NMR analysis of the alphaIIb beta3 cytoplasmic interaction suggests a mechanism for integrin regulation

Molecular mechanisms of catch bonds and their implications for platelet hemostasis

Adhesive molecular bonds between blood cells are essential for thrombosis and hemostasis as they provide means for platelet adhesion, aggregation, and signaling in flowing blood. According to the nowadays conventional definition, a "catch" bond is a type of non-covalent bio-molecular bridge, whose dissociation lifetime counter-intuitively increases with applied tensile force. Following recent experimental findings, such receptor-ligand protein bonds are vital to the blood cells involved in the prevention of bleeding (hemostatic response) and infection (immunity). In this review, we examine the up-to-date experimental discoveries and theoretical insights about catch bonds between the blood cells, their biomechanical principles at the molecular level, and their role in platelet thrombosis and hemostasis.

Migfilin: Cell Adhesion Effect and Comorbidities

Cell adhesion manifests as cell linkages to neighboring cells and/or the extracellular matrix (ECM). Migfilin is a widely expressed adhesion protein. It comprises three LIM domains in the C-terminal region and one proline-rich sequence in the N-terminal region. Through interplay with its various binding partners, such as Kindlin-2, Filamin, vasodilator-stimulated phosphoprotein (VASP) protein and the transcription factor CSX, Migfilin facilitates the dynamic association of connecting actomyosin fibers, orchestrating cell morphogenetic movement and cell adhesion, proliferation, migration, invasion, differentiation and signal transduction. In this review, to further elucidate the functional contributions of and pathogenesis induced by Migfilin, we focused on the structure of Migfilin and the targets which it directly binds with. We also summarized the role of Migfilin and its binding partners in the progression of different diseases and malignancies. As a possible candidate for coordinating various cellular processes and because of its association with both the pathogenesis and progression of certain tumors, Migfilin likely has utility as a therapeutic target against multiple diseases in the clinic.

Targeted Deletion of Kindlin-2 in Mouse Mammary Glands Inhibits Tumor Growth, Invasion, and Metastasis Downstream of a TGF-β/EGF Oncogenic Signaling Pathway

Breast cancer (BC) is one of the leading causes of cancer-related deaths due in part to its invasive and metastatic properties. Kindlin-2 (FERMT2) is associated with the pathogenesis of several cancers. Although the role of Kindlin-2 in regulating the invasion-metastasis cascade in BC is widely documented, its function in BC initiation and progression remains to be fully elucidated. Accordingly, we generated a floxed mouse strain by targeting the Fermt2 (K2^lox/lox) locus, followed by tissue-specific deletion of Kindlin-2 in the myoepithelial compartment of the mammary glands by crossing the K2^lox/lox mice with K14-Cre mice. Loss of Kindlin-2 in mammary epithelial cells (MECs) showed no deleterious effects on mammary gland development, fertility, and lactation in mice bearing Kindlin-2-deletion. However, in a syngeneic mouse model of BC, mammary gland, specific knockout of Kindlin-2 inhibited the growth and metastasis of murine E0771 BC cells inoculated into the mammary fat pads. However, injecting the E0771 cells into the lateral tail vein of Kindlin-2-deleted mice had no effect on tumor colonization in the lungs, thereby establishing a critical role of MEC Kindlin-2 in supporting BC tumor growth and metastasis. Mechanistically, we found the MEC Kindlin-2-mediated inhibition of tumor growth and metastasis is accomplished through its regulation of the TGF-β/ERK MAP kinase signaling axis. Thus, Kindlin-2 within the mammary gland microenvironment facilitates the progression and metastasis of BC.

Structural determinants of the integrin transmembrane domain required for bidirectional signal transmission across the cell membrane

Studying the tight activity regulation of platelet-specific integrin α_IIbβ₃ is foundational and paramount to our understanding of integrin structure and activation. α_IIbβ₃ is essential for the aggregation and adhesion function of platelets in hemostasis and thrombosis. Structural and mutagenesis studies have previously revealed the critical role of α_IIbβ₃ transmembrane (TM) association in maintaining the inactive state. Gain-of-function TM mutations were identified and shown to destabilize the TM association leading to integrin activation. Studies using isolated TM peptides have suggested an altered membrane embedding of the β₃ TM α-helix coupled with α_IIbβ₃ activation. However, controversies remain as to whether and how the TM α-helices change their topologies in the context of full-length integrin in native cell membrane. In this study, we utilized proline scanning mutagenesis and cysteine scanning accessibility assays to analyze the structure and function correlation of the α_IIbβ₃ TM domain. Our identification of loss-of-function proline mutations in the TM domain suggests the requirement of a continuous TM α-helical structure in transmitting activation signals bidirectionally across the cell membrane, characterized by the inside-out activation for ligand binding and the outside-in signaling for cell spreading. Similar results were found for α_Lβ₂ and α₅β₁ TM domains, suggesting a generalizable mechanism. We also detected a topology change of β₃ TM α-helix within the cell membrane, but only under conditions of cell adhesion and the absence of α_IIb association. Our data demonstrate the importance of studying the structure and function of the integrin TM domain in the native cell membrane.

Phosphorylation of Kindlins and the Control of Integrin Function

Integrins serve as conduits for the transmission of information between cells and their extracellular environment. Signaling across integrins is bidirectional, transducing both inside-out and outside-signaling. Integrin activation, a transition from a low affinity/avidity state to a high affinity/avidity state for cognate ligands, is an outcome of inside-signaling. Such activation is particularly important for the recognition of soluble ligands by blood cells but also influences cell-cell and cell-matrix interactions. Integrin activation depends on a complex series of interactions, which both accelerate and inhibit their interconversion from the low to the high affinity/avidity state. There are three components regarded as being most proximately involved in integrin activation: the integrin cytoplasmic tails, talins and kindlins. The participation of each of these molecules in integrin activation is highly regulated by post-translation modifications. The importance of targeted phosphorylation of integrin cytoplasmic tails and talins in integrin activation is well-established, but much less is known about the role of post-translational modification of kindlins. The kindlins, a three-member family of 4.1-ezrin-radixin-moesin (FERM)-domain proteins in mammals, bind directly to the cytoplasmic tails of integrin beta subunits. This commentary provides a synopsis of the emerging evidence for the role of kindlin phosphorylation in integrin regulation.

The MemMoRF database for recognizing disordered protein regions interacting with cellular membranes

Protein and lipid membrane interactions play fundamental roles in a large number of cellular processes (e.g. signalling, vesicle trafficking, or viral invasion). A growing number of examples indicate that such interactions can also rely on intrinsically disordered protein regions (IDRs), which can form specific reversible interactions not only with proteins but also with lipids. We named IDRs involved in such membrane lipid-induced disorder-to-order transition as MemMoRFs, in an analogy to IDRs exhibiting disorder-to-order transition upon interaction with protein partners termed Molecular Recognition Features (MoRFs). Currently, both the experimental detection and computational characterization of MemMoRFs are challenging, and information about these regions are scattered in the literature. To facilitate the related investigations we generated a comprehensive database of experimentally validated MemMoRFs based on manual curation of literature and structural data. To characterize the dynamics of MemMoRFs, secondary structure propensity and flexibility calculated from nuclear magnetic resonance chemical shifts were incorporated into the database. These data were supplemented by inclusion of sentences from papers, functional data and disease-related information. The MemMoRF database can be accessed via a user-friendly interface at https://memmorf.hegelab.org, potentially providing a central resource for the characterization of disordered regions in transmembrane and membrane-associated proteins.

Migfilin supports hemostasis and thrombosis through regulating platelet αIIbβ3 outside-in signaling

Elucidating the regulation mechanism of integrin αIIbβ3 is key to understand platelet biology and thrombotic diseases. Previous in vitro studies have implicated a role of migfilin in the support of platelet αIIbβ3 activation, however, contribution of migfilin to thrombosis and hemostasis in vivo and a detailed mechanism of migfilin in platelets are not known. In this study, with migfilin deletion (migfilin-/-) mice, we report that migfilin is a pivotal positive regulator of hemostasis and thrombosis. Migfilin-/- mice showed a nearly doubled tail-bleeding time and a prolonged occlusion time in Fecl3-induced mesenteric arteriolar thrombosis. Migfilin deficiency impedes platelet thrombi formation on collagen surface and impairs platelet aggregation and dense-granule secretion. Supported by characteristic functional readings and phosphorylation status of distinctive signaling molecules in the bidirectional signaling processes of αIIbβ3, the functional defects of migfilin-/- platelets appear to be mechanistically associated with a compromised outside-in signaling, rather than inside-out signaling. A synthesized cell-permeable migfilin peptide harboring filamin A binding sequence rescued the defective function and phosphorylation of signaling molecules of migfilin-/- platelets. Finally, migfilin does not influence the binding of filamin A and β3 subunit of αIIbβ3 in resting platelets, but hampers the re-association of filamin A and β3 during the conduct of outside-in signaling, suggesting that migfilin functions through regulating the interaction dynamics of αIIbβ3 and filamin A in platelets. Our study enhances the current understanding of platelet integrin αIIbβ3-mediated outside-in signaling and proves that migfilin is an important regulator for platelet activation, hemostasis and thrombosis.

Role of Kindlin-2 in cancer progression and metastasis

Cancer metastasis is a complex and multistep process whereby cancer cells escape the confines of the primary site to establish a new residency at distant sites. This multistep process is also known as the invasion-metastasis cascade. The biological and molecular mechanisms that control the invasion-metastasis cascade, which ultimately leads to the spread of cancer cells into distant sites, remain poorly understood. Kindlin-2 (K2) belongs to the 4.1-ezrin-ridixin-moesin (FERM) domain family of proteins, which interact with the cytoplasmic tails of β-integrin subunits, leading to the activation of extensive biological functions. These biological functions include cell migration, differentiation, cancer initiation, development, and invasion. In this review, we will discuss the various molecular signaling pathways that are regulated by K2 during the invasion-metastasis cascade of cancer tumors. These signaling pathways include TGFβ, Wnt/β-Catenin, Hedgehog, p53 and senescence, and cancer stem cell (CSC) maintenance. We will also discuss the molecular signaling pathways that regulate K2 function both at the transcriptional and the posttranslational levels. Finally, we will consider molecular mechanisms to specifically target K2 as novel therapeutic options for cancer treatment.

Structure of the SLy1 SAM homodimer reveals a new interface for SAM domain self-association

Sterile alpha motif (SAM) domains are protein interaction modules that are involved in a diverse range of biological functions such as transcriptional and translational regulation, cellular signalling, and regulation of developmental processes. SH3 domain-containing protein expressed in lymphocytes 1 (SLy1) is involved in immune regulation and contains a SAM domain of unknown function. In this report, the structure of the SLy1 SAM domain was solved and revealed that this SAM domain forms a symmetric homodimer through a novel interface. The interface consists primarily of the two long C-terminal helices, α5 and α5', of the domains packing against each other. The dimerization is characterized by a dissociation constant in the lower micromolar range. A SLy1 SAM domain construct with an extended N-terminus containing five additional amino acids of the SLy1 sequence further increases the stability of the homodimer, making the SLy1 SAM dimer two orders of magnitude more stable than previously studied SAM homodimers, suggesting that the SLy1 SAM dimerization is of functional significance. The SLy1 SAM homodimer contains an exposed mid-loop surface on each monomer, which may provide a scaffold for mediating interactions with other SAM domain-containing proteins via a typical mid-loop-end-helix interface.

Integrin α2β1 inhibits MST1 kinase phosphorylation and activates Yes-associated protein oncogenic signaling in hepatocellular carcinoma

The Hippo pathway regulates the down-stream target Yes-associated protein (YAP) to maintain organ homeostasis, which is commonly inactivated in many types of cancers. However, how cell adhesion dysregulates the Hippo pathway activating YAP oncogene in hepatocellular carcinoma (HCC) remains unclear. Our findings demonstrate that α2β1 integrin (but not other β1 integrins) expressed in HCC cells, after binding to collagen extracellular matrix, could inhibit MST1 kinase phosphorylation and activate YAP pro-oncogenic activities. Knockdown of integrin α2 gene (ITGA2) suppressed YAP targeted gene expression in vitro. α2β1 and collagen binding resulted in suppressing Hippo signaling of mammalian sterile 20-like kinase 1 (MST1) and Large tumor suppressor homolog 1 (LATS1) with concomitant activation of YAP-mediated connective tissue growth factor (CTGF) gene expression. In vitro kinase assay showed that MST1 is an immediate downstream target of integrin α2 with S1180 residue as the critical phosphorylation site. Clinical correlational analysis using a gene expression dataset of 228 HCC tumors revealed that ITGA2 expression was significantly associated with tumor progression, and co-expression with YAP targeted genes (AXL receptor tyrosine kinase, CTGF, cyclin D1, glypican 3, insulin like growth factor 1 receptor, and SRY-box 4) correlated with survivals of HCC patients. In conclusion, α2β1 integrin activation through cellular adhesion impacts the Hippo pathway in solid tumors and modulates MST1-YAP signaling cascade. Targeting integrin α2 holds promises for treating YAP-positive HCC.

Integrin αIIbβ3 outside-in signaling

Integrin α_IIbβ₃ is a highly abundant heterodimeric platelet receptor that can transmit information bidirectionally across the plasma membrane, and plays a critical role in hemostasis and thrombosis. Upon platelet activation, inside-out signaling pathways increase the affinity of α_IIbβ₃ for fibrinogen and other ligands. Ligand binding and integrin clustering subsequently stimulate outside-in signaling, which initiates and amplifies a range of cellular events driving essential platelet processes such as spreading, thrombus consolidation, and clot retraction. Integrin α_IIbβ₃ has served as an excellent model for the study of integrin biology, and it has become clear that integrin outside-in signaling is highly complex and involves a vast array of enzymes, signaling adaptors, and cytoskeletal components. In this review, we provide a concise but comprehensive overview of α_IIbβ₃ outside-in signaling, focusing on the key players involved, and how they cooperate to orchestrate this critical aspect of platelet biology. We also discuss gaps in the current understanding of α_IIbβ₃ outside-in signaling and highlight avenues for future investigation.

Redox regulation of the actin cytoskeleton and its role in the vascular system

The actin cytoskeleton is critical for form and function of vascular cells, serving mechanical, organizational and signaling roles. Because many cytoskeletal proteins are sensitive to reactive oxygen species, redox regulation has emerged as a pivotal modulator of the actin cytoskeleton and its associated proteins. Here, we summarize work implicating oxidants in altering actin cytoskeletal proteins and focus on how these alterations affect cell migration, proliferation and contraction of vascular cells. Finally, we discuss the role of oxidative modification of the actin cytoskeleton in vivo and highlight its importance for vascular diseases.

Implications of the differing roles of the β1 and β3 transmembrane and cytoplasmic domains for integrin function

Integrins are transmembrane receptors composed of α and β subunits. Although most integrins contain β1, canonical activation mechanisms are based on studies of the platelet integrin, αIIbβ3. Its inactive conformation is characterized by the association of the αIIb transmembrane and cytosolic domain (TM/CT) with a tilted β3 TM/CT that leads to activation when disrupted. We show significant structural differences between β1 and β3 TM/CT in bicelles. Moreover, the 'snorkeling' lysine at the TM/CT interface of β subunits, previously proposed to regulate αIIbβ3 activation by ion pairing with nearby lipids, plays opposite roles in β1 and β3 integrin function and in neither case is responsible for TM tilt. A range of affinities from almost no interaction to the relatively high avidity that characterizes αIIbβ3 is seen between various α subunits and β1 TM/CTs. The αIIbβ3-based canonical model for the roles of the TM/CT in integrin activation and function clearly does not extend to all mammalian integrins.

Regulation of integrins in platelets

Blood platelets prevent bleeding after trauma by forming occlusive aggregates at sites of vascular injury. Platelet aggregation is mediated by the integrin heterodimer αIIbβ3 and occurs when platelet agonists generated at the injury site convert αIIbβ3 from its resting to its active conformation. Active αIIbβ3 is then able to bind macromolecular ligands such as fibrinogen that crosslink adjacent platelets into hemostatic aggregates. Platelets circulate in a plasma milieu containing high concentrations of the principal αIIbβ3 ligand fibrinogen. Thus, αIIbβ3 activity is tightly regulated to prevent the spontaneous formation of platelet aggregates. αIIbβ3 activity is regulated at least three levels. First, intramolecular interactions involving motifs located in the membrane-proximal stalk regions, transmembrane domains, and the membrane-proximal cytosolic tails of αIIb and β3 maintain αIIbβ3 in its inactive conformation. Transmembrane domain interactions appear particularly important because disrupting these interactions causes constitutive αIIbβ3 activation. Second, the agonist-stimulated binding of the cytosolic proteins talin and kindlin-3 to the β3 cytosolic tail rapidly causes αIIbβ3 activation by disrupting the intramolecular interactions constraining αIIbβ3 activity. Third, the strength of ligand binding to active αIIbβ3 seems to be allosterically regulated. Thus, αIIbβ3 exists in a minimum of three interconvertible states: an inactive (resting) state that does not interact with ligands and two active ligand binding states that differ in their affinity for fibrinogen and in the mechanical stability of fibrinogen complexes they form.

Juxtamembrane contribution to transmembrane signaling

Signaling across the cell membrane mediated by transmembrane receptors plays an important role in diverse biological processes. Recent studies have indicated that, in a number of single-span transmembrane receptors, the intracellular juxtamembrane (JM) sequence linking the transmembrane helix with the rest of the cytoplasmic domain participates directly in the signaling process via several novel mechanisms. This review briefly highlights several modes of JM dynamics in the context of signal transduction that are shared by different types of transmembrane receptors.

Guided reconstitution of membrane protein fragments

Structural analysis by NMR of G protein-coupled receptors (GPCRs) has proven to be extremely challenging. To reduce the number of peaks in the NMR spectra by segmentally labeling a GPCR, we have developed a Guided Reconstitution method that includes the use of charged residues and Cys activation to drive heterodimeric disulfide bond formation. Three different cysteine-activating reagents: 5-5'-dithiobis(2-nitrobenzoic acid) [DTNB], 2,2'-dithiobis(5-nitropyridine) [DTNP], and 4,4'-dipyridyl disulfide [4-PDS] were analyzed to determine their efficiency in heterodimer formation at different pHs. Short peptides representing the N-terminal (NT) and C-terminal (CT) regions of the first extracellular loop (EL1) of Ste2p, the Saccharomyces cerevisiae alpha-factor mating receptor, were activated using these reagents and the efficiencies of activation and rates of heterodimerization were analyzed. Activation of NT peptides with DTNP and 4-PDS resulted in about 60% yield, but heterodimerization was rapid and nearly quantitative. Double transmembrane domain protein fragments were biosynthesized and used in Guided Reconstitution reactions. A 102-residue fragment, 2TM-tail [Ste2p(G31-I120C)], was heterodimerized with CT-EL1-tail(DTNP) at pH 4.6 with a yield of ∼75%. A 132-residue fragment, 2TMlong-tail [Ste2p(M1-I120C)], was expressed in both unlabeled and (15)N-labeled forms and used with a peptide comprising the third transmembrane domain, to generate a 180-residue segmentally labeled 3TM protein that was found to be segmentally labeled using [(15)N,(1)H]-HSQC analysis. Our data indicate that the Guided Reconstitution method would be applicable to the segmental labeling of a membrane protein with 3 transmembrane domains and may prove useful in the preparation of an intact reconstituted GPCR for use in biophysical analysis and structure determination.

Kindlin-2 directly binds actin and regulates integrin outside-in signaling

Bledzka et al. show that kindlin-2 binds actin via its F0 domain, and mutation of this site diminishes cell spreading, revealing a new mechanism by which kindlin-2 regulates cellular responses.

Reduced levels of kindlin-2 (K2) in endothelial cells derived from K2^+/− mice or C2C12 myoblastoid cells treated with K2 siRNA showed disorganization of their actin cytoskeleton and decreased spreading. These marked changes led us to examine direct binding between K2 and actin. Purified K2 interacts with F-actin in cosedimentation and surface plasmon resonance analyses and induces actin aggregation. We further find that the F0 domain of K2 binds actin. A mutation, LK⁴⁷/AA, within a predicted actin binding site (ABS) of F0 diminishes its interaction with actin by approximately fivefold. Wild-type K2 and K2 bearing the LK⁴⁷/AA mutation were equivalent in their ability to coactivate integrin αIIbβ3 in a CHO cell system when coexpressed with talin. However, K2-LK⁴⁷/AA exhibited a diminished ability to support cell spreading and actin organization compared with wild-type K2. The presence of an ABS in F0 of K2 that influences outside-in signaling across integrins establishes a new foundation for considering how kindlins might regulate cellular responses.

Selective integrin endocytosis is driven by interactions between the integrin α-chain and AP2

Integrins are heterodimeric cell-surface adhesion molecules comprising one of 18 possible α-chains and one of eight possible β-chains. They control a range of cell functions in a matrix- and ligand-specific manner. Integrins can be internalized by clathrin-mediated endocytosis (CME) through β subunit-based motifs found in all integrin heterodimers. However, whether specific integrin heterodimers can be selectively endocytosed was unknown. Here, we found that a subset of α subunits contain an evolutionarily conserved and functional YxxΦ motif directing integrins to selective internalization by the most abundant endocytic clathrin adaptor, AP2. We determined the structure of the human integrin α4-tail motif in complex with the AP2 C-μ2 subunit and confirmed the interaction by isothermal titration calorimetry. Mutagenesis of the motif impaired selective heterodimer endocytosis and attenuated integrin-mediated cell migration. We propose that integrins evolved to enable selective integrin-receptor turnover in response to changing matrix conditions.

Disulfide Bonds as Regulators of Integrin Function in Thrombosis and Hemostasis

SIGNIFICANCE

Disulfide bonds are generally viewed as structure-stabilizing elements in proteins, but some display an alternative functional role as redox switches. Functional disulfide bonds have recently emerged as important regulators of integrin function in thrombosis and hemostasis.

RECENT ADVANCES

Functional disulfide bonds were identified in the β subunit of the major platelet integrin αIIbβ3 and in other integrins involved in thrombus formation that is, αvβ3 and α2β1. Most of these functional bonds are located in the four epidermal growth factor-like domains of the integrins. Redox agents such as glutathione and nitric oxide and enzymatic thiol isomerase activity were shown to regulate the function of these integrins by disulfide bond reduction and thiol/disulfide exchange.

CRITICAL ISSUES

Increasing evidence suggests that thiol isomerases such as protein disulfide isomerase (PDI) and Erp57 directly bind to the β3 subunit of αIIbβ3 and αvβ3 and regulate their function during thrombus formation. αIIbβ3 also exhibits an endogenous thiol isomerase activity. The specific functional disulfide bonds identified in the β3 subunit might be the targets for both exogenous and endogenous thiol isomerase activity.

FUTURE DIRECTIONS

Targeting redox sites of integrins or redox agents and enzymes that regulate their function can provide a useful tool for development of anti-thrombotic therapy. Hence, inhibitors of PDI are currently studied for this purpose.

PIP2 and Talin Join Forces to Activate Integrin

Integrins are major players in cell adhesion and migration, and malfunctions in controlling their activity are associated with various diseases. Nevertheless, the details of integrin activation are not completely understood, and the role of lipids in the process is largely unknown. Herein, we show using atomistic molecular dynamics simulations that the interplay of phosphatidylinositol 4,5-bisphosphate (PIP2) and talin may directly alter the conformation of integrin αIIbβ3. Our results provide a new perspective on the role of PIP2 in integrin activation and indicate that the charged PIP2 lipid headgroup can perturb a clasp at the cytoplasmic face of the integrin heterodimer.

The Tyrosine Kinase c-Src Specifically Binds to the Active Integrin αIIbβ3 to Initiate Outside-in Signaling in Platelets

It is currently believed that inactive tyrosine kinase c-Src in platelets binds to the cytoplasmic tail of the β3 integrin subunit via its SH3 domain. Although a recent NMR study supports this contention, it is likely that such binding would be precluded in inactive c-Src because an auto-inhibitory linker physically occludes the β3 tail binding site. Accordingly, we have re-examined c-Src binding to β3 by immunoprecipitation as well as NMR spectroscopy. In unstimulated platelets, we detected little to no interaction between c-Src and β3. Following platelet activation, however, c-Src was co-immunoprecipitated with β3 in a time-dependent manner and underwent progressive activation as well. We then measured chemical shift perturbations in the (15)N-labeled SH3 domain induced by the C-terminal β3 tail peptide NITYRGT and found that the peptide interacted with the SH3 domain RT-loop and surrounding residues. A control peptide whose last three residues where replaced with those of the β1 cytoplasmic tail induced only small chemical shift perturbations on the opposite face of the SH3 domain. Next, to mimic inactive c-Src, we found that the canonical polyproline peptide RPLPPLP prevented binding of the β3 peptide to the RT- loop. Under these conditions, the β3 peptide induced chemical shift perturbations similar to the negative control. We conclude that the primary interaction of c-Src with the β3 tail occurs in its activated state and at a site that overlaps with PPII binding site in its SH3 domain. Interactions of inactive c-Src with β3 are weak and insensitive to β3 tail mutations.

αIIbβ3 variants defined by next-generation sequencing: predicting variants likely to cause Glanzmann thrombasthenia

Next-generation sequencing is transforming our understanding of human genetic variation but assessing the functional impact of novel variants presents challenges. We analyzed missense variants in the integrin αIIbβ3 receptor subunit genes ITGA2B and ITGB3 identified by whole-exome or -genome sequencing in the ThromboGenomics project, comprising ∼32,000 alleles from 16,108 individuals. We analyzed the results in comparison with 111 missense variants in these genes previously reported as being associated with Glanzmann thrombasthenia (GT), 20 associated with alloimmune thrombocytopenia, and 5 associated with aniso/macrothrombocytopenia. We identified 114 novel missense variants in ITGA2B (affecting ∼11% of the amino acids) and 68 novel missense variants in ITGB3 (affecting ∼9% of the amino acids). Of the variants, 96% had minor allele frequencies (MAF) < 0.1%, indicating their rarity. Based on sequence conservation, MAF, and location on a complete model of αIIbβ3, we selected three novel variants that affect amino acids previously associated with GT for expression in HEK293 cells. αIIb P176H and β3 C547G severely reduced αIIbβ3 expression, whereas αIIb P943A partially reduced αIIbβ3 expression and had no effect on fibrinogen binding. We used receiver operating characteristic curves of combined annotation-dependent depletion, Polyphen 2-HDIV, and sorting intolerant from tolerant to estimate the percentage of novel variants likely to be deleterious. At optimal cut-off values, which had 69-98% sensitivity in detecting GT mutations, between 27% and 71% of the novel αIIb or β3 missense variants were predicted to be deleterious. Our data have implications for understanding the evolutionary pressure on αIIbβ3 and highlight the challenges in predicting the clinical significance of novel missense variants.

The dual structural roles of the membrane distal region of the α-integrin cytoplasmic tail during integrin inside-out activation

Studies on the mechanism of integrin inside-out activation have been focused on the role of β-integrin cytoplasmic tails, which are relatively conserved and bear binding sites for the intracellular activators including talin and kindlin. Cytoplasmic tails for α-integrins share a conserved GFFKR motif at the membrane-proximal region and this forms a specific interface with the β-integrin membrane-proximal region to keep the integrin inactive. The α-integrin membrane-distal regions, after the GFFKR motif, are diverse both in length and sequence and their roles in integrin activation have not been well-defined. In this study, we report that the α-integrin cytoplasmic membrane-distal region contributes to maintaining integrin in the resting state and to integrin inside-out activation. Complete deletion of the α-integrin membrane-distal region diminished talin- and kindlin-mediated integrin ligand binding and conformational change. A proper length and suitable amino acids in α-integrin membrane-distal region was found to be important for integrin inside-out activation. Our data establish an essential role for the α-integrin cytoplasmic membrane-distal region in integrin activation and provide new insights into how talin and kindlin induce the high-affinity integrin conformation that is required for fully functional integrins.

Integrin αIIbβ3 transmembrane domain separation mediates bi-directional signaling across the plasma membrane

Integrins play an essential role in hemostasis, thrombosis, and cell migration, and they transmit bidirectional signals. Transmembrane/cytoplasmic domains are hypothesized to associate in the resting integrins; whereas, ligand binding and intracellular activating signals induce transmembrane domain separation. However, how this conformational change affects integrin outside-in signaling and whether the α subunit cytoplasmic domain is important for this signaling remain elusive. Using Chinese Hamster Ovary (CHO) cells that stably expressed different integrin αIIbβ3 constructs, we discovered that an αIIb cytoplasmic domain truncation led to integrin activation but not defective outside-in signaling. In contrast, preventing transmembrane domain separation abolished both inside-out and outside-in signaling regardless of removing the αIIb cytoplasmic tail. Truncation of the αIIb cytoplasmic tail did not obviously affect adhesion-induced outside-in signaling. Our research revealed that transmembrane domain separation is a downstream conformational change after the cytoplasmic domain dissociation in inside-out activation and indispensable for ligand-induced outside-in signaling. The result implicates that the β TM helix rearrangement after dissociation is essential for integrin transmembrane signaling. Furthermore, we discovered that the PI3K/Akt pathway is not essential for cell spreading but spreading-induced Erk1/2 activation is PI3K dependent implicating requirement of the kinase for cell survival in outside-in signaling.

Peptides derived from central turn motifs within integrin αIIb and αV cytoplasmic tails inhibit integrin activation

We previously found that peptides derived from the full length of integrin αIIb and αV cytoplasmic tails inhibited their parent integrin activation, respectively. Here we showed that the cell-permeable peptides corresponding to the conserved central turn motif within αIIb and αV cytoplasmic tails, myr-KRNRPPLEED (αIIb peptide) and myr-KRVRPPQEEQ (αV peptide), similarly inhibited both αIIb and αV integrin activation. Pre-treatment with αIIb or αV peptides inhibited Mn(2+)-activated αIIbβ3 binding to soluble fibrinogen as well as the binding of αIIbβ3-expressing Chinese Hamster Ovary cells to immobilized fibrinogen. Our turn peptides also inhibited adhesion of two breast cancer cell lines (MDA-MB-435 and MCF7) to αV ligand vitronectin. These results suggest that αIIb and αV peptides share a same mechanism in regulating integrin function. Using αIIb peptide as a model, we found that replacement of RPP with AAA significantly attenuated the inhibitory activity of αIIb peptide. Furthermore, we found that αIIb peptide specifically bound to β-tubulin in cells. Our work suggests that the central motif of α tails is an anchoring point for cytoskeletons during integrin activation and integrin-mediated cell adhesion, and its function depends on the turn structure at RPP. However, post-treatment of peptides derived from the full-length tail or from the turn motif did not reverse αIIb and αV integrin activation.

Talin-driven inside-out activation mechanism of platelet αIIbβ3 integrin probed by multimicrosecond, all-atom molecular dynamics simulations

Platelet aggregation is the consequence of the binding of extracellular bivalent ligands such as fibrinogen and von Willebrand factor to the high affinity, active state of integrin αIIbβ3. This state is achieved through a so-called "inside-out" mechanism characterized by the membrane-assisted formation of a complex between the F2 and F3 subdomains of intracellular protein talin and the integrin β3 tail. Here, we present the results of multi-microsecond, all-atom molecular dynamics simulations carried on the complete transmembrane (TM) and C-terminal (CT) domains of αIIbβ3 integrin in an explicit lipid-water environment, and in the presence or absence of the talin-1 F2 and F3 subdomains. These large-scale simulations provide unprecedented molecular-level insights into the talin-driven inside-out activation of αIIbβ3 integrin. Specifically, they suggest a preferred conformation of the complete αIIbβ3 TM/CT domains in a lipid-water environment, and testable hypotheses of key intermolecular interactions between αIIbβ3 integrin and the F2/F3 domains of talin-1. Notably, not only do these simulations give support to a stable left-handed reverse turn conformation of the αIIb juxtamembrane motif rather than a helical turn, but they raise the question as to whether TM helix separation is required for talin-driven integrin activation.

Redox-relevant aspects of the extracellular matrix and its cellular contacts via integrins

SIGNIFICANCE

The extracellular matrix (ECM) fulfills essential functions in multicellular organisms. It provides the mechanical scaffold and environmental cues to cells. Upon cell attachment, the ECM signals into the cells. In this process, reactive oxygen species (ROS) are physiologically used as signalizing molecules.

RECENT ADVANCES

ECM attachment influences the ROS-production of cells. In turn, ROS affect the production, assembly and turnover of the ECM during wound healing and matrix remodeling. Pathological changes of ROS levels lead to excess ECM production and increased tissue contraction in fibrotic disorders and desmoplastic tumors. Integrins are cell adhesion molecules which mediate cell adhesion and force transmission between cells and the ECM. They have been identified as a target of redox-regulation by ROS. Cysteine-based redox-modifications, together with structural data, highlighted particular regions within integrin heterodimers that may be subject to redox-dependent conformational changes along with an alteration of integrin binding activity.

CRITICAL ISSUES

In a molecular model, a long-range disulfide-bridge within the integrin β-subunit and disulfide bridges within the genu and calf-2 domains of the integrin α-subunit may control the transition between the bent/inactive and upright/active conformation of the integrin ectodomain. These thiol-based intramolecular cross-linkages occur in the stalk domain of both integrin subunits, whereas the ligand-binding integrin headpiece is apparently unaffected by redox-regulation.

FUTURE DIRECTIONS

Redox-regulation of the integrin activation state may explain the effect of ROS in physiological processes. A deeper understanding of the underlying mechanism may open new prospects for the treatment of fibrotic disorders.

A new view of integrin αIIbβ3 bound to membrane

In this issue of Blood, Choi et al report a 3-dimensional (3D) reconstruction model of integrin aIIbb3 in its latent state which challenges the existing paradigm and provides new insights into the mechanism of integrin activation.

Integrin cytoplasmic tail interactions

Integrins are heterodimeric cell surface adhesion receptors essential for multicellular life. They connect cells to the extracellular environment and transduce chemical and mechanical signals to and from the cell. Intracellular proteins that bind the integrin cytoplasmic tail regulate integrin engagement of extracellular ligands as well as integrin localization and trafficking. Cytoplasmic integrin-binding proteins also function downstream of integrins, mediating links to the cytoskeleton and to signaling cascades that impact cell motility, growth, and survival. Here, we review key integrin-interacting proteins and their roles in regulating integrin activity, localization, and signaling.

The mechanism of kindlin-mediated activation of integrin αIIbβ3

Increased ligand binding to cellular integrins ("activation") plays important roles in processes such as development, cell migration, extracellular matrix assembly, tumor metastasis, hemostasis, and thrombosis. Integrin activation encompasses both increased integrin monomer affinity and increased receptor clustering and depends on integrin-talin interactions. Loss of kindlins results in reduced activation of integrins. Kindlins might promote talin binding to integrins through a cooperative mechanism; however, kindlins do not increase talin association with integrins. Here, we report that, unlike talin head domain (THD), kindlin-3 has little effect on the affinity of purified monomeric αIIbβ3, and it does not enhance activation by THD. Furthermore, studies with ligands of varying valency show that kindlins primarily increase cellular αIIbβ3 avidity rather than monomer affinity. In platelets or nucleated cells, loss of kindlins markedly reduces αIIbβ3 binding to multivalent but not monovalent ligands. Finally, silencing of kindlins reduces the clustering of ligand-occupied αIIbβ3 as revealed by total internal reflection fluorescence and electron microscopy. Thus, in contrast to talins, kindlins have little primary effect on integrin αIIbβ3 affinity for monovalent ligands and increase multivalent ligand binding by promoting the clustering of talin-activated integrins.

Structure and dynamic properties of membrane proteins using NMR

Integral membrane proteins are one of the most challenging groups of macromolecules despite their apparent conformational simplicity. They manage and drive transport, circulate information, and participate in cellular movements via interactions with other proteins and through intricate conformational changes. Their structural and functional decoding is challenging and has imposed demanding experimental development. Solution nuclear magnetic resonance (NMR) spectroscopy is one of the techniques providing the capacity to make a significant difference in the deciphering of the membrane protein structure-function paradigm. The method has evolved dramatically during the last decade resulting in a plethora of new experiments leading to a significant increase in the scientific repertoire for studying membrane proteins. Besides solving the three-dimensional structures using state-of-the-art approaches, a large variety of developments of well-established techniques are available providing insight into membrane protein flexibility, dynamics, and interactions. Inspired by the speed of development in the application of new strategies, by invention of methods to measure solvent accessibility and describe low-populated states, this review seeks to introduce the vast possibilities solution NMR can offer to the study of membrane protein structure-function analyses with special focus on applicability.

Integrin α1 has a long helix, extending from the transmembrane region to the cytoplasmic tail in detergent micelles

Integrin proteins are very important adhesion receptors that mediate cell-cell and cell-extracellular matrix interactions. They play essential roles in cell signaling and the regulation of cellular shape, motility, and the cell cycle. Here, the transmembrane and cytoplasmic (TMC) domains of integrin α1 and β1 were over-expressed and purified in detergent micelles. The structure and backbone relaxations of α1-TMC in LDAO micelles were determined and analyzed using solution NMR. A long helix, extending from the transmembrane region to the cytoplasmic tail, was observed in α1-TMC. Structural comparisons of α1-TMC with reported αIIb-TMC domains indicated different conformations in the transmembrane regions and cytoplasmic tails. An NMR titration experiment indicated weak interactions between α1-TMC and β1-TMC through several α1-TMC residues located at its N-terminal juxta-transmembrane region and C-terminal extended helix region.

Integrin αIIbβ3: from discovery to efficacious therapeutic target

From the initial description of platelets in 1882, their propensity to aggregate and to contribute to thrombosis was apparent. Indeed, excessive platelet aggregation is associated with myocardial infarction and other thrombotic diseases whereas Glanzmann thrombasthenia, in which platelet aggregation is reduced, is a bleeding syndrome. Over the last half of the 20th century, many investigators have provided insights into the cellular and molecular basis for platelet aggregation. The major membrane protein on platelets, integrin αIIbβ3, mediates this response by rapidly transiting from its resting to an activated state in which it serves as a receptor for ligands that can bridge platelets together. Monoclonal antibodies, natural products, and small peptides were all shown to inhibit αIIbβ3 dependent platelet aggregation, and these inhibitors became the forerunners of antagonists that proceeded through preclinical testing and into large patient trials to treat acute coronary syndromes, particularly in the context of percutaneous coronary interventions. Three such αIIbβ3 antagonists, abciximab, eptifibatide, and tirofiban, received Food and Drug Administration approval. Over the past 15 years, millions of patients have been treated with these αIIbβ3 antagonists and many lives have been saved by their administration. With the side effect of increased bleeding and the development of new antithrombotic drugs, the use of αIIbβ3 antagonists is waning. Nevertheless, they are still widely used for the prevention of periprocedural thrombosis during percutaneous coronary interventions. This review focuses on the biology of αIIbβ3, the development of its antagonists, and some of the triumphs and shortcomings of αIIbβ3 antagonism.

The impaired viability of prostate cancer cell lines by the recombinant plant kallikrein inhibitor

BACKGROUND

Kallikreins play a pivotal role in establishing prostate cancer.

RESULTS

In contrast to the classical Kunitz plant inhibitor SbTI, the recombinant kallikrein inhibitor (rBbKIm) led to prostate cancer cell death, whereas fibroblast viability was not affected.

CONCLUSION

rBbKIm shows selective cytotoxic effect and angiogenesis inhibition against prostate cancer cells.

SIGNIFICANCE

New actions of rBbKIm may contribute to understanding the mechanisms of prostate cancer. Prostate cancer is the most common type of cancer, and kallikreins play an important role in the establishment of this disease. rBbKIm is the recombinant Bauhinia bauhinioides kallikreins inhibitor that was modified to include the RGD/RGE motifs of the inhibitor BrTI from Bauhinia rufa. This work reports the effects of rBbKIm on DU145 and PC3 prostate cancer cell lines. rBbKIm inhibited the cell viability of DU145 and PC3 cells but did not affect the viability of fibroblasts. rBbKIm caused an arrest of the PC3 cell cycle at the G0/G1 and G2/M phases but did not affect the DU145 cell cycle, although rBbKIm triggers apoptosis and cytochrome c release into the cytosol of both cell types. The differences in caspase activation were observed because rBbKIm treatment promoted activation of caspase-3 in DU145 cells, whereas caspase-9 but not caspase-3 was activated in PC3 cells. Because angiogenesis is important to the development of a tumor, the effect of rBbKIm in this process was also analyzed, and an inhibition of 49% was observed in in vitro endothelial cell capillary-like tube network formation. In summary, we demonstrated that different properties of the protease inhibitor rBbKIm may be explored for investigating the androgen-independent prostate cancer cell lines PC3 and DU145.

Skelemin in integrin α(IIb)β(3) mediated cell spreading

Skelemin, a myosin-associated protein in skeletal muscle, has been demonstrated to interact with integrin α(IIb)β(3) in nonmuscle cells during initial stages of cell spreading. The significance of this interaction and the role of skelemin in integrin signaling and cytoskeletal reorganization were investigated in this study. We established a series of Chinese hamster ovary cell lines expressing wild-type or mutant α(IIb)β(3) receptors in which skelemin binding residues at the membrane proximal region of integrin tails were mutated to alanine. Most cells displayed unimpaired adhesive capacity and spreading on immobilized fibrinogen at the early stages of cell spreading. In addition, they formed normal focal adhesions and stress fibers with no indication of impaired cell spreading. R995A/R997A/L1000A, H722A, and K716A exhibited the greatest cell spreading, which was associated with enhanced p-Src activation but was independent of FAK activation. Transfection of the cells with GFP-skelemin, containing only the C2 integrin binding domain, caused wild-type cells to round up, but had no effect on R995A/R997A/L1000A, H722A, and K716A cell spreading. Furthermore, the protrusions of the leading edge of K716A cells showed strong colocalization of talin with α(IIb)β(3) which was associated with a loss in skelemin binding. Thus, we propose that during early stages of cell spreading, skelemin exerts contractile force on cell spreading and modulates the attachment of cytoskeletal proteins and Src to integrin clusters.

Structural framework of c-Src activation by integrin β3

The integrin β3-mediated c-Src priming and activation, via the SH3 domain, is consistently associated with diseases, such as the formation of thrombosis and the migration of tumor cells. Conventionally, activation of c-Src is often induced by the binding of proline-rich sequences to its SH3 domain. Instead, integrin β3 uses R(760)GT(762) for priming and activation. Because of the lack of structural information, it is not clear where RGT will bind to SH3, and under what mechanism this interaction can prime/activate c-Src. In this study, we present a 2.0-Å x-ray crystal structure in which SH3 is complexed with the RGT peptide. The binding site lies in the "N"-Src loop of the SH3 domain. Structure-based site-directed mutagenesis showed that perturbation on the "N"-Src loop disrupts the interaction between the SH3 domain and the RGT peptide. Furthermore, the simulated c-Src:β3 complex based on the crystal structure of SH3:RGT suggests that the binding of the RGT peptide might disrupt the intramolecular interaction between the SH3 and linker domains, leading to the disengagement of Trp260:"C"-helix and further activation of c-Src.

β1 integrin NPXY motifs regulate kidney collecting-duct development and maintenance by induced-fit interactions with cytosolic proteins

Loss of β1 integrin expression inhibits renal collecting-system development. Two highly conserved NPXY motifs in the distal β1 tail regulate integrin function by associating with phosphtyrosine binding (PTB) proteins, such as talin and kindlin. Here, we define the roles of these two tyrosines in collecting-system development and delineate the structural determinants of the distal β1 tail using nuclear magnetic resonance (NMR). Mice carrying alanine mutations have moderate renal collecting-system developmental abnormalities relative to β1-null mice. Phenylalanine mutations did not affect renal collecting-system development but increased susceptibility to renal injury. NMR spectra in bicelles showed the distal β1 tail is disordered and does not interact with the model membrane surface. Alanine or phenylalanine mutations did not alter β1 structure or interactions between α and β1 subunit transmembrane/cytoplasmic domains; however, they did decrease talin and kindlin binding. Thus, these studies highlight the fact that the functional roles of the NPXY motifs are organ dependent. Moreover, the β1 cytoplasmic tail, in the context of the adjacent transmembrane domain in bicelles, is significantly different from the more ordered, membrane-associated β3 integrin tail. Finally, tyrosine mutations of β1 NPXY motifs induce phenotypes by disrupting their interactions with critical integrin binding proteins like talins and kindlins.

Structure and binding interface of the cytosolic tails of αXβ2 integrin

BACKGROUND

Integrins are signal transducer proteins involved in a number of vital physiological processes including cell adhesion, proliferation and migration. Integrin molecules are hetero-dimers composed of two distinct subunits, α and β. In humans, 18 α and 8 β subunits are combined into 24 different integrin molecules. Each of the subunit comprises a large extracellular domain, a single pass transmembrane segment and a cytosolic tail (CT). The CTs of integrins are vital for bidirectional signal transduction and in maintaining the resting state of the receptors. A large number of intracellular proteins have been found to interact with the CTs of integrins linking integrins to the cytoskeleton.

METHODOLOGY/PRINCIPAL FINDINGS

In this work, we have investigated structure and interactions of CTs of the leukocyte specific integrin αXβ2. We determined the atomic resolution structure of a myristoylated CT of αX in perdeuterated dodecylphosphocholine (DPC) by NMR spectroscopy. Our results reveal that the 35-residue long CT of αX adopts an α-helical conformation for residues F4-N17 at the N-terminal region. The remaining residues located at the C-terminal segment of αX delineate a long loop of irregular conformations. A segment of the loop maintains packing interactions with the helical structure by an extended non-polar surface of the αX CT. Interactions between αX and β2 CTs are demonstrated by (15)N-(1)H HSQC NMR experiments. We find that residues constituting the polar face of the helical conformation of αX are involved in interactions with the N-terminal residues of β2 CT. A docked structure of the CT complex indicates that a network of polar and/or salt-bridge interactions may sustain the heteromeric interactions.

CONCLUSIONS/SIGNIFICANCE

The current study provides important insights into the conservation of interactions and structures among different CTs of integrins.

Integrin αIIbβ3 inside-out activation: an in situ conformational analysis reveals a new mechanism

Integrins are a family of heterodimeric adhesion receptors that transmit signals bi-directionally across the plasma membranes. The transmembrane domain (TM) of integrin plays a critical role in mediating transition of the receptor from the default inactive to the active state on the cell surfaces. In this study, we successfully applied the substituted cysteine scanning accessibility method to determine the intracellular border of the integrin α(IIb)β(3) TM in the inactive and active states in living cells. We examined the aqueous accessibility of 75 substituted cysteines comprising the C terminus of both α(IIb) and β(3) TMs, the intracellular membrane-proximal regions, and the whole cytoplasmic tails, to the labeling of a membrane-permeable, cysteine-specific chemical biotin maleimide (BM). The active state of integrin α(IIb)β(3) heterodimer was generated by co-expression of activating partners with the cysteine-substituted constructs. Our data revealed that, in the inactive state, the intracellular lipid/aqueous border of α(IIb) TM was at Lys(994) and β(3) TM was at Phe(727) respectively; in the active state, the border of α(IIb) TM shifted to Pro(998), whereas the border of β(3) TM remained unchanged, suggesting that complex conformational changes occurred in the TMs upon α(IIb)β(3) inside-out activation. On the basis of the results, we propose a new inside-out activation mechanism for integrin α(IIb)β(3) and by inference, all of the integrins in their native cellular environment.

Membrane binding of the N-terminal ubiquitin-like domain of kindlin-2 is crucial for its regulation of integrin activation

Kindlin-2 belongs to an emerging class of regulators for heterodimeric (α/β) integrin adhesion receptors. By binding to integrin β cytoplasmic tail via its C-terminal FERM-like domain, kindlin-2 promotes integrin activation. Intriguingly, this activation process depends on the N terminus of kindlin-2 (K2-N) that precedes the FERM domain. The molecular function of K2-N is unclear. We present the solution structure of K2-N, which displays a ubiquitin fold similar to that observed in kindlin-1. Using chemical shift mapping and mutagenesis, we found that K2-N contains a conserved positively charged surface that binds to membrane enriched with negatively charged phosphatidylinositol-(4,5)-bisphosphate. We show that while wild-type kindlin-2 is capable of promoting integrin activation, such ability is significantly reduced for its membrane-binding defective mutant. These data suggest a membrane-binding function of the ubiquitin-like domain of kindlin-2, which is likely common for all kindlins to promote their localization to the plasma membrane and control integrin activation.

Structural basis for the activation of platelet integrin αIIbβ3 by calcium- and integrin-binding protein 1

Calcium and integrin binding protein 1 (CIB1) is a specific binding partner for the cytoplasmic domain of the αIIb subunit of the highly abundant platelet integrin αIIbβ3. This protein has been suggested to be involved in the regulation of the activation of αIIbβ3, a process leading to platelet aggregation and blood coagulation. In this work, the solution structure of the deuterated Ca(2+)-CIB1 protein complexed with an αIIb peptide was first determined through modern RDC-based NMR methods. Next, we generated a complex structure for CIB1 and the αIIb domain (Ca(2+)-CIB1/αIIb) using the program Haddock, which is based on experimental restraints obtained for the protein-peptide interface from cross-saturation NMR experiments. In this data-driven complex structure, the N-terminal α-helix of the cytoplasmic domain of αIIb is buried in the hydrophobic pocket of the C-lobe of Ca(2+)-CIB1. The C-terminal acidic tail of αIIb remains unstructured and likely interacts with several positively charged residues in the N-lobe of Ca(2+)-CIB1. A potential molecular mechanism for the CIB1-mediated activation of the platelet integrin could be proposed on the basis of the model structure of this protein complex. Another feature of this work is that, in the NMR cross-saturation experiments, we applied the selective radio frequency irradiation to the smaller binding partner (the αIIb peptide), and successfully detected the binding interface on the larger binding partner Ca(2+)-CIB1 through its selectively protonated methyl groups. This 'reverse' methodology has a broad potential to be employed to many other complexes where synthetic peptides and a suitably isotope-labeled medium- to large-sized protein are used to study protein-protein interactions.

Affinity of talin-1 for the β3-integrin cytosolic domain is modulated by its phospholipid bilayer environment

Binding of the talin-1 FERM (4.1/ezrin/radixin/moesin) domain to the β3 cytosolic tail causes activation of the integrin αIIbβ3. The FERM domain also binds to acidic phospholipids. Although much is known about the interaction of talin-1 with integrins and lipids, the relative contribution of each interaction to integrin regulation and possible synergy between them remain to be clarified. Here, we examined the thermodynamic interplay between FERM domain binding to phospholipid bilayers and to its binding sites in the β3 tail. We found that although both the F0F1 and F2F3 subdomains of the talin-1 FERM domain bind acidic bilayers, the full-length FERM domain binds with an affinity similar to F2F3, indicating that F0F1 contributes little to the overall interaction. When free in solution, the β3 tail has weak affinity for the FERM domain. However, appending the tail to acidic phospholipids increased its affinity for the FERM domain by three orders of magnitude. Nonetheless, the affinity of the FERM for the appended tail was similar to its affinity for binding to bilayers alone. Thus, talin-1 binding to the β3 tail is a ternary interaction dominated by a favorable surface interaction with phospholipid bilayers and set by lipid composition. Nonetheless, interactions between the FERM domain, the β3 tail, and lipid bilayers are not optimized for a high-affinity synergistic interaction, even at the membrane surface. Instead, the interactions appear to be tuned in such a way that the equilibrium between inactive and active integrin conformations can be readily regulated.

Structures and interaction analyses of integrin αMβ2 cytoplasmic tails

Integrins are heterodimeric (α and β subunits) signal transducer proteins involved in cell adhesions and migrations. The cytosolic tails of integrins are essential for transmitting bidirectional signaling and also implicated in maintaining the resting states of the receptors. In addition, cytosolic tails of integrins often undergo post-translation modifications like phosphorylation. However, the consequences of phosphorylation on the structures and interactions are not clear. The leukocyte-specific integrin αMβ2 is essential for myeloid cell adhesion, phagocytosis, and degranulation. In this work, we determined solution structures of the myristoylated cytosolic tail of αM and a Ser phosphorylated variant in dodecylphosphocholine micelles by NMR spectroscopy. Furthermore, the interactions between non-phosphorylated and phosphorylated αM tails with β2 tail were investigated by NMR and fluorescence resonance energy transfer (FRET). The three-dimensional structures of the 24-residue cytosolic tail of αM or phosphorylated αM are characterized by an N-terminal amphipathic helix and a loop at the C terminus. The residues at the loop are involved in packing interactions with the hydrophobic face of the helix. 15N-1H heteronuclear single quantum coherence experiments identified residues of αM and β2 tails that may be involved in the formation of a tail-tail heterocomplex. We further examined interactions between myristoylated β2 tail in dodecylphosphocholine micelles with dansylated αM tail peptides by FRET. These studies revealed enhanced interactions between αM or phosphorylated αM tails with β2 tail with Kd values ∼5.2±0.6 and ∼4.4±0.7 μm, respectively. Docked structures of tail-tail complexes delineated that the αM/β2 interface at the cytosolic region could be sustained by a network of polar interactions, ionic interactions, and/or hydrogen bonds.

Transmembrane communication: general principles and lessons from the structure and function of the M2 proton channel, K⁺ channels, and integrin receptors

Signal transduction across biological membranes is central to life. This process generally happens through communication between different domains and hierarchical coupling of information. Here, we review structural and thermodynamic principles behind transmembrane (TM) signal transduction and discuss common themes. Communication between signaling domains can be understood in terms of thermodynamic and kinetic principles, and complex signaling patterns can arise from simple wiring of thermodynamically coupled domains. We relate this to functions of several signal transduction systems: the M2 proton channel from influenza A virus, potassium channels, integrin receptors, and bacterial kinases. We also discuss key features in the structural rearrangements responsible for signal transduction in these systems.

Tyrosine phosphorylation as a conformational switch: a case study of integrin β3 cytoplasmic tail

Reversible protein phosphorylation is vital for many fundamental cellular processes. The actual impact of adding and removing phosphate group(s) is 3-fold: changes in the local/global geometry, alterations in the electrostatic potential and, as the result of both, modified protein-target interactions. Here we present a comprehensive structural investigation of the effects of phosphorylation on the conformational as well as functional states of a crucial cell surface receptor, α(IIb)β(3) integrin. We have analyzed phosphorylated (Tyr(747) and Tyr(759)) β(3) integrin cytoplasmic tail (CT) primarily by NMR, and our data demonstrate that under both aqueous and membrane-mimetic conditions, phosphorylation causes substantial conformational rearrangements. These changes originate from novel ionic interactions and revised phospholipid binding. Under aqueous conditions, the critical Tyr(747) phosphorylation prevents β(3)CT from binding to its heterodimer partner α(IIb)CT, thus likely maintaining an activated state of the receptor. This conclusion was tested in vivo and confirmed by integrin-dependent endothelial cells adhesion assay. Under membrane-mimetic conditions, phosphorylation results in a modified membrane embedding characterized by significant changes in the secondary structure pattern and the overall fold of β(3)CT. Collectively these data provide unique molecular insights into multiple regulatory roles of phosphorylation.