NMR analysis of structure and dynamics of the cytosolic tails of integrin alpha IIb beta 3 in aqueous solution

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

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

Structural analysis of receptors and actin polarity in platelet protrusions

During activation the platelet cytoskeleton is reorganized, inducing adhesion to the extracellular matrix and cell spreading. These processes are critical for wound healing and clot formation. Initially, this task relies on the formation of strong cellular-extracellular matrix interactions, exposed in subendothelial lesions. Despite the medical relevance of these processes, there is a lack of high-resolution structural information on the platelet cytoskeleton controlling cell spreading and adhesion. Here, we present in situ structural analysis of membrane receptors and the underlying cytoskeleton in platelet protrusions by applying cryoelectron tomography to intact platelets. We utilized three-dimensional averaging procedures to study receptors at the plasma membrane. Analysis of substrate interaction-free receptors yielded one main structural class resolved to 26 Å, resembling the α_IIbβ₃ integrin folded conformation. Furthermore, structural analysis of the actin network in pseudopodia indicates a nonuniform polarity of filaments. This organization would allow generation of the contractile forces required for integrin-mediated cell adhesion.

Differential Binding of Active and Inactive Integrin to Talin

Bi-directional signaling of integrins plays an important role in platelet and leukocyte function. Talin plays a key role in integrin bi-directional signaling and its binding to integrin is highly regulated. The precise regulation of the recruitment and binding of talin to integrin is still being elucidated. In particular, the recruitment of talin to integrin is controlled by the RAP-1 and RIAM/lamellipodin signaling axis and the affinity between talin and integrin is regulated by the conformation or protease cleavage of talin. However, whether the binding between integrin and talin is also regulated by integrin conformation has not been thoroughly explored before. In this work, we used biochemical binding assays to study the potential role of integrin conformational changes in integrin-talin interactions. Constitutively active integrin αIIbb3 binds markedly stronger to talin than inactive αIIbb3. Inactive αIIbb3 markedly increases its binding to talin once activated, regardless of how αIIbb3 is activated. Further, the increased binding to talin is b3 tail dependent. Our results suggest that integrin conformation is another regulatory mechanism for integrin-talin interaction.

Identification of indothiazinone as a natural antiplatelet agent

Cardiovascular disease, which is caused by unregulated platelet aggregation, is one of the main causes of deaths worldwide. Many studies have focused on natural products with antiplatelet effects as a safe alternative therapy to prevent the disease. In this context, an in-house chemical library was screened to find natural products capable of inhibiting the interaction between platelet integrin αIIbβ3 and fibrinogen, which is an essential step in platelet aggregation. On the basis of the screening results, indothiazinone, an alkaloid found in microbial cultures, was identified as a potential antiplatelet agent. Specifically, indothiazinone treatment significantly inhibited the binding of fibrinogen to Chinese hamster ovary cells expressing integrin αIIbβ3. It also restricted thrombin- and adenosine diphosphate-dependent spreading of human platelets on a fibrinogen matrix. More importantly, surface plasmon resonance and molecular dynamics studies suggested that indothiazinone suppressed talin-induced activation of integrin αIIbβ3 presumably by inhibiting talin-integrin interaction. In conclusion, these results suggest that indothiazinone can be used as a lead compound for the development of antiplatelet drugs with a novel mode of action.

A Conserved Ectodomain-Transmembrane Domain Linker Motif Tunes the Allosteric Regulation of Cell Surface Receptors

In many families of cell surface receptors, a single transmembrane (TM) α-helix separates ecto- and cytosolic domains. A defined coupling of ecto- and TM domains must be essential to allosteric receptor regulation but remains little understood. Here, we characterize the linker structure, dynamics, and resulting ecto-TM domain coupling of integrin αIIb in model constructs and relate it to other integrin α subunits by mutagenesis. Cellular integrin activation assays subsequently validate the findings in intact receptors. Our results indicate a flexible yet carefully tuned ecto-TM coupling that modulates the signaling threshold of integrin receptors. Interestingly, a proline at the N-terminal TM helix border, termed NBP, is critical to linker flexibility in integrins. NBP is further predicted in 21% of human single-pass TM proteins and validated in cytokine receptors by the TM domain structure of the cytokine receptor common subunit β and its P441A-substituted variant. Thus, NBP is a conserved uncoupling motif of the ecto-TM domain transition and the degree of ecto-TM domain coupling represents an important parameter in the allosteric regulation of diverse cell surface receptors.

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.

Vinculin regulates assembly of talin: β3 integrin complexes

Vinculin is a talin-binding protein that promotes integrin-mediated cell adhesion, but the mechanisms are not understood. Because talin is a direct activator of integrins, we asked whether and how vinculin regulates the formation of integrin: talin complexes. We report that VD1 (aa 1-258) and its talin-binding mutant, VD1A50I, bind directly and equally to several β integrin cytoplasmic tails (βCT). Results from competition assays show that VD1, but not VD1A50I, inhibits the interaction of talin (Tn) and talin rod (TnR), but not talin head (TnH) with β3CT. The inhibition observed could be the result of VD1 binding to one or more of the 11 vinculin binding sites (VBSs) in the TnR domain. Our studies demonstrate that VD1 binding to amino acids 482-911, a VBS rich region, in TnR perturbs the interaction of rod with β3CT. The integrin activation assays done using CHOA5 cells show that activated vinculin enhances αIIbβ3 integrin activation and that the effect is dependent on talin. The TnR domain however shows no integrin activation unlike TnH that shows enhanced integrin activation. The overall results indicate that activated vinculin promotes talin-mediated integrin activation by binding to accessible VBSs in TnR and thus displacing the TnR from the β3 subunit. The study presented, defines a novel direct interaction of VD1 with β3CT and provides an attractive explanation for vinculin's ability to potentiate integrin-mediated cell adhesion through directly binding to both TnR and the integrin cytoplasmic tail.

Cytoplasmic salt bridge formation in integrin αvß3 stabilizes its inactive state affecting integrin-mediated cell biological effects

Heterodimeric integrin receptors are mediators of cell adhesion, motility, invasion, proliferation, and survival. By this, they are crucially involved in (tumor) cell biological behavior. Integrins trigger signals bidirectionally across cell membranes: by outside-in, following binding of protein ligands of the extracellular matrix, and by inside-out, where proteins are recruited to ß-integrin cytoplasmic tails resulting in conformational changes leading to increased integrin binding affinity and integrin activation. Computational modeling and experimental/mutational approaches imply that associations of integrin transmembrane domains stabilize the low-affinity integrin state. Moreover, a cytoplasmic interchain salt bridge is discussed to contribute to a tight clasp of the α/ß-membrane-proximal regions; however, its existence and physiological relevance for integrin activation are still a controversial issue. In order to further elucidate the functional role of salt bridge formation, we designed mutants of the tumor biologically relevant integrin αvß3 by mutually exchanging the salt bridge forming amino acid residues on each chain (αvR995D and ß3D723R). Following transfection of human ovarian cancer cells with different combinations of wild type and mutated integrin chains, we showed that loss of salt bridge formation strengthened αvß3-mediated adhesion to vitronectin, provoked recruitment of cytoskeletal proteins, such as talin, and induced integrin signaling, ultimately resulting in enhanced cell migration, proliferation, and activation of integrin-related signaling molecules. These data support the notion of a functional relevance of integrin cytoplasmic salt bridge disruption during integrin activation.

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.

Deriving how far structural information is transmitted through parallel homodimeric coiled-coils: a correlation analysis of helical staggers

How local conformation is affected by local sequence is fairly well understood for alpha-helical coiled-coils, but less is known about how local conformation is influenced by distant features. Here, I describe an approach to detect such an effect, based on computing correlation coefficients of local out-of-register alignments, or so-called "staggers" between the helices, as a function of the axial distance between the staggers. This approach requires parallel homodimers, in which each stagger can occur with two "signs," where either one helix or the other is shifted towards the N terminus. The signs of such staggers separated by up to 12 residues are strongly correlated, indicating that the conformations of the ends of coiled-coils are commonly influenced by attached structures. Thus, the structures of coiled-coil residues aberrantly attached to alternative proteins, such as those resulting from leukemogenic chromosomal rearrangements, may be distinguishable from those in normal tissues, and in turn serve as targets of selective drug design. The signs of helical staggers separated by between 13 and 30 residues are moderately yet significantly correlated, indicating that some of the coiled-coils transmit this conformational feature axially for at least 45 Å. A positive, albeit noisy, correlation also exists among tropomyosin coiled-coils for signed staggers separated by the 40-residue actin repeat distance, consistent with the semi-flexible tropomyosin filament binding F-actin and regulating skeletal muscle contraction in a partially cooperative manner. Communication of the signs of axial staggers is explained in part by minimization of main-chain hydrogen bond deformations.

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.

Construction of covalent membrane protein complexes and high-throughput selection of membrane mimics

The association of transmembrane (TM) helices underlies membrane protein structure and folding. Structural studies of TM complexes are limited by complex stability and the often time-consuming selection of suitable membrane mimics. Here, methodology for the efficient, preparative scale construction of covalent TM complexes and the concomitant high-throughput selection of membrane mimics is introduced. For the employed integrin αIIbβ3 model system, the methodology identified phospholipid bicelles, including their specific composition, as the best membrane mimic. The method facilitates structure determination by NMR spectroscopy as exemplified by the measurement of previously inaccessible residual dipolar couplings and (15)N relaxation parameters.

Subcellular localization of talin is regulated by inter-domain interactions

Talin, which is composed of head (THD) and rod domains, plays an important role in cell adhesion events in diverse species including most metazoans and Dictyostelium discoideum. Talin is abundant in the cytosol; however, it mediates adhesion by associating with integrins in the plasma membrane where it forms a primary link between integrins and the actin cytoskeleton. Cells modulate the partitioning of talin between the plasma membrane and the cytosol to control cell adhesion. Here, we combine nuclear magnetic resonance spectroscopy (NMR) with subcellular fractionation to characterize two distinct THD-rod domain interactions that control the interaction of talin with the actin cytoskeleton or its localization to the plasma membrane. An interaction between a discrete vinculin-binding region of the rod (VBS1/2a; Tln1(482-787)), and the THD restrains talin from interacting with the plasma membrane. Furthermore, we show that vinculin binding to VBS1/2a results in talin recruitment to the plasma membrane. Thus, we have structurally defined specific inter-domain interactions between THD and the talin rod domain that regulate the subcellular localization of talin.

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.

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.

Multiscale simulations suggest a mechanism for integrin inside-out activation

Integrins are large cell-surface adhesion receptors that can be activated to a high affinity state by the formation of an intracellular complex between the integrin β-subunit tail, the membrane, and talin. The F2 and F3 subdomains of the talin head play a key role in formation of this complex. Here, activation of the integrin αIIb/β3 dimer by the talin head domain was probed using multiscale molecular dynamics simulations. A number of novel insights emerge from these studies, including (i) the importance of the integrin αIIb subunit F992 and F993 residues in stabilizing the "off" state of the αIIb/β3 dimer, (ii) a crucial role for negatively charged groups in the F2-F3/membrane interaction, (iii) binding of the talin F2-F3 domain to negatively charged lipid headgroups in the membrane induces a reorientation of the β transmembrane (TM) domain, (iv) an increase in the tilt angle of the β TM domain relative to the bilayer normal helps to destabilize the α/β TM interaction and promote a scissor-like movement of the integrin TM helices. These results, combined with various published experimental observations, suggest a model for the mechanism of inside-out activation of integrins by talin.

Regulation of integrin activation

Regulation of cell-cell and cell-matrix interaction is essential for the normal physiology of metazoans and is important in many diseases. Integrin adhesion receptors can rapidly increase their affinity (integrin activation) in response to intracellular signaling events in a process termed inside-out signaling. The transmembrane domains of integrins and their interactions with the membrane are important in inside-out signaling. Moreover, integrin activation is tightly regulated by a complex network of signaling pathways. Here, we review recent progress in understanding how the membrane environment can, in cooperation with integrin-binding proteins, regulate integrin activation.

Integrin structure, activation, and interactions

Integrins are large, membrane-spanning, heterodimeric proteins that are essential for a metazoan existence. All members of the integrin family adopt a shape that resembles a large "head" on two "legs," with the head containing the sites for ligand binding and subunit association. Most of the receptor dimer is extracellular, but both subunits traverse the plasma membrane and terminate in short cytoplasmic domains. These domains initiate the assembly of large signaling complexes and thereby bridge the extracellular matrix to the intracellular cytoskeleton. To allow cells to sample and respond to a dynamic pericellular environment, integrins have evolved a highly responsive receptor activation mechanism that is regulated primarily by changes in tertiary and quaternary structure. This review summarizes recent progress in the structural and molecular functional studies of this important class of adhesion receptor.

Overview: structural biology of integrins

Integrins are cell adhesion molecules that play important roles in many biological processes including hemostasis, immune responses, development, and cancer. Their adhesiveness is dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces outside-in signals from the extracellular domain to the cytoplasm. Advances in the past several years have shed light on structural basis for integrin regulation and signaling, especially how the large-scale reorientations of the ectodomain are related to the inter-domain and intra-domain shape shifting that changes ligand-binding affinity. Experiments have also shown how the conformational changes of the ectodomain are linked to changes in the α- and β-subunit transmembrane and cytoplasmic domains.

Structural diversity in integrin/talin interactions

The adhesion of integrins to the extracellular matrix is regulated by binding of the cytoskeletal protein talin to the cytoplasmic tail of the β-integrin subunit. Structural studies of this interaction have hitherto largely focused on the β3-integrin, one member of the large and diverse integrin family. Here, we employ NMR to probe interactions and dynamics, revealing marked structural diversity in the contacts between β1A, β1D, and β3 tails and the Talin1 and Talin2 isoforms. Coupled with analysis of recent structures of talin/β tail complexes, these studies elucidate the thermodynamic determinants of this heterogeneity and explain why the Talin2/β1D isoforms, which are co-localized in striated muscle, form an unusually tight interaction. We also show that talin/integrin affinity can be enhanced 1000-fold by deleting two residues in the β tail. Together, these studies illustrate how the integrin/talin interaction has been fine-tuned to meet varying biological requirements.

Structural basis of integrin transmembrane activation

Integrins are cell adhesion receptors that transmit bidirectional signals across plasma membrane and are crucial for many biological functions. Recent structural studies of integrin transmembrane (TM) and cytoplasmic domains have shed light on their conformational changes during integrin activation. A structure of the resting state was solved based on Rosetta computational modeling and experimental data using intact integrins on mammalian cell surface. In this structure, the alpha(IIb) GXXXG motif and their beta(3) counterparts of the TM domains associate with ridge-in-groove packing, and the alpha(IIb) GFFKR motif and the beta(3) Lys-716 in the cytoplasmic segments play a critical role in the alpha/beta association. Comparing this structure with the NMR structures of the monomeric alpha(IIb) and beta(3) (represented as active conformations), the alpha subunit helix remains similar after dissociation whereas beta subunit helix is tilted by embedding additional 5-6 residues into the lipid bilayer. These conformational changes are critical for integrin activation and signaling across the plasma membrane. We thus propose a new model of integrin TM activation in which the recent NMR structure of the alpha(IIb)beta(3) TM/cytoplasmic complex represents an intermediate or transient state, and the electrostatic interaction in the cytoplasmic region is important for priming the initial alpha/beta association, but not absolutely necessary for the resting state.

Structural basis of transmembrane domain interactions in integrin signaling

Cell surface receptors of the integrin family are pivotal to cell adhesion and migration. The activation state of heterodimeric alphabeta integrins is correlated to the association state of the single-pass alpha and beta transmembrane domains. The association of integrin alphaIIbbeta3 transmembrane domains, resulting in an inactive receptor, is characterized by the asymmetric arrangement of a straight (alphaIIb) and tilted (beta3) helix relative to the membrane in congruence to the dissociated structures. This allows for a continuous association interface centered on helix-helix glycine-packing and an unusual alphaIIb(GFF) structural motif that packs the conserved Phe-Phe residues against the beta3 transmembrane helix, enabling alphaIIb(D723)beta3(R995) electrostatic interactions. The transmembrane complex is further stabilized by the inactive ectodomain, thereby coupling its association state to the ectodomain conformation. In combination with recently determined structures of an inactive integrin ectodomain and an activating talin/beta complex that overlap with the alphabeta transmembrane complex, a comprehensive picture of integrin bi-directional transmembrane signaling has emerged.

The final steps of integrin activation: the end game

Cell-directed changes in the ligand-binding affinity ('activation') of integrins regulate cell adhesion and migration, extracellular matrix assembly and mechanotransduction, thereby contributing to embryonic development and diseases such as atherothrombosis and cancer. Integrin activation comprises triggering events, intermediate signalling events and, finally, the interaction of integrins with cytoplasmic regulators, which changes an integrin's affinity for its ligands. The first two events involve diverse interacting signalling pathways, whereas the final steps are immediately proximal to integrins, thus enabling integrin-focused therapeutic strategies. Recent progress provides insight into the structure of integrin transmembrane domains, and reveals how the final steps of integrin activation are mediated by integrin-binding proteins such as talins and kindlins.

Beta integrin tyrosine phosphorylation is a conserved mechanism for regulating talin-induced integrin activation

Integrins are large membrane-spanning receptors fundamental to cell adhesion and migration. Integrin adhesiveness for the extracellular matrix is activated by the cytoskeletal protein talin via direct binding of its phosphotyrosine-binding-like F3 domain to the cytoplasmic tail of the beta integrin subunit. The phosphotyrosine-binding domain of the signaling protein Dok1, on the other hand, has an inactivating effect on integrins, a phenomenon that is modulated by integrin tyrosine phosphorylation. Using full-length tyrosine-phosphorylated (15)N-labeled beta3, beta1A, and beta7 integrin tails and an NMR-based protein-protein interaction assay, we show that talin1 binds to the NPXY motif and the membrane-proximal portion of beta3, beta1A, and beta7 tails, and that the affinity of this interaction is decreased by integrin tyrosine phosphorylation. Dok1 only interacts weakly with unphosphorylated tails, but its affinity is greatly increased by integrin tyrosine phosphorylation. The Dok1 interaction remains restricted to the integrin NPXY region, thus phosphorylation inhibits integrin activation by increasing the affinity of beta integrin tails for a talin competitor that does not form activating membrane-proximal interactions with the integrin. Key residues governing these specificities were identified by detailed structural analysis, and talin1 was engineered to bind preferentially to phosphorylated integrins by introducing the mutation D372R. As predicted, this mutation affects talin1 localization in live cells in an integrin phosphorylation-specific manner. Together, these results indicate that tyrosine phosphorylation is a common mechanism for regulating integrin activation, despite subtle differences in how these integrins interact with their binding proteins.

Kindling the flame of integrin activation and function with kindlins

PURPOSE OF REVIEW

There are three kindlin family members in vertebrates, which have high-sequence homology and a common organization signature with a C-terminal 4.1, ezrin, radixin, moesin (FERM) domain bisected by a pleckstrin-homology domain. Although the cell and tissue distributions of the three kindlins differ, there is a consistent and close interrelationship between kindlins and integrins, and alterations of kindlin expression affect integrin-dependent functions. However, in-vivo data on the functions of the kindlins and their mechanisms of action have been lacking.

RECENT FINDINGS

Recent studies have shown that deficiencies of each of the three kindlins result in profound and distinct phenotypes, ranging from skin and intestinal defects (kindlin-1), embryonic lethality due to cardiac developmental problems (kindlin-2), to marked abnormalities in platelet, leukocyte and erythrocyte function (kindlin-3). A human disease characterized by bleeding, frequent infections and osteopetrosis has now been attributed to mutations in the gene encoding for kindlin-3. These defects are consistent with recent data showing that kindlins control integrin activation and function.

SUMMARY

The three members of the kindlin family have now been implicated as essential regulators of integrin function in individual cells and in whole organisms.

Structure of an integrin alphaIIb beta3 transmembrane-cytoplasmic heterocomplex provides insight into integrin activation

Heterodimeric integrin adhesion receptors regulate diverse biological processes including angiogenesis, thrombosis and wound healing. The transmembrane-cytoplasmic domains (TMCDs) of integrins play a critical role in controlling activation of these receptors via an inside-out signaling mechanism, but the precise structural basis remains elusive. Here, we present the solution structure of integrin alphaIIb beta3 TMCD heterodimer, which reveals a right-handed coiled-coil conformation with 2 helices intertwined throughout the transmembrane region. The helices extend into the cytoplasm and form a clasp that differs significantly from a recently published alphaIIb beta3 TMCD structure. We show that while a point mutation in the clasp interface modestly activates alphaIIb beta3, additional mutations in the transmembrane interface have a synergistic effect, leading to extensive integrin activation. Detailed analyses and structural comparison with previous studies suggest that extensive integrin activation is a highly concerted conformational transition process, which involves transmembrane coiled-coil unwinding that is triggered by the membrane-mediated alteration and disengagement of the membrane-proximal clasp. Our results provide atomic insight into a type I transmembrane receptor heterocomplex and the mechanism of integrin inside-out transmembrane signaling.

Calreticulin-independent regulation of the platelet integrin αIIbβ3by the KVGFFKR αIIb-cytoplasmic motif

The platelet integrin alphaIIbbeta3 alters conformation in response to platelet activation and ligand binding, although the molecular mechanisms involved are not known. We previously showed that a lipid modified peptide, corresponding to the membrane proximal 989KVGFFKR995 portion of the alphaIIb cytoplasmic tail, independently activates platelet alphaIIbbeta3. Calreticulin (CRT) is a potential integrin regulatory protein based on its interaction with the highly conserved alpha-integrin sequence KxGFFKR. We therefore examined the possible interaction of calreticulin and alphaIIbbeta3 in human platelets. We demonstrate that calreticulin in platelets is localised to the granulomere. In contrast, the known integrin-binding protein talin accumulates at the periphery of spreading platelets and colocalises with alphaIIbbeta3 during the process of adhesion. An interaction between calreticulin and alphaIIbbeta3 could not be demonstrated using co-immunoprecipitation techniques under various platelet activation states, even in the presence of covalent chemical crosslinkers. Thus, calreticulin does not functionally interact with the major integrin in human platelets. In order to identify proteins that interact with the integrin KVGFFKR motif we then used a peptide 'pull-down' assay from platelet lysates with biotinylated peptides and demonstrate that only the alphaIIb and beta3 subunits selectively and individually interact with this sequence. This interaction is divalent cation-dependent, has high-affinity, and occurs both with purified alphaIIbbeta3 complex and with electroeluted alpha and beta subunits. Thus, our data show that the conserved integrin KVGFFKR domain interacts primarily with the alpha and beta cytoplasmic tails and not with CRT in human platelets.

The structure and function of platelet integrins

Integrins are a ubiquitous family of non-covalently associated alpha/beta transmembrane heterodimers linking extracellular ligands to intracellular signaling pathways [1] [Cell, 2002; 110: 673]. Platelets contain five integrins, three beta1 integrins that mediate platelet adhesion to the matrix proteins collagen, fibronectin and laminin, and the beta3 integrins alphavbeta3 and alphaIIbbeta3 [2] [J Clin Invest, 2005; 115: 3363]. While there are only several hundred alphavbeta3 molecules per platelet, alphavbeta3 mediates platelet adhesion to osteopontin and vitronectin in vitro [3] [J Biol Chem, 1997; 272: 8137]; whether this occurs in vivo remains unknown. By contrast, the 80,000 alphaIIbbeta3 molecules on agonist-stimulated platelets bind fibrinogen, von Willebrand factor, and fibronectin, mediating platelet aggregation when the bound proteins crosslink adjacent platelets [2] [J Clin Invest, 2005; 115: 3363]. Although platelet integrins are poised to shift from resting to active conformations, tight regulation of their activity is essential to prevent the formation of intravascular thrombi. This review focuses on the structure and function of the intensively studied beta3 integrins, in particular alphaIIbbeta3, but reference will be made to other integrins where relevant.

The structure of a receptor with two associating transmembrane domains on the cell surface: integrin alphaIIbbeta3

Structures of intact receptors with single-pass transmembrane domains are essential to understand how extracellular and cytoplasmic domains regulate association and signaling through transmembrane domains. A chemical and computational method to determine structures of the membrane regions of such receptors on the cell surface is developed here and validated with glycophorin A. An integrin heterodimer structure reveals association over most of the lengths of the alpha and beta transmembrane domains and shows that the principles governing association of hetero and homo transmembrane dimers differ. A turn at the Gly of the juxtamembrane GFFKR motif caps the alpha TM helix and brings the two Phe of GFFKR into the alpha/beta interface. A juxtamembrane Lys residue in beta also has an important role in the interface. The structure shows how transmembrane association/dissociation regulates integrin signaling. A joint ectodomain and membrane structure shows that substantial flexibility between the extracellular and TM domains is compatible with TM signaling.

The structure of the integrin alphaIIbbeta3 transmembrane complex explains integrin transmembrane signalling

Heterodimeric integrin adhesion receptors regulate cell migration, survival and differentiation in metazoa by communicating signals bi-directionally across the plasma membrane. Protein engineering and mutagenesis studies have suggested that the dissociation of a complex formed by the single-pass transmembrane (TM) segments of the alpha and beta subunits is central to these signalling events. Here, we report the structure of the integrin alphaIIbbeta3 TM complex, structure-based site-directed mutagenesis and lipid embedding estimates to reveal the structural event that underlies the transition from associated to dissociated states, that is, TM signalling. The complex is stabilized by glycine-packing mediated TM helix crossing within the extracellular membrane leaflet, and by unique hydrophobic and electrostatic bridges in the intracellular leaflet that mediate an unusual, asymmetric association of the 24- and 29-residue alphaIIb and beta3 TM helices. The structurally unique, highly conserved integrin alphaIIbbeta3 TM complex rationalizes bi-directional signalling and represents the first structure of a heterodimeric TM receptor complex.

Interactions of platelet integrin alphaIIb and beta3 transmembrane domains in mammalian cell membranes and their role in integrin activation

Clustering and occupancy of platelet integrin alpha(IIb)beta(3) (GPIIb-IIIa) generate biologically important signals: conversely, intracellular signals increase the integrins' affinity, leading to integrin activation; both forms of integrin signaling play important roles in hemostasis and thrombosis. Indirect evidence implicates interactions between integrin alpha and beta transmembrane domains (TMDs) and cytoplasmic domains in integrin signaling; however, efforts to directly identify these associations have met with varying and controversial results. In this study, we develop mini-integrin affinity capture and use it in combination with nuclear magnetic resonance spectroscopy to show preferential heterodimeric association of integrin alpha(IIb)beta(3) TMD tails via specific TMD interactions in mammalian cell membranes in lipid bicelles. Furthermore, charge reversal mutations at alpha(IIb)(R995)beta(3)(D723) confirm a proposed salt bridge and show that it stabilizes the TMD-tail association; talin binding to the beta(3) tail, which activates the integrin, disrupts this association. These studies establish the preferential heterodimeric interactions of integrin alpha(IIb)beta(3) TMD tails in mammalian cell membranes and document their role in integrin signaling.

Integrin activation

Agonist stimulation of integrin receptors, composed of transmembrane alpha and beta subunits, leads cells to regulate integrin affinity ('activation'), a process that controls cell adhesion and migration, and extracellular matrix assembly. A final step in integrin activation is the binding of talin to integrin beta cytoplasmic domains. We used forward, reverse and synthetic genetics to engineer and order integrin activation pathways of a prototypic integrin, platelet alphaIIbbeta3. PMA activated alphaIIbbeta3 only after expression of both PKCalpha (protein kinase Calpha) and talin at levels approximating those in platelets. Inhibition of Rap1 GTPase reduced alphaIIbbeta3 activation, whereas expression of constitutively active Rap1A(G12V) bypassed the requirement for PKCalpha. Overexpression of a Rap effector, RIAM (Rap1-GTP-interacting adaptor molecule), activated alphaIIbbeta3 and bypassed the requirement for PKCalpha and Rap1. In addition, shRNA (short hairpin RNA)-mediated knockdown of RIAM blocked talin interaction with and activation of integrin alphaIIbbeta3. Rap1 activation caused the formation of an 'activation complex' containing talin and RIAM that redistributed to the plasma membrane and activated alphaIIbbeta3. The central finding was that this Rap1-induced formation of an 'integrin activation complex' leads to the unmasking of the integrin-binding site on talin, resulting in integrin activation.

Integrin conformational regulation: uncoupling extension/tail separation from changes in the head region by a multiresolution approach

Integrin-dependent adhesion and signaling are regulated by conformational changes whose details remain controversial. Crystallography revealed bent shapes for resting and primed integrin ectodomains, whereas large, ligand-induced rearrangements in other constructs suggested extension, "opening," and tail separation. We have used experimental/computed hydrodynamics to discriminate among different alpha(v)beta(3) and alpha(IIb)beta(3) atomic models built on X-ray, NMR, and EM data. In contrast with X-ray structures and EM maps, hydrodynamics indicate that resting integrins are already extended. Furthermore, the hydrodynamics of an alpha(v)beta(3) ectodomain-fibronectin fragment complex support opening via additional head region conformational changes (hybrid domain swing-out), but without tail separation. Likewise, frictional changes induced by priming agents in full-length alpha(IIb)beta(3) correlate well with the swing-out coupled to a simple transmembrane helix shift in an extended, electron tomography-based model. Extension and immediate tail separation are then uncoupled from head region rearrangements following activation, thus underscoring integrins' delicate, finely tuned plasticity.

Transmembrane and cytoplasmic domains in integrin activation and protein-protein interactions (review)

Integrins are heterodimeric membrane-spanning adhesion receptors that are essential for a wide range of biological functions. Control of integrin conformational states is required for bidirectional signalling across the membrane. Key components of this control mechanism are the transmembrane and cytoplasmic domains of the alpha and beta subunits. These domains are believed to interact, holding the integrin in the inactive state, while inside-out integrin activation is accompanied by domain separation. Although there are strong indications for domain interactions, the majority of evidence is insufficient to precisely define the interaction interface. The current best model of the complex, derived from computational calculations with experimental restraints, suggests that integrin activation by the cytoplasmic protein talin is accomplished by steric disruption of the alpha/beta interface. Better atomic-level resolution structures of the alpha/beta transmembrane/cytoplasmic domain complex are still required for the resting state integrin to corroborate this. Integrin activation is also controlled by competitive interactions involving the cytoplasmic domains, particularly the beta-tails. The concept of the beta integrin tail as a focal adhesion interaction 'hub' for interactions and regulation is discussed. Current efforts to define the structure and affinity of the various complexes formed by integrin tails, and how these interactions are controlled, e.g. by phosphorylation and localization, are described.

Functional analysis of the cytoplasmic domain of the integrin {alpha}1 subunit in endothelial cells

Integrin alpha1beta1, the major collagen type IV receptor, is expressed by endothelial cells and plays a role in both physiologic and pathologic angiogenesis. Because the molecular mechanisms whereby this collagen IV receptor mediates endothelial cell functions are poorly understood, truncation and point mutants of the integrin alpha1 subunit cytoplasmic tail (amino acids 1137-1151) were generated and expressed into alpha1-null endothelial cells. We show that alpha1-null endothelial cells expressing the alpha1 subunit, which lacks the entire cytoplasmic tail (mutant alpha1-1136) or expresses all the amino acids up to the highly conserved GFFKR motif (mutant alpha1-1143), have a similar phenotype to parental alpha1-null cells. Pro(1144) and Leu(1145) were shown to be necessary for alpha1beta1-mediated endothelial cell proliferation; Lys(1146) for adhesion, migration, and tubulogenesis and Lys(1147) for tubulogenesis. Integrin alpha1beta1-dependent endothelial cell proliferation is primarily mediated by ERK activation, whereas migration and tubulogenesis require both p38 MAPK and PI3K/Akt activation. Thus, distinct amino acids distal to the GFFKR motif of the alpha1 integrin cytoplasmic tail mediate activation of selective downstream signaling pathways and specific endothelial cell functions.

Structure of the integrin beta3 transmembrane segment in phospholipid bicelles and detergent micelles

Integrin adhesion receptors transduce bidirectional signals across the plasma membrane, with the integrin transmembrane domains acting as conduits in this process. Here, we report the first high-resolution structure of an integrin transmembrane domain. To assess the influence of the membrane model system, structure determinations of the beta3 integrin transmembrane segment and flanking sequences were carried out in both phospholipid bicelles and detergent micelles. In bicelles, a 30-residue linear alpha-helix, encompassing residues I693-H772, is adopted, of which I693-I721 appear embedded in the hydrophobic bicelle core. This relatively long transmembrane helix implies a pronounced helix tilt within a typical lipid bilayer, which facilitates the snorkeling of K716's charged side chain out of the lipid core while simultaneously immersing hydrophobic L717-I721 in the membrane. A shortening of bicelle lipid hydrocarbon tails does not lead to the transfer of L717-I721 into the aqueous phase, suggesting that the reported embedding represents the preferred beta3 state. The nature of the lipid headgroup affected only the intracellular part of the transmembrane helix, indicating that an asymmetric lipid distribution is not required for studying the beta3 transmembrane segment. In the micelle, residues L717-I721 are also embedded but deviate from linear alpha-helical conformation in contrast to I693-K716, which closely resemble the bicelle structure.

NPXY motifs in the beta1 integrin cytoplasmic tail are required for functional reovirus entry

Reovirus cell entry is mediated by attachment to cell surface carbohydrate and junctional adhesion molecule A (JAM-A) and internalization by beta1 integrin. The beta1 integrin cytoplasmic tail contains two NPXY motifs, which function in recruitment of adaptor proteins and clathrin for endocytosis and serve as sorting signals for internalized cargo. As reovirus infection requires disassembly in the endocytic compartment, we investigated the role of the beta1 integrin NPXY motifs in reovirus internalization. In comparison to wild-type cells (beta1+/+ cells), reovirus infectivity was significantly reduced in cells expressing mutant beta1 integrin in which the NPXY motifs were altered to NPXF (beta1+/+Y783F/Y795F cells). However, reovirus displayed equivalent binding and internalization levels following adsorption to beta1+/+ cells and beta1+/+Y783F/Y795F cells, suggesting that the NPXY motifs are essential for transport of reovirus within the endocytic pathway. Reovirus entry into beta1+/+ cells was blocked by chlorpromazine, an inhibitor of clathrin-mediated endocytosis, while entry into beta1+/+Y783F/Y795F cells was unaffected. Furthermore, virus was distributed to morphologically distinct endocytic organelles in beta1+/+ and beta1+/+Y783F/Y795F cells, providing further evidence that the beta1 integrin NPXY motifs mediate sorting of reovirus in the endocytic pathway. Thus, NPXY motifs in the beta1 integrin cytoplasmic tail are required for functional reovirus entry, which indicates a key role for these sequences in endocytosis of a pathogenic virus.

The N-terminal domains of talin cooperate with the phosphotyrosine binding-like domain to activate beta1 and beta3 integrins

The activation of integrin adhesion receptors from low to high affinity in response to intracellular cues controls cell adhesion and signaling. Binding of the cytoskeletal protein talin to the beta3 integrin cytoplasmic tail is required for beta3 activation, and the integrin-binding PTB-like F3 domain of talin is sufficient to activate beta3 integrins. Here we report that, whereas the conserved talin-integrin interaction is also required for beta1 activation, and talin F3 binds beta1 and beta3 integrins with comparable affinity, expression of the talin F3 domain is not sufficient to activate beta1 integrins. beta1 integrin activation could, however, be detected following expression of larger talin fragments that included the N-terminal and F1 domains, and mutagenesis indicates that these domains cooperate with talin F3 to mediate beta1 activation. This effect is not due to increased affinity for the integrin beta tail and we hypothesize that the N-terminal domains function by targeting or orienting talin in such a way as to optimize the interaction with the integrin tail. Analysis of beta3 integrin activation indicates that inclusion of the N-terminal and F1 domains also enhances F3-mediated beta3 activation. Our results therefore reveal a role for the N-terminal and F1 domains of talin during integrin activation and highlight differences in talin-mediated activation of beta1 and beta3 integrins.

The antithrombotic potential of selective blockade of talin-dependent integrin alpha IIb beta 3 (platelet GPIIb-IIIa) activation

In vitro studies indicate that binding of talin to the beta(3) integrin cytoplasmic domain (tail) results in integrin alpha(IIb)beta(3) (GPIIb-IIIa) activation. Here we tested the importance of talin binding for integrin activation in vivo and its biological significance by generating mice harboring point mutations in the beta(3) tail. We introduced a beta(3)(Y747A) substitution that disrupts the binding of talin, filamin, and other cytoplasmic proteins and a beta(3)(L746A) substitution that selectively disrupts interactions only with talin. Platelets from animals homozygous for each mutation showed impaired agonist-induced fibrinogen binding and platelet aggregation, providing proof that inside-out signals that activate alpha(IIb)beta(3) require binding of talin to the beta(3) tail. beta(3)(L746A) mice were resistant to both pulmonary thromboembolism and to ferric chloride-induced thrombosis of the carotid artery. Pathological bleeding, measured by the presence of fecal blood and development of anemia, occurred in 53% of beta(3)(Y747A) and virtually all beta(3)-null animals examined. Remarkably, less than 5% of beta(3)(L746A) animals exhibited this form of bleeding. These results establish that alpha(IIb)beta(3) activation in vivo is dependent on the interaction of talin with the beta(3) integrin cytoplasmic domain. Furthermore, they suggest that modulation of beta(3) integrin-talin interactions may provide an attractive target for antithrombotics and result in a reduced risk of pathological bleeding.

CD98hc (SLC3A2) Interaction with the Integrin β Subunit Cytoplasmic Domain Mediates Adhesive Signaling

In mammals, beta1 integrin adhesion receptors generate signals that mediate cell spreading, migration, proliferation, and survival. CD98, a heterodimeric transmembrane protein, physically associates with certain integrin beta subunit cytoplasmic domains (tails) via its heavy chain, CD98hc (SLC3A2), and loss of CD98hc impairs integrin signaling. Here we have used the lack of CD98hc interaction with the Drosophila integrin betaPS tail for a homology scanning analysis that implicated the C-terminal 8 residues of beta3 (Thr(755)-Thr(802)) in CD98hc binding. We then identified point mutations in the beta3 C terminus (T755K and T758M) that abolish CD98hc association and a double mutation in the corresponding residues in the betaPS tail (K839T,M842T), which resulted in gain of CD98hc interaction. Furthermore, the loss of function beta3(T755K) mutation or the gain of function beta3/betaPS(K839T,M842T) led to a loss or gain of integrin-mediated cell spreading, respectively. Thus, we have identified critical integrin residues required for CD98hc interaction and in doing so have shown that CD98c interaction with the integrin beta tail is required for its ability to mediate integrin signaling. These studies also provide new insights into how CD98hc may cooperate with other cytoplasmic domain binding proteins to modulate integrin functions and into the evolution of integrin signaling.

Platelet integrin alpha(IIb)beta(3): activation mechanisms

Integrin alpha(IIb)beta(3) plays a critical role in platelet aggregation, a central response in hemostasis and thrombosis. This function of alpha(IIb)beta(3) depends upon a transition from a resting to an activated state such that it acquires the capacity to bind soluble ligands. Diverse platelet agonists alter the cytoplasmic domain of alpha(IIb)beta(3) and initiate a conformational change that traverses the transmembrane region and ultimately triggers rearrangements in the extracellular domain to permit ligand binding. The membrane-proximal regions of alpha(IIb) and beta(3) cytoplasmic tails, together with the transmembrane segments of the subunits, contact each other to form a complex which restrains the integrin in the resting state. It is unclasping of this complex that induces integrin activation. This clasping/unclasping process is influenced by multiple cytoplasmic tail binding partners. Among them, talin appears to be a critical trigger of alpha(IIb)beta(3) activation, but other binding partners, which function as activators or suppressors, are likely to act as co-regulators of integrin activation.

Structural basis of integrin regulation and signaling

Integrins are cell adhesion molecules that mediate cell-cell, cell-extracellular matrix, and cell-pathogen interactions. They play critical roles for the immune system in leukocyte trafficking and migration, immunological synapse formation, costimulation, and phagocytosis. Integrin adhesiveness can be dynamically regulated through a process termed inside-out signaling. In addition, ligand binding transduces signals from the extracellular domain to the cytoplasm in the classical outside-in direction. Recent structural, biochemical, and biophysical studies have greatly advanced our understanding of the mechanisms of integrin bidirectional signaling across the plasma membrane. Large-scale reorientations of the ectodomain of up to 200 A couple to conformational change in ligand-binding sites and are linked to changes in alpha and beta subunit transmembrane domain association. In this review, we focus on integrin structure as it relates to affinity modulation, ligand binding, outside-in signaling, and cell surface distribution dynamics.