Eight integrin α-β heterodimers recognize ligands with an Arg-Gly-Asp (RGD) motif. However, the structural mechanism by which integrins differentiate among extracellular proteins with RGD motifs is not understood. Here, crystal structures, mutations and peptide-affinity measurements show that αVβ6 binds with high affinity to a RGDLXXL/I motif within the prodomains of TGF-β1 and TGF-β3. The LXXL/I motif forms an amphipathic α-helix that binds in a hydrophobic pocket in the β6 subunit. Elucidation of the basis for ligand binding specificity by the integrin β subunit reveals contributions by three different βI-domain loops, which we designatespecificity-determining loops (SDLs) 1, 2 and 3. Variation in a pair of single key residues in SDL1 and SDL3 correlates with the variation of the entire β subunit in integrin evolution, thus suggesting a paradigmatic role in overall β-subunit function.
Micronemal protein 2 (MIC2) is the key adhesin that supports glidingmotility and host cell invasion by Toxoplasma gondii. With a von Willebrand factor A (VWA) domain and six thrombospondin repeat domains (TSR1-6) in its ectodomain, MIC2 connects to the parasite actomyosin system through its cytoplasmic tail. MIC2-associated protein (M2AP) binds noncovalently to the MIC2 ectodomain. MIC2 and M2AP are stored in micronemes as proforms. We find that the MIC2-M2AP ectodomain complex is a highly elongated 1:1 monomer with M2AP bound to the TSR6 domain. Crystal structures of N-terminal fragments containing the VWA and TSR1 domains for proMIC2 and MIC2 reveal a closed conformation of the VWA domain and how it associates with the TSR1 domain. A long, proline-rich, disulfide-bonded pigtail loop in TSR1 overlaps the VWA domain. Mannose α-C-linked to Trp-276 in TSR1 has an unusual (1)C4 chair conformation. The MIC2 VWA domain includes a mobile α5-helix and a 22-residue disordered region containing two disulfide bonds in place of an α6-helix. A hydrophobic residue in the prodomain binds to a pocket adjacent to the α7-helix that pistons in opening of the VWA domain to a putative high-affinity state.
When blood vessels are cut, the forces in the bloodstream increase and change character. The dark side of these forces causes hemorrhage and death. However, vonWillebrandfactor (VWF), with help from our circulatory system and platelets, harnesses the same forces to form a hemostatic plug. Force and VWF function are so closely intertwined that, like members of the Jedi Order in the movie Star Wars who learn to use "the Force" to do good, VWF may be considered the Jediknight of the bloodstream. The long length of VWF enables responsiveness to flow. The shape of VWF is predicted to alter from irregularly coiled to extended thread-like in the transition from shear to elongational flow at sites of hemostasis and thrombosis. Elongational force propagated through the length of VWF in its thread-like shape exposes its monomers for multimeric binding to platelets and subendothelium and likely also increases affinity of the A1 domain for platelets. Specialized domains concatenate and compact VWF during biosynthesis. A2 domain unfolding by hydrodynamic force enables postsecretion regulation of VWF length. Mutations in VWF in vonWillebranddisease contribute to and are illuminated by VWF biology. I attempt to integrate classic studies on the physiology of hemostatic plug formation into modern molecular understanding, and point out what remains to be learned.
Carefully soaking crystals with Arg-Gly-Asp (RGD) peptides, we captured eight distinct RGD-bound conformations of the αIIbβ3integrinheadpiece. Starting from the closed βI domain conformation, we saw six intermediate βI conformations and finally the fully open βI with the hybrid domain swung out in the crystal lattice. The β1-α1 backbone that hydrogen bonds to the Asp side chain of RGD was the first element to move followed by adjacent to metal ion-dependent adhesion site Ca(2+), α1 helix, α1' helix, β6-α7 loop, α7 helix, and hybrid domain. We define in atomic detail how conformational change was transmitted over long distances in integrins, 40 Å from the ligand binding site to the opposite end of the βI domain and 80 Å to the far end of the hybrid domain. During these movements, RGD slid in its binding groove toward αIIb, and its Arg side chain became ordered. RGD concentration requirements in soaking suggested a >200-fold higher affinity after opening. The thermodynamic cycle shows how higher affinity pays the energetic cost of opening.
Mucosaladdressincelladhesionmolecule (MAdCAM) binds integrin α4β7. Their interaction directs lymphocyte homing to mucosa-associated lymphoid tissues. The interaction between the two immunoglobulin superfamily (IgSF) domains of MAdCAM and integrin α4β7 is unusual in its ability to mediate either rolling adhesion or firm adhesion of lymphocytes on vascular surfaces. We determined four crystal structures of the IgSF domains of MAdCAM to test for unusual structural features that might correlate with this functional diversity. Higher resolution 1.7- and 1.4-Å structures of the IgSF domains of MAdCAM in a previously described crystal lattice revealed two alternative conformations of the integrin-bindingloop, which were deformed by large lattice contacts. New crystal forms in the presence of two different Fabs to MAdCAM demonstrate a shift in IgSF domain topology from the I2- to I1-set, with a switch ofintegrin-bindingloop from CC' to CD. The I1-setfold and CD loop appear biologically relevant. The different conformations seen in crystal structures suggest that the integrin-bindingloop of MAdCAM is inherently flexible. This contrasts with rigidity of the corresponding loops in vascular celladhesionmolecule, intercellular adhesionmolecule (ICAM)-1, ICAM-2, ICAM-3, and ICAM-5 and may reflect a specialization of MAdCAM to mediate both rolling and firm adhesion by binding to different α4β7 integrin conformations.
Natalizumab antibody to α4-integrins is used in therapy of multiple sclerosis and Crohn's disease. A crystal structure of the Fab bound to an α4 integrin β-propeller and thigh domain fragment shows that natalizumab recognizes human-mouse differences on the circumference of the β-propeller domain. The epitope is adjacent to but outside of a ligand-binding groove formed at the interface with the β-subunit βI domain and shows no difference in structure when bound to Fab. Competition between Fab and the ligand vascular cell adhesion molecule (VCAM) for binding to cell surface α4β1 shows noncompetitive antagonism. In agreement, VCAM docking models suggest that binding of domain 1 of VCAM to α4-integrins is unimpeded by the Fab, and that bound Fab requires a change in orientation between domains 1 and 2 of VCAM for binding to α4β1. Mapping of species-specific differences onto α4β1 and α4β7 shows that their ligand-binding sites are highly conserved. Skewing away from these conserved regions of the epitopes recognized by current therapeutic function-blocking antibodies has resulted in previously unanticipated mechanisms of action.
How is massive conformational change in integrins achieved on a rapid timescale? We report crystal structures of a metastable, putative transition state of integrin αXβ2. The αXβ2 ectodomain is bent; however, a lattice contact stabilizes its ligand-binding αI domain in a high affinity, open conformation. Much of the αI α7 helix unwinds, loses contact with the αI domain, and reshapes to form an internal ligand that binds to the interface between the β propeller and βI domains. Lift-off of the αI domain above this platform enables a range of extensional and rotational motions without precedent in allosteric machines. Movements of secondary structure elements in the β2 βI domain occur in an order different than in β3 integrins, showing that integrin β subunits can be specialized to assume different intermediate states between closed and open. Mutations demonstrate that the structure trapped here is metastable and can enable rapid equilibration between bent and extended-open integrin conformations and up-regulation of leukocyte adhesiveness.
vonWillebrand Factor (VWF) is an ultralong, concatameric, and adhesive glycoprotein. On short time scales, adhesiveness for platelets is activated by elongation of VWF by altered hydrodynamics at sites of hemostasis. Over longer time scales, the length of VWF is regulated by ADAMTS13 (a disintegrin and metalloprotease with a thrombospondin type 1 motif, member 13), cleavage by which in the VWF A2domain is dependent on elongational force. Patients with vonWillebranddisease type 2A present with increased bleeding due to mutations within the VWF A2domain that enhance cleavage. We tested using temperature and force the hypothesis that vonWillebranddiseasemutations disrupt A2 force sensing by destabilizing the folded state. Mutations R1597W, M1528V, and E1638K reduced A2 thermal stability by 10-18 °C. M1528V and E1638K showed a marked further decrease in stability upon calcium removal. In contrast, R1597W, which resides within the A2 calcium-binding loop, exhibited similar stability in the presence and absence of calcium. Using single molecule optical tweezers and R1597W, we measured the force dependence of unfolding and refolding kinetics. In the presence of calcium, the R1597W mutation slowed the rate of refolding but had no effect on unfolding. The three mutations highlight the calcium-binding loop (R1597W), the hydrophobic core around the vicinal disulfide (M1528V), and hydrogen bonds to the α4-less loop (E1638K), as structural features critically important to the function of A2 as a force sensor in regulating thrombogenic activity in the vasculature.
Basic mechanisms by which cellular barriers sense and respond to integrity disruptions remain poorly understood. Despite its tenuous structure and constitutive exposure to disruptive strains, the vascular endothelium exhibits robust barrier function. We show that in response to micrometer-scale disruptions induced by transmigrating leukocytes, endothelial cells generate unique ventrallamellipodia that propagate via integrins toward and across these "micro-wounds" to close them. This novel actin remodeling activity progressively healed multiple micro-wounds in succession and changed direction during this process. Mechanical probe-induced micro-wounding of both endothelia and epithelia suggests that ventrallamellipodia formed as a response to force imbalance and specifically loss of isometric tension. Ventrallamellipodia were enriched in the Rac1 effectors cortactin, IQGAP, and p47Phox and exhibited localized production of hydrogen peroxide. Together with Apr2/3, these were functionally required for effective micro-wound healing. We propose that barrier disruptions are detected as local release of isometric tension/force unloading, which is directly coupled to reactive oxygen species-dependent self-restorative actin remodeling dynamics.
Von Willebrand factor (VWF) is a large, multimeric plasma glycoprotein that critically mediates hemostasis at sites of vascular injury. Very large VWF multimers have the greatest thrombogenic activity, which is attenuated by cleavage in the A2 domain by the metalloproteinase ADAMTS13. ADAMTS13 proteolysis requires mechanical force to expose the scissile bond and is regulated by a calcium-binding site within A2. In this study, we characterized the interaction between VWF A2 and calcium by examining the effect of calcium on VWF A2 stability and mechanical unfolding and refolding. Isothermal calorimetry yielded a calcium binding K(d) = 3.8 ± 1.0 μM and reversible thermal denaturation showed that 5 mM calcium stabilized the unfolding transition from 56.7 ± 0.1 to 69.1 ± 0.1 °C. Using optical tweezers to apply tensile force to single domains, we found that calcium did not affect VWF A2 unfolding, but rather enhanced refolding kinetics fivefold, resulting in a 0.9 kcal/mol stabilization in the folding activation energy in the presence of calcium. Taken together, our data demonstrate that VWF binds calcium at physiologic calcium concentrations and that calcium stabilizes VWF A2 by accelerating refolding.
Glycoprotein-A repetitions predominant protein (GARP) associates with latent transforming growth factor-β (proTGFβ) on the surface of T regulatory cells and platelets; however, whether GARP functions in latent TGFβ activation and the structural basis of coassociation remain unknown. We find that Cys-192 and Cys-331 of GARP disulfide link to the TGFβ1 prodomain and that GARP with C192A and C331A mutations can also noncovalently associate with proTGFβ1. Noncovalent association is sufficiently strong for GARP to outcompete latent TGFβ-binding protein for binding to proTGFβ1. Association between GARP and proTGFβ1 prevents the secretion of TGFβ1. Integrin α(V)β(6) and to a lesser extent α(V)β(8) are able to activate TGFβ from the GARP-proTGFβ1 complex. Activation requires the RGD motif of latent TGFβ, disulfide linkage between GARP and latent TGFβ, and membrane association of GARP. Our results show that GARP is a latent TGFβ-binding protein that functions in regulating the bioavailability and activation of TGFβ.
We study a mechanism by which dimerization of the EGF receptor (EGFR) cytoplasmic domain is transmitted to the ectodomain. Therapeutic and other small molecule antagonists to the kinase domain that stabilize its active conformation, but not those that stabilize an inactive conformation, stabilize ectodomain dimerization. Inhibitor-induced dimerization requires an asymmetric kinase domain interface associated with activation. EGF and kinase inhibitors stimulate formation of identical dimer interfaces in the EGFR transmembrane domain, as shown by disulfide cross-linking. Disulfide cross-linking at an interface in domain IV in the ectodomain was also stimulated similarly; however, EGF but not inhibitors stimulated cross-linking in domain II. Inhibitors similarly induced noncovalent dimerization in nearly full-length, detergent-solubilized EGFR as shown by gel filtration. EGFR ectodomain deletion resulted in spontaneous dimerization, whereas deletion of exons 2-7, in which extracellular domains III and IV are retained, did not. In EM, kinase inhibitor-induced dimers lacked any well defined orientation between the ectodomain monomers. Fab of the therapeutic antibody cetuximab to domain III confirmed a variable position and orientation of this domain in inhibitor-induced dimers but suggested that the C termini of domain IV of the two monomers were in close proximity, consistent with dimerization in the transmembrane domains. The results provide insights into the relative energetics of intracellular and extracellular dimerization in EGFR and have significance for physiologic dimerization through the asymmetric kinase interface, bidirectional signal transmission in EGFR, and mechanism of action of therapeutics.
The lymphocyte homing receptor integrin α(4)β(7) is unusual for its ability to mediate both rolling and firm adhesion. α(4)β(1) and α(4)β(7) are targeted by therapeutics approved for multiple sclerosis and Crohn's disease. Here, we show by electron microscopy and crystallography how two therapeutic Fabs, a small molecule (RO0505376), and mucosal adhesion molecule-1 (MAdCAM-1) bind α(4)β(7). A long binding groove at the α(4)-β(7) interface for immunoglobulin superfamily domains differs in shape from integrin pockets that bind Arg-Gly-Asp motifs. RO0505376 mimics an Ile/Leu-Asp motif in α(4) ligands, and orients differently from Arg-Gly-Asp mimics. A novel auxiliary residue at the metal ion-dependent adhesion site in α(4)β(7) is essential for binding to MAdCAM-1 in Mg(2+) yet swings away when RO0505376 binds. A novel intermediate conformation of the α(4)β(7) headpiece binds MAdCAM-1 and supports rolling adhesion. Lack of induction of the open headpiece conformation by ligand binding enables rolling adhesion to persist until integrin activation is signaled.
Sporozoite gliding motility and invasion of mosquito and vertebrate host cells in malaria is mediated by thrombospondin repeat anonymous protein (TRAP). Tandem von Willebrand factor A (VWA) and thrombospondin type I repeat (TSR) domains in TRAP connect through proline-rich stalk, transmembrane, and cytoplasmic domains to the parasite actin-dependent motility apparatus. We crystallized fragments containing the VWA and TSR domains fromPlasmodium vivax and Plasmodium falciparum in different crystal lattices. TRAP VWA domains adopt closed and open conformations, and bind a Mg2+ ion at a metal ion–dependent adhesion site implicated in ligand binding. Metal ion coordination in the open state is identical to that seen in the open high-affinity state of integrin I domains. The closed VWA conformation associates with a disordered TSR domain. In contrast, the open VWA conformation crystallizes with an extensible β ribbon and ordered TSR domain. The extensible β ribbon is composed of disulfide-bonded segments N- and C-terminal to the VWA domain that are largely drawn out of the closed VWA domain in a 15 Å movement to the open conformation. The extensible β ribbon and TSR domain overlap at a conserved interface. The VWA, extensible β ribbon, and TSR domains adopt a highly elongated overall orientation that would be stabilized by tensile force exerted across a ligand-receptor complex by the actin motility apparatus of the sporozoite. Our results provide insights into regulation of “stick-and-slip” parasite motility and for development of sporozoite subunit vaccines.
Intermolecular contacts between integrin LFA-1 (alpha(L)beta(2)) and ICAM-1 derive solely from the integrin alpha(L) I domain and the first domain (D1) of ICAM-1. This study presents a crystal structure of the engineered complex of the alpha(L) I domain and ICAM-1 D1. Previously, we engineered the I domain for high affinity by point mutations that were identified by a directed evolution approach. In order to examine alpha(L) I domain allostery between the C-terminal alpha7-helix (allosteric site) and the metal-ion dependent adhesion site (active site), we have chosen a high affinity variant without mutations directly influencing either the position of the alpha7-helix or the active sites. In our crystal, the alpha(L) I domain was found to have a high affinity conformation to D1 with its alpha7-helix displaced downward away from the binding interface, recapitulating a current understanding of the allostery in the I domain and its linkage to neighboring domains of integrins in signaling. To enable soluble D1 of ICAM-1 to fold on its own, we also engineered D1 to be functional by mutations, which were found to be those that would convert hydrogen bond networks in the solvent-excluded core into vdW contacts. The backbone structure of the beta-sandwich fold and the epitope for I domain binding of the engineered D1 were essentially identical to those of wild-type D1. Most deviations in engineered D1 were found in the loops at the N-terminal region that interacts with human rhinovirus (HRV). Structural deviation found in engineered D1 was overall in agreement with the function of engineered D1 observed previously, i.e., full capacity binding to alpha(L) I domain but reduced interaction with HRV.
Adaptive immunity requires that T cells efficiently scan diverse cell surfaces to identify cognate Ag. However, the basic cellular mechanisms remain unclear. In this study, we investigated this process using vascular endothelial cells, APCs that possess a unique and extremely advantageous, planar morphology. High-resolution imaging revealed that CD4 memory/effector T cells dynamically probe the endothelium by extending submicron-scale, actin-rich "invadosome/podosome-like protrusions" (ILPs). The intimate intercellular contacts enforced by ILPs consistently preceded and supported T cell activation in response to endothelial MHC class II/Ag. The resulting calcium flux stabilized dense arrays of ILPs (each enriched in TCR, protein kinase C-theta, ZAP70, phosphotyrosine, and HS1), forming what we term a podo-synapse. Similar findings were made using CD8 CTLs on endothelium. Furthermore, careful re-examination of both traditional APC models and professional APCs suggests broad relevance for ILPs in facilitating Ag recognition. Together, our results indicate that ILPs function as sensory organelles that serve as actuators of immune surveillance.
Developmental endothelial cell locus-1 (Del-1) glycoprotein is secreted by endothelial cells and a subset of macrophages. Del-1plays a regulatory role in vascular remodeling and functions in innate immunity through interaction with integrin α(V)β(3). Del-1contains 3 epidermal growth factor (EGF)-like repeats and 2 discoidin-like domains. An Arg-Gly-Asp (RGD) motif in the second EGF domain (EGF2) mediates adhesion by endothelial cells and phagocytes. We report the crystal structure of its 3 EGF domains. TheRGD motif of EGF2 forms a type II' β turn at the tip of a long protruding loop, dubbed the RGDfinger. Whereas EGF2 and EGF3 constitute a rigid rod via an interdomain calcium ion binding site, the long linker between EGF1 and EGF2 lends considerable flexibility to EGF1. Two unique O-linked glycans and 1 N-linked glycan locate to the opposite side of EGF2 from the RGD motif. These structural features favor integrinbinding of the RGDfinger. Mutagenesis data confirm the importance of having the RGD motif at the tip of the RGDfinger. A database search for EGF domain sequences shows that this RGDfinger is likely an evolutionary insertion and unique to the EGF domain of Del-1 and its homologue milk fat globule-EGF 8.
In the present study, we re-annotated von Willebrand factor (VWF), assigned its entire sequence to specific modules, and related these modules to structure using electron microscopy (EM). The D domains are assemblies of smaller modules visible as lobes in EM. Modules in the D-domain assemblies include von Willebrand D, 8-cysteine, trypsin inhibitor-like, E or fibronectin type 1-like domains, and a unique D4N module in D4. The D1-D2 prodomain shows 2 large connected assemblies, each containing smaller lobes. The previous B and C regions of VWF are re-annotated as 6 tandem von Willebrand C (VWC) and VWC-like domains. These 6 VWC domains correspond to 6 elongated domains that associate in pairs at acidic pH in the stem region of VWF dimeric bouquets. This correspondence is demonstrated by binding of integrin αIIbβ3 to the fourth module seen in EM, VWC4, which bears the VWF Arg-Gly-Asp motif. The C-terminal cystine knot domain dimerizes end-to-end in a manner predicted by homology to TGF-β and orients approximately perpendicular to the VWC domains in dimeric bouquets. Homologies of domains in VWF to domains in other proteins allow many disulfide bonds to be tentatively assigned, which may have functional implications.