The talin rod IBS2 alpha-helix interacts with the beta3 integrin cytoplasmic tail membrane-proximal helix by establishing charge complementary salt bridges

Talin establishes a major link between integrins and actin filaments and contains two distinct integrin binding sites: one, IBS1, located in the talin head domain and involved in integrin activation and a second, IBS2, that maps to helix 50 of the talin rod domain and is essential for linking integrin beta subunits to the cytoskeleton ( Moes, M., Rodius, S., Coleman, S. J., Monkley, S. J., Goormaghtigh, E., Tremuth, L., Kox, C., van der Holst, P. P., Critchley, D. R., and Kieffer, N. (2007) J. Biol. Chem. 282, 17280-17288 ). Through the combined approach of mutational analysis of the beta3 integrin cytoplasmic tail and the talin rod IBS2 site, SPR binding studies, as well as site-specific antibody inhibition experiments, we provide evidence that the integrin beta3-talin rod interaction relies on a helix-helix association between alpha-helix 50 of the talin rod domain and the membrane-proximal alpha-helix of the beta3 integrin cytoplasmic tail. Moreover, charge complementarity between the highly conserved talin rod IBS2 lysine residues and integrin beta3 glutamic acid residues is necessary for this interaction. Our results support a model in which talin IBS2 binds to the same face of the beta3 subunit cytoplasmic helix as the integrin alphaIIb cytoplasmic tail helix, suggesting that IBS2 can only interact with the beta3 subunit following integrin activation.     

Amino acid sequence of a novel integrin beta 4 subunit and primary expression of the mRNA in epithelial cells.

Using the polymerase chain reaction, we have isolated cDNA clones that encode a new integrin beta subunit--beta 4. Its cDNA, which is 5676 bp in length, has one long coding sequence (5256 bp), a polyadenylation signal and a poly(A) tail. The deduced sequence of 1752 amino acids is unique among the integrin beta subunits. It contains a putative signal sequence as well as a transmembrane domain that divides the molecule into an extracellular domain at the N-terminal side and a cytoplasmic domain at the C-terminal side. The extracellular domain exhibits a 4-fold repeat of cysteine-rich motif similar to those of other integrin beta subunits. Certain features of the extracellular domain, however, are unique to the beta 4 subunit sequence. Of the 56 conserved cysteine residues found within the extracellular domain of other mature beta subunits, eight such residues are deleted from the beta 4 subunit sequence. The cytoplasmic domain is much larger (approximately 1000 amino acids) than those of other beta subunits (approximately 50 amino acids) and has no significant homology with them. A protein homology search revealed that the beta 4 subunit cytoplasmic domain has four repeating units that are homologous to the type III repetition exhibited by fibronectin. The beta 4 subunit mRNA was expressed primarily in epithelial cells. The restricted expression and the new structural features distinguish the integrin beta 4 subunit from other integrin beta subunits.     

Cloning and sequence analysis of beta-4 cDNA: an integrin subunit that contains a unique 118 kd cytoplasmic domain

The alpha 6 beta 4 complex is a member of the integrin superfamily of adhesion receptors. A human keratinocyte lambda gt11 cDNA library was screened using a monoclonal antibody directed against the beta 4 subunit. Two cDNAs were selected and subsequently used to isolate a complete set of overlapping cDNA clones. The beta 4 subunit consists of 1778 amino acids with a 683 amino acid extracellular domain, a 23 amino acid transmembrane domain and an exceptionally long cytoplasmic domain of 1072 residues. The deduced amino-terminal sequence is in good agreement with the published amino-terminal sequence of purified beta 4. The extracellular domain contains five potential N-linked glycosylation sites and four cysteine-rich homologous repeat sequences. The extracellular part of the beta 4 subunit sequence shows 35% identify with other integrin beta subunits, but is the most different among this class of molecules. The transmembrane region is poorly conserved, whereas the cytoplasmic domain shows no substantial identity in any region to the cytoplasmic tails of the known beta sequences or to other protein sequences. The exceptionally long cytoplasmic domain suggests distinct interactions of the beta 4 subunit with cytoplasmic proteins.     

Structure of the membrane domain of subunit b of the Escherichia coli F0F1 ATP synthase.

The structure of the N-terminal transmembrane domain (residues 1-34) of subunit b of the Escherichia coli F0F1-ATP synthase has been solved by two-dimensional 1H NMR in a membrane mimetic solvent mixture of chloroform/methanol/H2O (4:4:1). Residues 4-22 form an alpha-helix, which is likely to span the hydrophobic domain of the lipid bilayer to anchor the largely hydrophilic subunit b in the membrane. The helical structure is interrupted by a rigid bend in the region of residues 23-26 with alpha-helical structure resuming at Pro-27 at an angle offset by 20 degrees from the transmembrane helix. In native subunit b, the hinge region and C-terminal alpha-helical segment would connect the transmembrane helix to the cytoplasmic domain. The transmembrane domains of the two subunit b in F0 were shown to be close to each other by cross-linking experiments in which single Cys were substituted for residues 2-21 of the native subunit and b-b dimer formation tested after oxidation with Cu(II)(phenanthroline)2. Cys residues that formed disulfide cross-links were found with a periodicity indicative of one face of an alpha-helix, over the span of residues 2-18, where Cys at positions 2, 6, and 10 formed dimers in highest yield. A model for the dimer is presented based upon the NMR structure and distance constraints from the cross-linking data. The transmembrane alpha-helices are positioned at a 23 degrees angle to each other with the side chains of Thr-6, Gln-10, Phe-14, and Phe-17 at the interface between subunits. The change in direction of helical packing at the hinge region may be important in the functional interaction of the cytoplasmic domains.     

Transmembrane domain helix packing stabilizes integrin alphaIIbbeta3 in the low affinity state

Regulated changes in the affinity of integrin adhesion receptors ("activation") play an important role in numerous biological functions including hemostasis, the immune response, and cell migration. Physiological integrin activation is the result of conformational changes in the extracellular domain initiated by the binding of cytoplasmic proteins to integrin cytoplasmic domains. The conformational changes in the extracellular domain are likely caused by disruption of intersubunit interactions between the alpha and beta transmembrane (TM) and cytoplasmic domains. Here, we reasoned that mutation of residues contributing to alpha/beta interactions that stabilize the low affinity state should lead to integrin activation. Thus, we subjected the entire intracellular domain of the beta3 integrin subunit to unbiased random mutagenesis and selected it for activated mutants. 25 unique activating mutations were identified in the TM and membrane-proximal cytoplasmic domain. In contrast, no activating mutations were identified in the more distal cytoplasmic tail, suggesting that this region is dispensable for the maintenance of the inactive state. Among the 13 novel TM domain mutations that lead to integrin activation were several informative point mutations that, in combination with computational modeling, suggested the existence of a specific TM helix-helix packing interface that maintains the low affinity state. The interactions predicted by the model were used to identify additional activating mutations in both the alpha and beta TM domains. Therefore, we propose that helical packing of the alpha and beta TM domains forms a clasp that regulates integrin activation.     

Crosslinking between alpha and beta subunits defines the orientation and spatial relationship of some of the transmembrane helices of the proton-translocating pyridine nucleotide transhydrogenase of Escherichia coli

The proton-translocating pyridine nucleotide transhydrogenase of Escherichia coli is composed of two types of subunits, alpha and beta, organized as an alpha(2)beta(2) tetramer. The protein contains three recognizable domains, of which domain II is the transmembrane region of the molecule containing the pathway for proton translocation. Domain II is composed of four transmembrane helices at the carboxyl-terminus of the alpha subunit and either eight or nine transmembrane helices at the amino-terminal region of the beta subunit. We have introduced pairs of cysteine residues into a cysteine-free transhydrogenase by site-directed mutagenesis. Disulfide bond formation between some of these cysteine residues occurred spontaneously or on treatment with cupric 1, 10-phenanthrolinate. Analysis of crosslinked products confirmed that there are nine transmembrane helices in the domain II region of the beta subunit. The proximity to one another of several of the transmembrane helices was determined. Thus, helices 2 and 4 are close to helix 6 (nomenclature of Meuller and Rydstr?m, J. Biol. Chem. 274, 19072-19080, 1999), and helix 3 and the carboxyl-terminal eight residues of the alpha subunit are close to helix 7. In the alpha(2)beta(2) tetramer, helices 2 and 4 of one alpha subunit are close to the same pair of transmembrane helices of the other alpha subunit, and helix 6 of one beta subunit is close to helix 6 of the other beta subunit.     

Threonine phosphorylation sites in the beta 2 and beta 7 leukocyte integrin polypeptides

The cytoplasmic domains of integrins play a key role in a variety of integrin-mediated events including adhesion, migration, and signaling. The molecular mechanisms that enhance integrin function are still incompletely understood. Because protein kinases are known to be involved in the signaling and the activation of integrins, the role of phosphorylation has been studied by several groups. The beta(2) leukocyte integrin subunit has previously been shown to become phosphorylated in leukocytes on cytoplasmic serine and functionally important threonine residues. We have now mapped the phosphorylated threonine residues in activated T cells. After phorbol ester stimulation, all three threonine residues (758-760) of the threonine triplet became phosphorylated but only two at a time. CD3 stimulation leads to a strong threonine phosphorylation of the beta(2) integrin, but differed from phorbol ester activation in that phosphorylation occurred only on threonine 758. The other leukocyte-specific integrin, beta(7), has also been shown to need the cytoplasmic domain and leukocyte-specific signal transduction elements for integrin activation. Cell activation with phorbol ester, and interestingly, through the TCR-CD3 complex, caused beta(7) integrin binding to VCAM-1. Additionally, cell activation led to increased phosphorylation of the beta(7) subunit, and phosphoamino acid analysis revealed that threonine residues became phosphorylated after cell activation. Sequence analysis by manual radiosequencing by Edman degradation established that threonine phosphorylation occurred in the same threonine triplet as in beta(2) phosphorylation.     

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.     

Association of the membrane proximal regions of the alpha and beta subunit cytoplasmic domains constrains an integrin in the inactive state

The adhesiveness of integrins is regulated through a process termed "inside-out" signaling. To understand the molecular mechanism of integrin inside-out signaling, we generated K562 stable cell lines that expressed LFA-1 (alpha(L)beta(2)) or Mac-1 (alpha(M)beta(2)) with mutations in the cytoplasmic domain. Complete truncation of the beta(2) cytoplasmic domain, but not a truncation that retained the membrane proximal eight residues, resulted in constitutive activation of alpha(L)beta(2) and alpha(M)beta(2), demonstrating the importance of this membrane proximal region in the regulation of integrin adhesive function. Furthermore, replacement of the alpha(L) and beta(2) cytoplasmic domains with acidic and basic peptides that form an alpha-helical coiled coil caused inactivation of alpha(L)beta(2). Association of these artificial cytoplasmic domains was directly demonstrated. By contrast, replacement of the alpha(L) and beta(2) cytoplasmic domains with two basic peptides that do not form an alpha-helical coiled coil activated alpha(L)beta(2). Induction of ligand binding by the activating cytoplasmic domain mutations correlated with the induction of activation epitopes in the extracellular domain. Our data demonstrate that cytoplasmic, membrane proximal association between integrin alpha and beta subunits, constrains an integrin in the inactive conformation.     

Cloning and expression of a divergent integrin subunit beta 8

Rabbit and human cDNA clones have been identified that encode a novel integrin beta subunit. The sequences that encode this subunit, which has been designated as beta 8, were isolated initially from rabbit placental cDNA libraries using an oligonucleotide probe derived from a highly conserved region of integrin beta subunit sequences. The rabbit clone was used to isolate human beta 8 cDNA clones from human placental and MG-63 osteosarcoma cell libraries. The putative beta 8 polypeptides, which comprise 769 and 768 residues in human and rabbit, respectively, show a high degree of inter-species conservation (approximately 90% identity). In contrast, beta 8 is distinct from the other integrin beta subunits. At the amino acid level human beta 8 ranges from 31 to 37% identity with human beta 1-7. The domain structure of beta 8 is typical of the integrin beta subunits. Human beta 8 has a 42-residue N-terminal signal peptide, a large extracellular domain (approximately 639 residues) that contains four cysteine-rich repeats, a transmembrane domain (approximately 30 residues), and a C-terminal cytoplasmic domain (approximately 58 residues). There are several structural features that are unique to the beta 8 polypeptide, as compared with the other integrin beta subunits. Six of the 56 cysteine residues that are conserved within the extracellular domains of beta 1, beta 2, beta 3, beta 5, beta 6, and the beta subunit from Drosophila are absent in the beta 8 polypeptide. Also, the cytoplasmic domain of the beta 8 subunit shares no homology with the cytoplasmic regions of any of the other integrin beta subunits. Northern analysis demonstrated an approximately 8-kilobase beta 8 mRNA in rabbit placenta, kidney, brain, ovary, and uterus. PCR analysis revealed that beta 8 mRNA is also present in several transformed human cell lines. The beta 8 polypeptide has been transiently expressed in 293 human embryonic kidney cells. A polyclonal antipeptide antibody specific for beta 8 and a polyclonal antibody that recognizes alpha v epitopes were used to show that beta 8 can complex with the endogenous alpha v subunit in 293 cells and that the resulting integrin is expressed as a cell surface complex.     

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 α_IIbβ₃ constructs, we discovered that an α_IIb 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 α_IIb cytoplasmic tail. Truncation of the α_IIb 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.     

The Talin head domain binds to integrin beta subunit cytoplasmic tails and regulates integrin activation.

The beta subunit cytoplasmic domains of integrin adhesion receptors are necessary for the connection of these receptors to the actin cytoskeleton. The cytoplasmic protein, talin, binds to beta integrin cytoplasmic tails and actin filaments, hence forming an integrin-cytoskeletal linkage. We used recombinant structural mimics of beta(1)A, beta(1)D and beta(3) integrin cytoplasmic tails to characterize integrin-binding sites within talin. Here we report that an integrin-binding site is localized within the N-terminal talin head domain. The binding of the talin head domain to integrin beta tails is specific in that it is abrogated by a single point mutation that disrupts integrin localization to talin-rich focal adhesions. Integrin-cytoskeletal interactions regulate integrin affinity for ligands (activation). Overexpression of a fragment of talin containing the head domain led to activation of integrin alpha(IIb)beta(3); activation was dependent on the presence of both the talin head domain and the integrin beta(3) cytoplasmic tail. The head domain of talin thus binds to integrins to form a link to the actin cytoskeleton and can thus regulate integrin function.     

Structure of the integrin alphaIIb transmembrane segment

Integrin cell-adhesion receptors transduce signals bidirectionally across the plasma membrane via the single-pass transmembrane segments of each alpha and beta subunit. While the beta3 transmembrane segment consists of a linear 29-residue alpha-helix, the structure of the alphaIIb transmembrane segment reveals a linear 24-residue alpha-helix (Ile-966 -Lys-989) followed by a backbone reversal that packs Phe-992-Phe-993 against the transmembrane helix. The length of the alphaIIb transmembrane helix implies the absence of a significant transmembrane helix tilt in contrast to its partnering beta3 subunit. Sequence alignment shows Gly-991-Phe-993 to be fully conserved among all 18 human integrin alpha subunits, suggesting that their unusual structural motif is prototypical for integrin alpha subunits. The alphaIIb transmembrane structure demonstrates a level of complexity within the membrane that is beyond simple transmembrane helices and forms the structural basis for assessing the extent of structural and topological rearrangements upon alphaIIb-beta3 association, i.e. integrin transmembrane signaling.     

Complete amino acid sequence of a novel integrin beta subunit (beta 6) identified in epithelial cells using the polymerase chain reaction

The integrin family of adhesion receptors consists of several heterodimeric glycoproteins, each composed of one alpha and one beta subunit. Three different mammalian beta subunits, beta 1, beta 2, and beta 3, have been sequenced, but recent evidence suggests the existence of several others. Amplification of guinea pig airway epithelial cell cDNA with oligonucleotide primers designed to recognize consensus integrin beta subunit sequences led to the identification of a novel partial cDNA sequence. Clones containing portions of this sequence were used to screen cDNA libraries constructed from the human pancreatic carcinoma cell line FG-2 and identified a series of overlapping clones encoding the full-length sequence of the human homologue of this protein. This sequence of 788 amino acids is 43, 38, and 47% identical to the sequences of beta 1, beta 2, and beta 3, respectively. Features shared between this novel protein and the previously sequenced beta subunits include the positions of all 56 cysteine residues in the extracellular domain, the single putative transmembrane domain, and the short putative cytoplasmic domain. However, a unique 11-amino acid extension at the carboxyl terminus, not present in any of the other beta subunits, is suggestive of distinctive interactions with cytoplasmic components. Comparison of the human and guinea pig sequences reveals a high degree (94%) of cross-species conservation. Because this protein is clearly distinct from the two other recently described integrins beta 4 and beta 5, we propose to designate it beta 6.     

CD98hc (SLC3A2) interaction with the integrin beta 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.     

Dynamic changes in the osteoclast cytoskeleton in response to growth factors and cell attachment are controlled by beta3 integrin

The beta3 integrin cytoplasmic domain, and specifically S752, is critical for integrin localization and osteoclast (OC) function. Because growth factors such as macrophage colony-stimulating factor and hepatocyte growth factor affect integrin activation and function via inside-out signaling, a process requiring the beta integrin cytoplasmic tail, we examined the effect of these growth factors on OC precursors. To this end, we retrovirally expressed various beta3 integrins with cytoplasmic tail mutations in beta3-deficient OC precursors. We find that S752 in the beta3 cytoplasmic tail is required for growth factor-induced integrin activation, cytoskeletal reorganization, and membrane protrusion, thereby affecting OC adhesion, migration, and bone resorption. The small GTPases Rho and Rac mediate cytoskeletal reorganization, and activation of each is defective in OC precursors lacking a functional beta3 subunit. Activation of the upstream mediators c-Src and c-Cbl is also dependent on beta3. Interestingly, although the FAK-related kinase Pyk2 interacts with c-Src and c-Cbl, its activation is not disrupted in the absence of functional beta3. Instead, its activation is dependent upon intracellular calcium, and on the beta2 integrin. Thus, the beta3 cytoplasmic domain is responsible for activation of specific intracellular signals leading to cytoskeletal reorganization critical for OC function.     

Cooperative role of the membrane-proximal and -distal residues of the integrin beta3 cytoplasmic domain in regulation of talin-mediated alpha IIb beta3 activation

Integrin cytoplasmic tails regulate integrin activation that is required for high affinity binding with ligands. The interaction of the integrin beta subunit tail with a cytoplasmic protein, talin, largely contributes to integrin activation. Here we report the cooperative interaction of the beta3 membrane-proximal and -distal residues in regulation of talin-mediated alpha IIb beta3 activation. Because a chimeric integrin, alpha IIb beta3/beta1, in which the beta3 tail was replaced with the beta1 tail was constitutively active, we searched for the residues responsible for integrin activation among the residues that differed between the beta3 and beta1 tails. Single amino acid substitutions of Ile-719 and Glu-749 in the beta3 membrane-proximal and -distal regions, respectively, with the corresponding beta1 residues or alanine rendered alphaIIbbeta3 constitutively active. The I719M/E749S double mutant had the same ligand binding activity as alpha IIb beta3/beta1. These beta3 mutations also induced alphaVbeta3 activation. Conversely, substitution of Met-719 or Ser-749 in the beta1 tail with the corresponding beta3 tail residue (M719I or S749E) inhibited alpha IIb beta3/beta1 activation, and the M719I/S749E double mutant inhibited ligand binding to a level comparable with that of the wild-type alpha IIb beta3. Knock down of talin by short hairpin RNA inhibited the I719M- and E749S-induced alpha IIb beta3 activation. These results suggest that the beta3 membrane-proximal and -distal residues cooperatively regulate talin-mediated alpha IIb beta3 activation.     

Conformational Changes in Talin on Binding to Anionic Phospholipid Membranes Facilitate Signaling by Integrin Transmembrane Helices

Integrins are heterodimeric (αβ) cell surface receptors that are activated to a high affinity state by the formation of a complex involving the α/β integrin transmembrane helix dimer, the head domain of talin (a cytoplasmic protein that links integrins to actin), and the membrane. The talin head domain contains four sub-domains (F0, F1, F2 and F3) with a long cationic loop inserted in the F1 domain. Here, we model the binding and interactions of the complete talin head domain with a phospholipid bilayer, using multiscale molecular dynamics simulations. The role of the inserted F1 loop, which is missing from the crystal structure of the talin head, PDB:3IVF, is explored. The results show that the talin head domain binds to the membrane predominantly via cationic regions on the F2 and F3 subdomains and the F1 loop. Upon binding, the intact talin head adopts a novel V-shaped conformation which optimizes its interactions with the membrane. Simulations of the complex of talin with the integrin α/β TM helix dimer in a membrane, show how this complex promotes a rearrangement, and eventual dissociation of, the integrin α and β transmembrane helices. A model for the talin-mediated integrin activation is proposed which describes how the mutual interplay of interactions between transmembrane helices, the cytoplasmic talin protein, and the lipid bilayer promotes integrin inside-out activation.     

Protein phosphatase 1 associates with the integrin alphaIIb subunit and regulates signaling

Regulation of integrin activation occurs by specific interactions among cytoplasmic proteins and integrin alpha and beta cytoplasmic tails. We report that the catalytic subunit of protein phosphatase 1 (PP1c) constitutively associates with the prototypic integrin alphaIIbbeta3 in platelets and in cell lines overexpressing the integrin. PP1c binds directly to the cytoplasmic domain of integrin alphaIIb subunit containing a conserved PP1c binding motif 989KVGF992. Anchored PP1c is inactive, while thrombin-induced platelet aggregation or fibrinogen-alphaIIbbeta3 engagement caused PP1c dissociation and concomitant activation as revealed by dephosphorylation of PP1c substrate, myosin light chain. Inhibition of ligand binding to activated alphaIIbbeta3 blocks PP1c dissociation and represses PP1c activation. These studies reveal a previously unrecognized role for integrins whereby the alpha subunit cytoplasmic tail localizes the machinery for initiating and temporally maintaining the regulatory signaling activity of a phosphatase.     

A specific interface between integrin transmembrane helices and affinity for ligand

Conformational communication across the plasma membrane between the extracellular and intracellular domains of integrins is beginning to be defined by structural work on both domains. However, the role of the α and β subunit transmembrane domains and the nature of signal transmission through these domains have been elusive. Disulfide bond scanning of the exofacial portions of the integrin α_IIβ and β₃ transmembrane domains reveals a specific heterodimerization interface in the resting receptor. This interface is lost rather than rearranged upon activation of the receptor by cytoplasmic mutations of the α subunit that mimic physiologic inside-out activation, demonstrating a link between activation of the extracellular domain and lateral separation of transmembrane helices. Introduction of disulfide bridges to prevent or reverse separation abolishes the activating effect of cytoplasmic mutations, confirming transmembrane domain separation but not hinging or piston-like motions as the mechanism of transmembrane signaling by integrins.