Ligand Binding to Integrins

Abstract

ligand competent metal ion-dependent adhesion site monoclonal antibody intracellular adhesion molecule vascular cell adhesion molecule mucosal addressin cell adhesion molecule The “integrin” terminology was applied in a 1987 review article (1.Hynes R.O. Cell. 1987; 48: 549-550Abstract Full Text PDF PubMed Scopus (3579) Google Scholar) to describe a family of structurally, immunochemically, and functionally related cell-surface heterodimeric receptors, which integrated the extracellular matrix with the intracellular cytoskeleton to mediate cell migration and adhesion. The three original β subunits (β1, β2, and β3) identified have now expanded to eight, and the number of α subunits stands at 17. These subunits interact noncovalently in a restricted manner to form more than 20 family members. The diversity of integrins is expanded further by alternative splicing, post-translational modifications, and interactions with other cell-surface and intracellular molecules (2.Green L.J. Mould A.P. Humphries M.J. Int. J. Biochem. Cell Biol. 1998; 30: 179-184Crossref PubMed Scopus (48) Google Scholar, 3.Porter J.C. Hogg N. Trends Cell Biol. 1998; 8: 390-396Abstract Full Text Full Text PDF PubMed Scopus (208) Google Scholar, 4.De Melker A.A. Sonnenberg A. Bioessays. 1999; 21: 499-509Crossref PubMed Scopus (109) Google Scholar). The number of integrins and the remarkable breadth of their cellular distribution support the statement that the phenotype of virtually every cell is uniquely influenced by its display of integrins. Over the past 13 years, more than 14,000 scientific articles have dealt with various aspects of integrin biology and almost 1,000 have appeared in theJournal of Biological Chemistry. This article examines a central aspect of integrin biology: ligand recognition and the structural basis for this function. A hallmark of the integrins is the ability of individual family members to recognize multiple ligands. Indeed, the extent of the integrin family pales in comparison with the number of their ligands. Table Isummarizes the major extracellular ligands of integrins; the listing is undoubtedly incomplete. The list includes a large number of extracellular matrix proteins (bone matrix proteins, collagens, fibronectins, fibrinogen, laminins, thrombospondins, vitronectin, and von Willebrand factor), reflecting the primary function of integrins in cell adhesion to extracellular matrices. Many “counter-receptors” are ligands, reflecting the role of integrins in mediating cell-cell interactions. Included are numerous microorganisms, which utilize integrins to gain entry into cells. There are direct and multiple linkages between integrins and host defense systems, created by their recognition of hemostatic and complement factors. The preference of any given integrin among its ligands is determined by relative affinity, availability within a specific microenvironment, and the conformational state of the ligand, which controls exposure of its integrin recognition sequence.Table IIntegrin extracellular ligandsLigandIntegrinAdenovirus penton base proteinαvβ3, αvβ5Bone sialoproteinαvβ3, αvβ5Borrelia burgdorferiαIIbβ3Candida albicansαMβ2Collagensα1β1, α2β1, α11β1, αIbβ3Denatured collagenα5β1, αvβ3, αIIbβ3Cytotactin/tenascin-Cα8β1, α9β1, αvβ3, αvβ6DecorsinαIIbβ3Disintegrinsαvβ3, αIIbβ3E cadherinαEβ7Echovirus 1α2β1Epiligrinα3β1Factor XαMβ2Fibronectinα2β1, α3β1, α4β1, α4β7, α5β1, α8β1, αvβ1, αvβ3, αvβ5, αvβ6, αvβ8, αIIbβ3Fibrinogenα5β1, αMβ2, αvβ3, αxβ2, αIIbβ3HIV Tat proteinαvβ3, αvβ5iC3bαMβ2, αxβ2ICAM-1αLβ2, αMβ2ICAM-2,3,4,5αLβ2Invasinα3β1, α4β1, α5β1, α6β1Lamininα1β1, α2β1, α6β1, α7β1, α6β4, αvβ3MAdCAM-1α4β7Matrix metalloproteinase-2αvβ3Neutrophil inhibitory factorαMβ2Osteopontinαvβ3PlasminogenαIIbβ3Prothrombinαvβ3, αIIbβ3Sperm fertilinα6β1Thrombospondinα3β1, αvβ3, αIIbβ3VCAM-1α4β1, α4β7Vitronectinαvβ1, αvβ3, αvβ5, αIIbβ3von Willebrand factorαvβ3, αIIbβ3 Open table in a new tab A primary goal of many structure-function analyses in the integrin field has been the reduction of macromolecular ligands to minimal recognition sequences. This endeavor has been highly successful, and many bioactive amino acid sequences have been teased out of large extracellular matrix proteins (5.Ruoslahti E. Annu. Rev. Cell Biol. 1996; 12: 697-715Crossref Scopus (2633) Google Scholar). The prototypic example is the RGD sequence. RGD was originally identified as the sequence in fibronectin that engages the fibronectin receptor, integrin α5β1, but now is known to serve as a recognition motif in multiple ligands for several different integrins (see Table II). Although RGD peptides inhibit ligand binding to integrins with an RGD recognition specificity (Table II), these receptors can discriminate among RGD-containing ligands. The context of the RGD sequence (flanking residues, three-dimensional presentation, and individual features of the integrin binding pockets) determine whether productive interactions occur (6.Haas T.A. Plow E.F. Curr. Opin. Cell Biol. 1994; 6: 656-662Crossref PubMed Scopus (273) Google Scholar). As an illustrative example of the nuances of the RGD recognition specificity, whereas both of the β3 integrins, αIIbβ3 and αVβ3, recognize fibrinogen, which contains multiple RGD sequences, and RGD peptides inhibit the binding of fibrinogen to these integrins, both integrins can recognize other sequences in fibrinogen (7.Yokoyama K. Zhang X.-P. Medved L. Takada Y. Biochemistry. 1999; 38: 5872-5877Crossref PubMed Scopus (75) Google Scholar, 8.Farrell D.H. Thiagarajan P. Chung D.W. Davie E.W. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 10729-10732Crossref PubMed Scopus (307) Google Scholar). Thus, recognition of this seemingly simple tripeptide sequence is complex. A second set of fibronectin sequences also has received considerable attention: those recognized by α4β1. Originally, the CS-1 sequence, which resides in an alternatively spliced segment of fibronectin, was determined to be a recognition site, but now several additional fibronectin sequences have been identified that interact with α4β1 (9.Wayner E.A. Garcia-Pardo A. Humphries M.J. McDonald J.A. Carter W.G. J. Cell Biol. 1989; 109: 1321-1330Crossref PubMed Scopus (773) Google Scholar, 10.Mould A.P. Komoriya A. Yamada K.M. Humphries M.J. J. Biol. Chem. 1991; 266: 3579-3585Abstract Full Text PDF PubMed Google Scholar, 11.Domı́nguez-Jiménez C. Sánchez-Aparicio P. Albar J.P. Garcı́a-Pardo A. Cell Adhes. Commun. 1996; 4: 251-267Crossref PubMed Scopus (18) Google Scholar). Multiple recognition sites also exist in fibrinogen for αMβ2 (12.Ugarova T.P. Solovjov D.A. Zhang L. Loukinov D.I. Yee V.C. Medved L.V. Plow E.F. J. Biol. Chem. 1998; 273: 22519-22527Abstract Full Text Full Text PDF PubMed Scopus (138) Google Scholar). Two generalizations can be derived from these examples: 1) integrin recognition specificities can often be reduced to small peptide sequences; and 2) peptide inhibition studies need to be complemented with other approaches to assess the role of specific sequences in ligand recognition by integrins.Table IIIntegrin recognition sequencesRecognition sequenceLigandIntegrinRGDAdenovirus penton base protein, bone sialoprotein, collagen, decorsin, disintegrins, fibrinogen, fibronectin, prothrombin, tenascin, thrombospondin, vitronectin, von Willebrand factorα3β1, α5β1, α8β1, αvβ1, αvβ3, αvβ5, αvβ6, αIIbβ3HHLGGAKQAGDVγ-Chain of fibrinogenαIIbβ3GPRα-Chain of fibrinogenαxβ2P1 peptideγ-Chain of fibrinogenαMβ2P2 peptideγ-Chain of fibrinogenαMβ2AEIDGIELTenascinα9β1QIDSVCAM-1α4β1LDTMAdCAM-1α4β7CS-1 peptideFibronectinα4β1, α4β7CS-5 peptideFibronectinα4β1IDAPSFibronectinα4β1ICAM peptidesICAM-1, -2, -3αLβ2, αMβ2DLXXLTenascinαvβ6GFOGER aO, hydroxyproline.Collagenα1β1, α2β1a O, hydroxyproline. Open table in a new tab Each integrin heterodimer contains 3–5 divalent cation binding sites of relatively low affinity (μm−1 to mm−1), and the bound cations exert profound effects on integrin function. Collectively, these bound divalent ions can act as effectors, promoting ligand binding; as antagonists, inhibiting ligand binding; and as selectors, changing the ligand binding specificity. One proposal to explain the influential role of cations on integrin function is that ligand and divalent cation share a common binding pocket on the integrin. This hypothesis was supported by data showing that RGD ligands could displace two receptor-bound metal ions and that divalent ion and RGD peptide could bind, in a mutually exclusive manner, a peptide from the β3 subunit (13.D'Souza S.E. Haas T.A. Piotrowicz R.S. Byers-Ward V. McGrath D.E. Soule H.R. Cierniewski C.S. Plow E.F. Smith J.W. Cell. 1994; 79: 659-667Abstract Full Text PDF PubMed Scopus (214) Google Scholar). Thus, a “displacement model” was proposed, in which RGD ligands initially form a ternary complex with receptor-bound divalent ion; then, as contacts between RGD and integrin stabilize, the divalent ion may be displaced. Recently, this model was extended to other integrins (14.Dickeson S.K. Bhattacharyya-Pakrasei M. Mathis N.L. Schlesinger P.H. Santoro S.A. Biochemistry. 1998; 37: 11280-11288Crossref PubMed Scopus (33) Google Scholar); collagen displaced Tb3+bound to the I domain of the α2 subunit. Dissection of the ligand binding reaction into ligand association and dissociation steps provided further insights into the roles of divalent ions in integrin function (15.Hu D.D. Barbas III, C.F. Smith J.W. J. Biol. Chem. 1996; 271: 21745-21751Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar). Using surface plasmon resonance, the β3 integrins were shown to contain two classes of ion binding sites. One class must be occupied for ligand to bind, ligand-competent (LC)1 sites; and the second class has an inhibitory effect on ligand binding, I sites. The I site(s) display specificity for Ca2+ and increase the rate of ligand dissociation. Because the I sites are allosteric to the ligand binding pocket, they can bind Ca2+even when ligand is prebound to integrin, providing a potential mechanism for the release of pre-existing cell-matrix contacts. Thus, it is the coordination between the LC and I cation binding sites that regulates the ligand binding event. There are at least two structurally distinct classes of ion binding motifs within integrins. A series of EF-hand-like domains are present in each of the integrin α subunits (16.Tuckwell D.S. Brass A. Humphries M.J. Biochem. J. 1992; 285: 325-331Crossref PubMed Scopus (101) Google Scholar). The integrin EF-hand loops lack a glutamate that is found at the 12th position in virtually all other EF-hand loops and is one of the ligands for Ca2+. The absence of this residue in integrins is likely to explain their lower affinity and selectivity for divalent ions. Two studies have examined the ion and ligand binding function of recombinant fragments containing the integrin EF-hands. Gulino et al. (17.Gulino D. Boudignon C. Zhang L.Y. Concord E. Rabiet M.J. Marguerie G. J. Biol. Chem. 1992; 267: 1001-1007Abstract Full Text PDF PubMed Google Scholar) produced a fragment composed of the four EF-hand sites within the αIIbsubunit and found that it contained two affinity classes for Ca2+, which could also bind Mg2+ and Mn2+ and fibrinogen, a physiologic ligand for this integrin. These observations are generally consistent with results obtained from Ca2+ binding studies on the purified integrin (18.Rivas G.A. Gonzalez-Rodriguez J. Biochem. J. 1991; 276: 35-40Crossref PubMed Scopus (55) Google Scholar) and synthetic peptides corresponding to the individual loops of each EF-hand (19.Cierniewski C.S. Haas T.A. Smith J.W. Plow E.F. Biochemistry. 1994; 33: 12238-12246Crossref PubMed Scopus (37) Google Scholar). The EF-hand domains of the α5 integrin also contain two affinity classes of ion binding sites and can bind fibronectin and RGD peptides (20.Baneres J.L. Roquet F. Green M. LeCalvez H. Parello J. J. Biol. Chem. 1998; 273: 24744-24753Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar). All four EF-hands were required for ligand binding, even though each pair of EF-hands was able to bind divalent ion. The second type of cation binding site in integrins is a metal ion-dependent adhesion site (MIDAS) motif. The first evidence for a unique cation binding motif came from mutagenesis studies of the I domain of the αM subunit (21.Michishita M. Videm V. Arnaout M.A. Cell. 1993; 72: 857-867Abstract Full Text PDF PubMed Scopus (346) Google Scholar). Soon thereafter, the I domain of the αM subunit and other integrin α subunits were crystallized (22.Lee J.-O. Rieu P. Arnaout M.A. Liddington R. Cell. 1995; 80: 631-638Abstract Full Text PDF PubMed Scopus (816) Google Scholar, 23.Qu A. Leahy D.J. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 10277-10281Crossref PubMed Scopus (291) Google Scholar, 24.Emsley J. King S.L. Bergelson J.M. Liddington R.C. J. Biol. Chem. 1997; 272: 28512-28517Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar). In Fig.1, the crystal structures of two I domains are displayed with the MIDAS motif at their upper surface. Within the MIDAS motif, five separate residues coordinate the divalent ion. The first three are closely spaced within a DXSXS motif; the fourth is a threonine separated from the DXSXS in the primary structure by ∼70 residues; and the fifth coordinating ligand is an aspartate about 100 residues downstream of the DXSXS. In the crystal structure, two of the αMI domains were linked via a Mg2+ ion in the MIDAS motif, and a glutamate from one I domain donated a sixth coordinating ligand to the Mg2+bound in an adjacent I domain. This quirk in the crystal structure provided evidence that metal ion bound to the MIDAS can ligand with carboxylates donated from another protein, consistent with the cation displacement model. Indeed, this finding has prompted the hypothesis that such a structure is a snapshot of I domain bound with “ligand” and led to the prevailing notion that integrins bind to their ligands by “grabbing an Asp” (25.Bergelson J.M. Hemler M.E. Curr. Biol. 1995; 5: 615-617Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar). All integrin β subunits may contain an ion binding site homologous to a MIDAS motif. This proposition stems from early work showing that a naturally occurring mutation of Y119D in integrin αIIbβ3 led to a receptor with abnormal ligand and cation binding functions (26.Loftus J.C. O'Toole T.E. Plow E.F. Glass A. Frelinger A.L. Ginsberg M.H. Science. 1990; 249: 915-918Crossref PubMed Scopus (390) Google Scholar), and it was proposed that this residue was part of an EF-hand. In retrospect, this ion binding site is more likely to be a MIDAS motif with Asp-119 being the first residue of the DXSXS motif. of any of these residues within the β2, subunits ligand binding to integrins McDonald S. Smith J.W. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google and Takada Y. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). Although the DXSXS motif to ligand with the residues that the fourth and fifth coordinating ligands in the β subunit MIDAS and McDonald S. Smith J.W. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, Takada Y. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar, Liddington R.C. M.J. J.C. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). The three-dimensional structure of a β subunit may be for is structural to the I Ca2+ binding site that is found on integrins. All of the ion binding sites that have been the EF-hand sites in the α subunits and the MIDAS motifs in the β to ion binding, are LC sites (15.Hu D.D. Barbas III, C.F. Smith J.W. J. Biol. Chem. 1996; 271: 21745-21751Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar, McDonald S. Smith J.W. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, D.D. S. N. Smith J.W. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). One of the more the of this I site is that the ion binding site within the integrin β subunits can into an EF-hand a MIDAS domain. may act as a MIDAS when Mn2+ but Ca2+ could an EF-hand the of divalent ions on cell adhesion and ligand binding to integrins, on the physiologic role of the ion binding sites is One is physiologic is the adhesion of The to the bone surface J. D. S.L. D.A. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar). to the bone surface must be to as bone is the of Ca2+ the and and of This effect may be by the allosteric I Ca2+ binding site on the of Ca2+ and Mg2+ is in the of J. 1995; PubMed Scopus Google Scholar). the the of Mg2+ and the of from the of to Because Mg2+ generally cell adhesion and Ca2+ is generally this increase in may be the ion binding sites also may a role in integrin Mn2+ the of multiple integrins for their ligands. Mn2+ to MIDAS and two of the MIDAS can be by the manner in which the bound metal is J.-O. Arnaout M.A. Liddington R.C. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, R. Rieu P. D. Arnaout M.A. J. Cell Biol. 1998; PubMed Scopus Google Scholar). Thus, Mn2+ could structural in the I which in hypothesis that is by the binding of Ca2+ to the allosteric I of this site by Ca2+ the integrin in a The of four different integrin receptors, αMβ2 and These integrins are in virtually every aspect of the adhesion to and the of and The of the integrins is by the of these integrins to (see G.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, and of and to and Recently, it has to the function of individual integrin receptors in in αMβ2 and The integrins are the of a separate review in this The on their I domains and their role in ligand The α subunits of all integrins contain of amino the I A domain. I domains are found in several other integrin α subunits and other proteins, such as and complement I domains mediate and in integrins, they are in the binding of ligands (14.Dickeson S.K. Bhattacharyya-Pakrasei M. Mathis N.L. Schlesinger P.H. Santoro S.A. Biochemistry. 1998; 37: 11280-11288Crossref PubMed Scopus (33) Google Scholar, J.-O. Rieu P. Arnaout M.A. Liddington R. Cell. 1995; 80: 631-638Abstract Full Text PDF PubMed Scopus (816) Google Scholar, Hogg N. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar). I those in integrin α can be as recombinant and can bind ligands. though I domains are highly homologous to each they are highly for of ligands. the a I domain can recognize multiple and structurally ligands (see ligand of αMβ2 in Table Thus, it is the individual amino acid within the highly structural of the I domains that ligand specificity. The crystal structures of several integrin and I domains have now been J.-O. Rieu P. Arnaout M.A. Liddington R. Cell. 1995; 80: 631-638Abstract Full Text PDF PubMed Scopus (816) Google Scholar, 23.Qu A. Leahy D.J. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 10277-10281Crossref PubMed Scopus (291) Google Scholar, 24.Emsley J. King S.L. Bergelson J.M. Liddington R.C. J. Biol. Chem. 1997; 272: 28512-28517Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar). Each of five β by α which are by loops (see The I domains of the integrins contain the cation binding MIDAS motif. In the I domains of αMβ2 and the binding for several ligands has been to the upper in to the bound cation R. Rieu P. D. Arnaout M.A. J. Cell Biol. 1998; PubMed Scopus Google Scholar, L. Plow E.F. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). in other of the I domains the I domains can exert allosteric on ligand binding R. Rieu P. D. Arnaout M.A. J. Cell Biol. 1998; PubMed Scopus Google Scholar, L. Plow E.F. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus (48) Google Scholar, C. C. T.A. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). Although the I domains the ligand binding functions of their integrins, other of the α subunits ligand As in αMβ2 a an the I domain but in the αM subunit ligand binding V. M. R.C. J. 1996; Google Scholar); and the EF-hand in and α2β1, integrins with I domains in their α to ligand recognition P. J. Hogg N. J. 1994; PubMed Scopus Google Scholar, S.K. Santoro S.A. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). The αM and other α contains a which is in of ligands, and may the function of the I domain V. J. 1996; PubMed Scopus Google Scholar). The role of the β subunit in ligand binding to the integrins is the of the β subunit in ligand recognition has been and mutagenesis S.L. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar), its direct in ligand has to be in the which ligand binding, may exert effect on ligand binding Plow E.F. Zhang L. J. 1998; Google Scholar). The central role that in has on αIIbβ3 recognition sequences within its ligands and the ligand within the Many of the insights from these studies to the several of the and both β3 integrins share a RGD recognition specificity. A second recognition specificity of to the function of αIIbβ3 is the of the fibrinogen M. S. J. Biochem. Commun. PubMed Scopus Google Scholar). Each sequence contains an acid that is for an with receptor-bound cation (26.Loftus J.C. O'Toole T.E. Plow E.F. Glass A. Frelinger A.L. Ginsberg M.H. Science. 1990; 249: 915-918Crossref PubMed Scopus (390) Google Scholar). The two recognition peptides inhibit the binding of each other to αIIbβ3 but may bind to separate but linked sites D.D. S. N. Smith J.W. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, C.S. M. Haas T.A. J. Zhang L. M. Plow E.F. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). data have the of potential ligand sites within the of inhibitory of and the of recombinant of αIIbβ3 have that the minimal ligand binding fragment contains the of each subunit J.C. Smith J.W. Ginsberg M.H. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar). The specificity of αIIbβ3 for ligands was to the residues of J.C. Ginsberg M.H. J.A. Smith J.W. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). studies have the of ligand in the of S.E. Ginsberg M.H. T.A. Plow E.F. J. Biol. Chem. 1990; Full Text PDF PubMed Google Scholar) and S.A. Cell. 1987; 48: Full Text PDF PubMed Scopus Google Scholar, S.E. Ginsberg M.H. Plow E.F. J. Biol. Chem. 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Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, M. Concord E. J. M. A. Marguerie G. Gulino D. 1996; PubMed Google Scholar). of the ligand contacts in have a role for its As a recombinant fragment of containing the four EF-hand-like sequences fibrinogen in a and manner (17.Gulino D. Boudignon C. Zhang L.Y. Concord E. Rabiet M.J. Marguerie G. J. Biol. Chem. 1992; 267: 1001-1007Abstract Full Text PDF PubMed Google Scholar). This of has been further in ligand binding ligand peptides within the second cation binding S.E. Ginsberg M.H. T.A. Plow E.F. J. Biol. Chem. 1990; Full Text PDF PubMed Google Scholar). from this inhibit fibrinogen binding to αIIbβ3 and bind fibrinogen in a manner S.E. Ginsberg M.H. Plow E.F. 1991; PubMed Scopus Google Scholar, J. Biol. Chem. 1992; 267: Full Text PDF PubMed Google Scholar). these data a of the cation binding domains in ligand interactions. a form of to which contain any of the divalent cation with β3 and recognized ligand D.S. S. D. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). have been at a structural model of integrin α A model that the the of a domain T.A. Proc. Natl. Acad. Sci. U. S. A. 1997; PubMed Scopus Google Scholar). These domains contain in a a with known have their sites at the of the adjacent loops in with this the to in one and in a second both to be at the of the have been in ligand binding by mutagenesis A. M. Takada Y. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar, Ginsberg M.H. J.C. 1999; PubMed Google Scholar). The of residues for ligand binding to α4β1 and the model Takada Y. Biochem. J. 1995; PubMed Scopus Google Scholar, A.P. L. Humphries M.J. J. Biol. Chem. 1998; 273: Full Text Full Text PDF PubMed Scopus Google Scholar). this model to the data the direct of the and α5 cation binding motifs in ligand The model these motifs on the lower surface of the from the ligand on the upper surface of the The data that the ligand binding pocket of of both the and β3 subunits and is consistent with the that binding of macromolecular ligands multiple in the model 2) is that αIIbβ3 has at least two distinct ligand binding domains that are linked and αIIbβ3 conformational and ligand it be to these the individual ligand binding domains as as these the manner in which the and each other to a structural basis for ligand binding to