca2+-binding motifs. CBRM1/20 binds to Mac-1 in the presence of Ca2+ or Sr2+ with an EC50 of 0.2 mM. Mg2+ or Mn2+ cannot substitute. Antibodies to other epitopes on the Mac-1 beta-propeller domain bind in the absence of calcium. mAb CBRM1/20 does not block ligand binding. Thus, the region on the lower surface of the beta-propeller domain to which mAb CBRM1/20 binds does not bind ligand and, furthermore, cannot bind other integrin domains, such as those of the beta-subunit.

The fibrinogen-binding MSCRAMM (clumping factor) of Staphylococcus aureus has a Ca2+-dependent inhibitory site

The clumping factor (ClfA) is a cell surface-associated protein of Staphylococcus aureus that promotes binding of fibrinogen or fibrin to the bacterial cell. Previous studies have shown that ClfA and the platelet integrin alphaIIbbeta3 recognize the same domain at the extreme C terminus of the fibrinogen gamma-chain. alphaIIbbeta3 interaction with this domain is known to occur in close proximity to a Ca2+-binding EF-hand structure in the alpha-subunit. Analysis of the primary structure of ClfA indicated the presence of a potential Ca2+-binding EF-hand-like motif at residues 310-321 within the fibrinogen-binding domain. Deletion mutagenesis and site-directed mutagenesis of this EF-hand in recombinant truncated ClfA proteins (Clf40, residues 40-559; and Clf41, residues 221-559) resulted in a significant reduction of affinity for native fibrinogen and a fibrinogen gamma-chain peptide. Furthermore, Ca2+ (or Mn2+) could inhibit the binding of the fibrinogen gamma-chain peptide to Clf40-(40-559) and the adhesion of S. aureus cells to immobilized fibrinogen with an IC50 of 2-3 mM. In contrast, Mg2+ (or Na+) at similar concentrations had no effect on the ClfA-fibrinogen interaction. Far-UV CD analysis of Clf40-(40-559) and Clf41-(221-559) in the presence of metal ions indicated Ca2+- and Mn2+-induced differences in secondary structure. These data suggest that Ca2+ binds to an inhibitory site(s) within ClfA and induces a conformational change that is incompatible with binding to the C terminus of the gamma-chain of fibrinogen. Mutagenesis studies indicate that the Ca2+-dependent inhibitory site is located within the EF-hand motif at residues 310-321.

Asp-698 and Asp-811 of the integrin alpha4-subunit are critical for the formation of a functional heterodimer

The amino acid motif LDV is the principal binding site for alpha4 integrins in fibronectin, and homologous motifs are recognized in vascular cell adhesion molecule-1 and MAdCAM-1. Three conserved LDV motifs (LDV-1 to 3) occur in the ectodomain of the human and mouse alpha4-subunit, the functions of which are unknown. We demonstrate here that alpha4-transfected fibroblasts with mutation in LDV-1 (D489N) behaved like alpha4-wild type but that LDV-2 (D698N) and LDV-3 (D811N) mutants were impaired in binding and spreading on alpha4-specific substrates. On the RGD-containing fibronectin fragment FN-120 there was an inverse behavior; now the alpha4-wild type and the LDV-1 mutant could not adhere whereas the two other mutants could. The beta1 chain was critical for the differential integrin response. Biochemical analysis demonstrated that the LDV-2 and -3 mutations reduced the strength of the alpha4beta1 association, favored the formation of alpha5beta1, and prevented the expression of alpha4beta7 on the cell surface. Our results indicate that LDV-2 and LDV-3 are critical for the formation of a functional heterodimer. The presence of similar amino acid motifs in ligands and the alpha4-subunit suggest that metal coordination plays an important role in integrin-ligand binding as well as for heterodimer formation.

Defining extracellular integrin alpha-chain sites that affect cell adhesion and adhesion strengthening without altering soluble ligand binding

It was previously shown that mutations of integrin alpha4 chain sites, within putative EF-hand-type divalent cation-binding domains, each caused a marked reduction in alpha4beta1-dependent cell adhesion. Some reports have suggested that alpha-chain "EF-hand" sites may interact directly with ligands. However, we show here that mutations of three different alpha4 "EF-hand" sites each had no effect on binding of soluble monovalent or bivalent vascular cell adhesion molecule 1 whether measured indirectly or directly. Furthermore, these mutations had minimal effect on alpha4beta1-dependent cell tethering to vascular cell adhesion molecule 1 under shear. However, EF-hand mutants did show severe impairments in cellular resistance to detachment under shear flow. Thus, mutation of integrin alpha4 "EF-hand-like" sites may impair 1) static cell adhesion and 2) adhesion strengthening under shear flow by a mechanism that does not involve alterations of initial ligand binding.

Sequence analysis of rat integrin alpha E1 and alpha E2 subunits: tissue expression reveals phenotypic similarities between intraepithelial lymphocytes and dendritic cells in lymph

The integrin alpha OX-62 subunit is defined by the OX-62 monoclonal antibody that was raised against rat dendritic cells in lymph (veiled cells) and shows properties similar to those of human alpha E2 that is predominantly expressed on intraepithelial lymphocytes. To clone alpha OX-62, rat probes generated using primers specific for the human alpha E sequence were used to screen rat T cell cDNA libraries. cDNA clones encoding two similar but not identical alpha subunits that are closely related to but distinct from human alpha E were isolated. alpha E1 is predicted to be the rat homolog of mouse alpha M290 and alpha E2 corresponds to rat alpha OX-62. Immunofluorescence analysis revealed that mouse alpha E1 and rat alpha E2 are expressed in dendritic epidermal T cells in the skin, intraepithelial lymphocytes in the small intestine and in cells with a dendritic morphology present at sites where gamma delta T cells occur in lymphoid organs. Unexpectedly, alpha E2 is co-expressed with intracellular CD3-delta and a 33-kDa CD3 chain but not the T cell receptor in veiled cells. These findings suggest that veiled cells may be derived from a lymphoid precursor. Furthermore, veiled cells show phenotypic similarities to intraepithelial lymphocytes.

Identification and characterization of functional cation coordination sites in platelet endothelial cell adhesion molecule-1

We have employed 45CaCl2 binding studies, terbium (Tb3+) luminescence spectroscopy, and electrospray mass spectroscopy (ESI-MS) to identify divalent metal binding properties of soluble recombinant human PECAM-1 (srPECAM-1), and to define unique cation binding domains using short, linear peptide sequences from the protein. PECAM-1 was found to directly interact with 45CaCl2, binding 2.3 nmol of Ca2+/nmol of srPECAM-1 with a Kd of 1.17 nM. PECAM-1 was found to contain high-affinity cation binding sites involving amino acids Asp443, Asp444, and Glu446 of Ig-domain 5 and residues Glu487, Glu490, Asp491, Glu538, Glu540, and Glu542 of Ig-domain 6. The PECAM cation binding sites demonstrated broad specificity for all divalent cations, with Mn2+ having a higher affinity than Ca2+ or Mg2+. Direct binding of Tb3+ to these PECAM peptides was confirmed by ESI-MS. Modeling studies predict that the six cation binding residues within Ig-domain 6 are proximal to each other in three-dimensional space, and may form a single cation coordination site. The identification of cation binding sites in PECAM-1 will direct further work in examining its cation-dependent roles in cellular signaling.

Folding of the beta-propeller domain of the integrin alphaL subunit is independent of the I domain and dependent on the beta2 subunit

We have studied the folding during biosynthesis of the lymphocyte function-associated antigen 1 (LFA-1) alphaL subunit using mAb to epitopes that map to seven different regions within the amino acid sequence. The N-terminal portion of alphaL is predicted to contain a beta-propeller domain, consisting of seven beta-sheets, and an I domain that is predicted to be inserted between beta-sheet 2 and beta-sheet 3 of the beta-propeller. The I domain of alphaL folds before association with the beta2 subunit, as shown by immunoprecipitation of the unassociated alphaL subunit by mAbs specific for four different sequence elements within the I domain. By contrast, the beta-propeller domain is not folded in unassociated alphaL after a chase of as long as 12 h after synthesis, but does fold upon association with beta2. This is shown with mAbs to regions of alphaL, that precede and follow the I domain in the primary structure. A mAb that maps near the junction of the C terminus of the I domain with the beta-propeller domain suggests that this region is partially folded before subunit association. The results show that the I domain and beta-propeller domains fold independently of one another, and suggest that the beta-propeller domain bears an interface for association with the beta subunit.