Magnesium Activates Microsecond Dynamics to Regulate Integrin-Collagen Recognition.

Integrin receptors bind collagen via metal-mediated interactions that are modulated by magnesium (Mg2+) levels in the extracellular matrix. Nuclear magnetic resonance-based relaxation experiments, isothermal titration calorimetry, and adhesion assays reveal that Mg2+ functions as both a structural anchor and dynamic switch of the α1ß1 integrin I domain (α1I). Specifically, Mg2+ binding activates micro- to millisecond timescale motions of residues distal to the binding site, particularly those surrounding the salt bridge at helix 7 and near the metal ion-dependent adhesion site. Mutagenesis of these residues impacts α1I functional activity, thereby suggesting that Mg-bound α1I dynamics are important for collagen binding and consequent allosteric rearrangement of the low-affinity closed to high-affinity open conformation. We propose a multistep recognition mechanism for α1I-Mg-collagen interactions involving both conformational selection and induced-fit processes. Our findings unravel the multifaceted role of Mg2+ in integrin-collagen recognition and assist in elucidating the molecular mechanisms by which metals regulate protein-protein interactions.

Targeting α1 inserted domain (I) of α1ß1 integrin by Lebetin 2 from M. lebetina transmediterranea venom decreased tumorigenesis and angiogenesis.

Through the recent development of knowledge in biotechnology and bioinformatics, snake venoms are widely used to develop new drugs to treat diseases such as hypertension and cancer. We have previously reported that Lebetin 2 isolated from Macrovipera lebetina transmediterranea venom displays a potent anti-platelet activity and exerts a cardioprotective effect in ischemia-reperfusion (IR) injury model. Here, we report that Lebetin 2 possess an anti-tumor effect by targeting the integrin receptor function. It was thus able to inhibit both adhesion and migration of pheochromocytoma cells (PC12) and α1ß1 integrin-expressing CHO cells (CHO-α1) to type I and IV collagens. Moreover, this peptide affects proliferation of PC12 cells by modulating AKT phosphorylation. Furthermore, Lebetin 2 exhibits a potent anti-angiogenic effect as assessed in vitro and ex vivo, using both the embryo chick chorioallantoic membrane model (CAM) and rat aortic ring assay. Interestingly, the interaction mode of Lebetin 2 with the integrin α1ß1, assessed in silico, showed that the peptide represents a steric obstruction preventing the collagen from enforcing the interactions with the integrin.

The binding capacity of α1ß1-, α2ß1- and α10ß1-integrins depends on non-collagenous surface macromolecules rather than the collagens in cartilage fibrils.

Interactions of cells with supramolecular aggregates of the extracellular matrix (ECM) are mediated, in part, by cell surface receptors of the integrin family. These are important molecular components of cell surface-suprastructures regulating cellular activities in general. A subfamily of ß1-integrins with von Willebrand-factor A-like domains (I-domains) in their α-chains can bind to collagen molecules and, therefore, are considered as important cellular mechano-receptors. Here we show that chondrocytes strongly bind to cartilage collagens in the form of individual triple helical molecules but very weakly to fibrils formed by the same molecules. We also find that chondrocyte integrins α1ß1-, α2ß1- and α10ß1-integrins and their I-domains have the same characteristics. Nevertheless we find integrin binding to mechanically generated cartilage fibril fragments, which also comprise peripheral non-collagenous material. We conclude that cell adhesion results from binding of integrin-containing adhesion suprastructures to the non-collagenous fibril periphery but not to the collagenous fibril cores. The biological importance of the well-investigated recognition of collagen molecules by integrins is unknown. Possible scenarios may include fibrillogenesis, fibril degradation and/or phagocytosis, recruitment of cells to remodeling sites, or molecular signaling across cytoplasmic membranes. In these circumstances, collagen molecules may lack a fibrillar organization. However, other processes requiring robust biomechanical functions, such as fibril organization in tissues, cell division, adhesion, or migration, do not involve direct integrin-collagen interactions.

Biological activity of peptide-conjugated polyion complex matrices consisting of alginate and chitosan.

Peptide-conjugated polysaccharide matrices using bioactive laminin-derived peptides are useful biomaterials for tissue and cell engineering. Here, we demonstrate an easy handling preparation method for peptide-polysaccharide matrices using polyion complex with both alginate and chitosan. First, aldehyde-alginate was synthesized by oxidization of alginate using NaIO4 , and then, reacted with Cys-peptides. Next, the peptide-alginate solution was added to a chitosan-coated plate, and the peptide-polyion complex matrices (peptide-PCMs) were prepared. The peptide-PCMs using an integrin αvß3-binding peptide (A99a: ALRGDN, mouse laminin α1 chain 1145-1150) and an integrin α2ß1-binding peptide (EF1XmR: RLQLQEGRLHFXFD, X = Nle, mouse laminin α1 chain 2751-2763) showed strong cell attachment activity in a dose-dependent manner. When we examined the effect of various spacers on the biological activity of A99a-PCM, hydrophobic and long spacers enhanced the cell attachment activity. Further, the A99a-PCM with the spacers strongly promoted neurite outgrowth. The polyion complex method is an easy way to obtain insolubilized matrix and is widely applicable for various polysaccharides. The peptide-PCM is useful as a biomaterial for cell and tissue engineering.

The synthesis and coupling of photoreactive collagen-based peptides to restore integrin reactivity to an inert substrate, chemically-crosslinked collagen.

Collagen is frequently advocated as a scaffold for use in regenerative medicine. Increasing the mechanical stability of a collagen scaffold is widely achieved by cross-linking using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS). However, this treatment consumes the carboxylate-containing amino acid sidechains that are crucial for recognition by the cell-surface integrins, abolishing cell adhesion. Here, we restore cell reactivity to a cross-linked type I collagen film by covalently linking synthetic triple-helical peptides (THPs), mimicking the structure of collagen. These THPs are ligands containing an active cell-recognition motif, GFOGER, a high-affinity binding site for the collagen-binding integrins. We end-stapled peptide strands containing GFOGER by coupling a short diglutamate-containing peptide to their N-terminus, improving the thermal stability of the resulting THP. A photoreactive Diazirine group was grafted onto the end-stapled THP to allow covalent linkage to the collagen film upon UV activation. Such GFOGER-derivatized collagen films showed restored affinity for the ligand-binding I domain of integrin α2ß1, and increased integrin-dependent cell attachment and spreading of HT1080 and Rugli cell lines, expressing integrins α2ß1 and α1ß1, respectively. The method we describe has wide application, beyond collagen films or scaffolds, since the photoreactive diazirine will react with many organic carbon skeletons.

In Situ Quantification of Surface Chemistry in Porous Collagen Biomaterials.

Cells inside a 3D matrix (such as tissue extracellular matrix or biomaterials) sense their insoluble environment through specific binding interactions between their adhesion receptors and ligands present on the matrix surface. Despite the critical role of the insoluble matrix in cell regulation, there exist no widely-applicable methods for quantifying the chemical stimuli provided by a matrix to cells. Here, we describe a general-purpose technique for quantifying in situ the density of ligands for specific cell adhesion receptors of interest on the surface of a 3D matrix. This paper improves significantly the accuracy of the procedure introduced in a previous publication by detailed marker characterization, optimized staining, and improved data interpretation. The optimized methodology is utilized to quantify the ligands of integrins α 1 ß 1, α 2 ß 1 on two kinds of matched porous collagen scaffolds, which are shown to possess significantly different ligand density, and significantly different ability to induce peripheral nerve regeneration in vivo. Data support the hypothesis that cell adhesion regulates contractile cell phenotypes, recently shown to be inversely related to organ regeneration. The technique provides a standardized way to quantify the surface chemistry of 3D matrices, and a means for introducing matrix effects in quantitative biological models.

Influence of canonical structure determining residues on antibody affinity and stability.

A number of light and heavy chain canonical residue core redesigns were made in a therapeutic antibody (AQC2, anti-VLA1) Fab to explore the consequences to binding affinity and stability. These positions are all loop supporting, primarily CDR1 residues which do not directly contact the antigen. Structure based methods were used with and without consensus sequence information. 30 constructs were made, 24 expressed, and 70% of the designs using consensus sequence information retained binding affinity. Some success maintaining stability with more extreme redesigns suggests a surprising tolerance to mutation, though it often comes at the cost of loss of binding affinity and presumed loop conformation changes. In concordance with the expected need to present an ordered surface for binding, a relationship between decreased affinity and decreased stability was observed. Overpacking the core tends to destabilize the molecule and should be avoided.

Molecular mechanism of T-cell protein tyrosine phosphatase (TCPTP) activation by mitoxantrone.

T-cell protein tyrosine phosphatase (TCPTP) is a ubiquitously expressed non-receptor protein tyrosine phosphatase. It is involved in the negative regulation of many cellular signaling pathways. Thus, activation of TCPTP could have important therapeutic applications in diseases such as cancer and inflammation. We have previously shown that the α-cytoplasmic tail of integrin α1ß1 directly binds and activates TCPTP. In addition, we have identified in a large-scale high-throughput screen six small molecules that activate TCPTP. These small molecule activators include mitoxantrone and spermidine. In this study, we have investigated the molecular mechanism behind agonist-induced TCPTP activation. By combining several molecular modeling and biochemical techniques, we demonstrate that α1-peptide and mitoxantrone activate TCPTP via direct binding to the catalytic domain, whereas spermidine does not interact with the catalytic domain of TCPTP in vitro. Furthermore, we have identified a hydrophobic groove surrounded by negatively charged residues on the surface of TCPTP as a putative binding site for the α1-peptide and mitoxantrone. Importantly, these data have allowed us to identify a new molecule that binds to TCPTP, but interestingly cannot activate its phosphatase activity. Accordingly, we describe here mechanism of TCPTP activation by mitoxantrone, the cytoplasmic tail of α1-integrin, and a mitoxantrone-like molecule at the atomic level. These data provide invaluable insight into the development of novel TCPTP activators, and may facilitate the rational discovery of small-molecule cancer therapeutics.

Enhancing integrin α1 inserted (I) domain affinity to ligand potentiates integrin α1ß1-mediated down-regulation of collagen synthesis.

Integrin α1ß1 binding to collagen IV, which is mediated by the α1-inserted (I) domain, down-regulates collagen synthesis. When unligated, a salt bridge between Arg(287) and Glu(317) is thought to keep this domain in a low affinity conformation. Ligand binding opens the salt bridge leading to a high-affinity conformation. How modulating integrin α1ß1 affinity alters collagen homeostasis is unknown. To address this question, we utilized a thermolysin-derived product of the α1α2α1 network of collagen IV (α1α2α1(IV) truncated protomer) that selectively binds integrin α1ß1. We show that an E317A substitution enhanced binding to the truncated protomer, consistent with a previous finding that this substitution eliminates the salt bridge. Surprisingly, we show that an R287A substitution did not alter binding, whereas R287E/E317R substitutions enhanced binding to the truncated protomer. NMR spectroscopy and molecular modeling suggested that eliminating the Glu(317) negative charge is sufficient to induce a conformational change toward the open state. Thus, the role played by Glu(317) is largely independent of the salt bridge. We further show that cells expressing E317A or R287E/E317R substitutions have enhanced down-regulation of collagen IV synthesis, which is mediated by the ERK/MAPK pathway. In conclusion, we have demonstrated that modulating the affinity of the extracellular α1 I domain to collagen IV enhances outside-in signaling by potentiating ERK activation and enhancing the down-regulation of collagen synthesis.

Mapping of potent and specific binding motifs, GLOGEN and GVOGEA, for integrin α1ß1 using collagen toolkits II and III.

Integrins are well characterized cell surface receptors for extracellular matrix proteins. Mapping integrin-binding sites within the fibrillar collagens identified GFOGER as a high affinity site recognized by α2ß1, but with lower affinity for α1ß1. Here, to identify specific ligands for α1ß1, we examined binding of the recombinant human α1 I domain, the rat pheochromocytoma cell line (PC12), and the rat glioma Rugli cell line to our collagen Toolkit II and III peptides using solid-phase and real-time label-free adhesion assays. We observed Mg(2+)-dependent binding of the α1 I domain to the peptides in the following rank order: III-7 (GLOGEN), II-28 (GFOGER), II-7 and II-8 (GLOGER), II-18 (GAOGER), III-4 (GROGER). PC12 cells showed a similar profile. Using antibody blockade, we confirmed that binding of PC12 cells to peptide III-7 was mediated by integrin α1ß1. We also identified a new α1ß1-binding activity within peptide II-27. The sequence GVOGEA bound weakly to PC12 cells and strongly to activated Rugli cells or to an activated α1 I domain, but not to the α2 I domain or to C2C12 cells expressing α2ß1 or α11ß1. Thus, GVOGEA is specific for α1ß1. Although recognized by both α2ß1 and α11ß1, GLOGEN is a better ligand for α1ß1 compared with GFOGER. Finally, using biosensor assays, we show that although GLOGEN is able to compete for the α1 I domain from collagen IV (IC(50) ∼3 µm), GFOGER is much less potent (IC(50) ∼90 µm), as shown previously. These data confirm the selectivity of GFOGER for α2ß1 and establish GLOGEN as a high affinity site for α1ß1.

Structure of collagen receptor integrin α(1)I domain carrying the activating mutation E317A.

We have analyzed the structure and function of the integrin α(1)I domain harboring a gain-of-function mutation E317A. To promote protein crystallization, a double variant with an additional C139S mutation was used. In cell adhesion assays, the E317A mutation promoted binding to collagen. Similarly, the double mutation C139S/E317A increased adhesion compared with C139S alone. Furthermore, soluble α(1)I C139S/E317A was a higher avidity collagen binder than α(1)I C139S, indicating that the double variant represents an activated form. The crystal structure of the activated variant of α(1)I was solved at 1.9 Å resolution. The E317A mutation results in the unwinding of the αC helix, but the metal ion has moved toward loop 1, instead of loop 2 in the open α(2)I. Furthermore, unlike in the closed αI domains, the metal ion is pentacoordinated and, thus, prepared for ligand binding. Helix 7, which has moved downward in the open α(2)I structure, has not changed its position in the activated α(1)I variant. During the integrin activation, Glu(335) on helix 7 binds to the metal ion at the metal ion-dependent adhesion site (MIDAS) of the ß(1) subunit. Interestingly, in our cell adhesion assays E317A could activate collagen binding even after mutating Glu(335). This indicates that the stabilization of helix 7 into its downward position is not required if the α(1) MIDAS is already open. To conclude, the activated α(1)I domain represents a novel conformation of the αI domain, mimicking the structural state where the Arg(287)-Glu(317) ion pair has just broken during the integrin activation.

NMR structure and dynamics of recombinant wild type and mutated jerdostatin, a selective inhibitor of integrin α1ß1.

NMR analysis of four recombinant jerdostatin molecules was assessed to define the structural basis of two naturally occurring gain-of-function events: C-terminal dipeptide processing and mutation of the active residue K21 to arginine. Removal of the highly mobile and a bulky C-terminal dipeptide produced pronounced chemical shift changes in the sequentially unconnected but spatially nearby α(1)ß(1) inhibitory loop. Analysis of chemical shift divergence and (15)N backbone relaxation dynamics indicated differences in motions in the picosecond to nanosecond time scale, and the higher T(2) rate of S25, S26, and H27 of rJerK21 point to a slowdown in the microsecond to millisecond motions of these residues when compared with rJerR21. The evidence presented in this article converges on the hypothesis that dynamic differences between the α(1)ß(1) recognition loops of rJerR21 and rJerK21 may influence the thermodynamics of their receptor recognition and binding. A decrease in the µs-ms time scale may impair the binding affinity by reducing the rate of possible conformations that the rJerK21 can adopt in this time scale.

Integrins alpha1beta1 and alpha2beta1 are receptors for the rotavirus enterotoxin.

Rotavirus NSP4 is a viral enterotoxin capable of causing diarrhea in neonatal mice. This process is initiated by the binding of extracellular NSP4 to target molecule(s) on the cell surface that triggers a signaling cascade leading to diarrhea. We now report that the integrins alpha1beta1 and alpha2beta1 are receptors for NSP4. NSP4 specifically binds to the alpha1 and alpha2 I domains with apparent K(d) = 1-2.7 muM. Binding is mediated by the I domain metal ion-dependent adhesion site motif, requires Mg(2+) or Mn(2+), is abolished with EDTA, and an NSP4 point mutant, E(120)A, fails to bind alpha2 integrin I domain. NSP4 has two distinct integrin interaction domains. NSP4 amino acids 114-130 are essential for binding to the I domain, and NSP4 peptide 114-135 blocks binding of the natural ligand, collagen I, to integrin alpha2. NSP4 amino acids 131-140 are not associated with the initial binding to the I domain, but elicit signaling that leads to the spreading of attached C2C12-alpha2 cells, mouse myoblast cells stably expressing the human alpha2 integrin. NSP4 colocalizes with integrin alpha2 on the basolateral surface of rotavirus-infected polarized intestinal epithelial (Caco-2) cells as well as surrounding noninfected cells. NSP4 mutants that fail to bind or signal through integrin alpha2 were attenuated in diarrhea induction in neonatal mice. These results indicate that NSP4 interaction with integrin alpha1 and alpha2 is an important component of enterotoxin function and rotavirus pathogenesis, further distinguishing this viral virulence factor from other microbial enterotoxins.

Collagen XVI harbors an integrin alpha1 beta1 recognition site in its C-terminal domains.

Collagen XVI is integrated tissue-dependently into distinct fibrillar aggregates, such as D-banded cartilage fibrils and fibrillin-1-containing microfibrils. In skin, the distribution of collagen XVI overlaps that of the collagen-binding integrins alpha1 beta1 and alpha2 beta1. Basal layer keratinocytes express integrin alpha2 beta1, whereas integrin alpha1 beta1 occurs in smooth muscle cells surrounding blood vessels, in hair follicles, and on adipocytes. Cells bearing the integrins alpha1 beta1 and alpha2 beta1 attach and spread on recombinant collagen XVI. Furthermore, collagen XVI induces the recruitment of these integrins into focal adhesion plaques, a principal step in integrin signaling. Of potential physiological relevance, these integrin-collagen XVI interactions may connect cells with specialized fibrils, thus contributing to the organization of fibrillar and cellular components within connective tissues. In cell-free binding assays, collagen XVI is more avidly bound by alpha1 beta1 integrin than by alpha2 beta1 integrin. Both integrins interact with collagen XVI via the A domain of their alpha subunits. A tryptic collagen XVI fragment comprising the collagenous domains 1-3 is recognized by alpha1 beta1 integrin. Electron microscopy of complexes of alpha1 beta1 integrin with this tryptic collagen XVI fragment or with full-length collagen XVI revealed a unique alpha1 beta1 integrin-binding site within collagen XVI located close to its C-terminal end.

An arthritogenic alphavirus uses the alpha1beta1 integrin collagen receptor.

Ross River (RR) virus is an alphavirus endemic to Australia and New Guinea and is the aetiological agent of epidemic polyarthritis or RR virus disease. Here we provide evidence that RR virus uses the collagen-binding alpha1beta1 integrin as a cellular receptor. Infection could be inhibited by collagen IV and antibodies specific for the beta1 and alpha1 integrin proteins, and fibroblasts from alpha1-integrin-/- mice were less efficiently infected than wild-type fibroblasts. Soluble alpha1beta1 integrin bound immobilized RR virus, and peptides representing the alpha1beta1 integrin binding-site on collagen IV inhibited virus binding to cells. We speculate that two highly conserved regions within the cell-receptor binding domain of E2 mimic collagen and provide access to cellular collagen-binding receptors.

Synthetic heterotrimeric collagen peptides as mimics of cell adhesion sites of the basement membrane.

Collagen type IV forms a network in the basement membrane into which other constituents of the tissue are incorporated. It also provides cell-adhesion sites that are specifically recognized by cell-surface receptors, i.e., the integrins. Different from the ubiquitous sequential RGD adhesion motif found in most of the matrix proteins, in collagen type IV, the responsible binding sites for alpha1beta1 integrin have been identified as Asp461 of the two alpha1 chains and Arg461 of the alpha2 chain. Because of the heterotrimeric character of this collagen, the spatial geometry of the binding epitope depends not only on the triple-helical fold, but decisively even on the stagger of the chains. To investigate the effects of chain registration on the conformational properties and binding affinities of this adhesion epitope, two synthetic heterotrimeric collagen peptides consisting of the identical three chains were assembled by an artificial cystine knot in two different registers, i.e., in the most plausible alpha2alpha1alpha1' and less probable alpha1alpha2alpha1' chain alignment. A detailed conformational characterization of both trimers allowed to correlate their different binding affinities for alpha1beta1 integrin with the degree of local plasticity of the two different triple helices. Optimal local breathing of the rod-shaped collagens is apparently crucial for selective recognition by proteins interacting with these main components of the extracellular matrix.

Jararhagin-derived RKKH peptides induce structural changes in alpha1I domain of human integrin alpha1beta1.

Integrin alpha(1)beta(1) is one of four collagen-binding integrins in humans. Collagens bind to the alphaI domain and in the case of alpha(2)I collagen binding is competitively inhibited by peptides containing the RKKH sequence and derived from the metalloproteinase jararhagin of snake venom from Bothrops jararaca. In alpha(2)I, these peptides bind near the metal ion-dependent adhesion site (MIDAS), where a collagen (I)-like peptide is known to bind; magnesium is required for binding. Published structures of the ligand-bound "open" conformation of alpha(2)I differs significantly from the "closed" conformation seen in the structure of apo-alpha(2)I near MIDAS. Here we show that two peptides, CTRKKHDC and CARKKHDC, derived from jararhagin also bind to alpha(1)I and competitively inhibit collagen I binding. Furthermore, calorimetric and fluorimetric measurements show that the structure of the complex of alpha(1)I with Mg(2+) and CTRKKHDC differs from structure in the absence of peptide. A comparison of the x-ray structure of apo-alpha(1)I ("closed" conformation) and a model structure of the alpha(1)I ("open" conformation) based on the closely related structure of alpha(2)I reveals that the binding site is partially blocked to ligands by Glu(255) and Tyr(285) in the "closed" structure, whereas in the "open" structure helix C is unwound and these residues are shifted, and the "RKKH" peptides fit well when docked. The "open" conformation of alpha(2)I resulting from binding a collagen (I)-like peptide leads to exposure of hydrophobic surface, also seen in the model of alpha(1)I and shown experimentally for alpha(1)I using a fluorescent hydrophobic probe.

Prolyl hydroxylation of collagen type I is required for efficient binding to integrin alpha 1 beta 1 and platelet glycoprotein VI but not to alpha 2 beta 1.

Collagen is a potent adhesive substrate for cells, an event essentially mediated by the integrins alpha 1 beta 1 and alpha 2 beta 1. Collagen fibrils also bind to the integrin alpha 2 beta 1 and the platelet receptor glycoprotein VI to activate and aggregate platelets. The distinct triple helical recognition motifs for these receptors, GXOGER and (GPO)n, respectively, all contain hydroxyproline. Using unhydroxylated collagen I produced in transgenic plants, we investigated the role of hydroxyproline in the receptor-binding properties of collagen. We show that alpha 2 beta 1 but not alpha 1 beta 1 mediates cell adhesion to unhydroxylated collagen. Soluble recombinant alpha 1 beta 1 binding to unhydroxylated collagen is considerably reduced compared with bovine collagens, but binding can be restored by prolyl hydroxylation of recombinant collagen. We also show that platelets use alpha 2 beta 1 to adhere to the unhydroxylated recombinant molecules, but the adhesion is weaker than on fully hydroxylated collagen, and the unhydroxylated collagen fibrils fail to aggregate platelets. Prolyl hydroxylation is thus required for binding of collagen to platelet glycoprotein VI and to cells by alpha 1 beta 1. These observations give new insights into the molecular basis of collagen-receptor interactions and offer new selective applications for the recombinant unhydroxylated collagen I.

Crystal structure of the alpha1beta1 integrin I domain in complex with an antibody Fab fragment.

The alpha1beta1 (VLA-1) integrin is a cell-surface receptor for collagen and laminin and has been implicated in biological pathways involved in several pathological processes. These processes may be inhibited by the monoclonal antibody AQC2, which binds with high affinity to human alpha1beta1 integrin. To understand the structural basis of the inhibition we determined the crystal structure of the complex of a chimeric rat/human I domain of the alpha1beta1 integrin and the Fab fragment of humanized AQC2 antibody. The structure of the complex shows that the antibody blocks the collagen binding site of the I domain. An aspartate residue, from the CDR3 loop of the antibody heavy chain, coordinates the MIDAS metal ion in a manner similar to that of a glutamate residue from collagen. Substitution of the aspartate residue by alanine or arginine results in significant reduction of antibody binding affinity. Interestingly, although the mode of metal ion coordination resembles that of the open conformation, the I domain maintains an overall closed conformation previously observed only for unliganded I domains.