Phosphorylation of the integrin alpha 4 cytoplasmic domain regulates paxillin binding

alpha4 integrins are essential for embryogenesis, hematopoiesis, inflammation, and immune response possibly because alpha4 integrins have distinct signaling properties from other integrins. Specifically, the alpha4 cytoplasmic domain binds tightly to paxillin, a signaling adaptor protein, leading to increased cell migration and an altered cytoskeletal organization that results in reduced cell spreading. The alpha4 tail contains potential phosphorylation sites clustered in its core paxillin binding region. We now report that the alpha4 tail is phosphorylated in vitro and in vivo. Furthermore, Ser(988) is a major phosphorylation site. Using antibodies specific for Ser(988)-phosphorylated alpha4, we found the stoichiometry of alpha4 phosphorylation varied in different cells. However, >60% of alpha4 was phosphorylated in Jurkat T cells. Phosphorylation at Ser(988) blocked paxillin binding to the alpha4 tail. A phosphorylation-mimicking mutant of alpha4 (alpha4S988D) blocked paxillin binding and reversed the inhibitory effect of alpha4 on cell spreading. Consequently, alpha4 phosphorylation is a biochemical mechanism to modulate paxillin binding to alpha4 integrins with consequent regulation of alpha4 integrin-dependent cellular functions.     

Spermidine/spermine N1-acetyltransferase specifically binds to the integrin alpha9 subunit cytoplasmic domain and enhances cell migration

The integrin alpha9beta1 is expressed on migrating cells, such as leukocytes, and binds to multiple ligands that are present at sites of tissue injury and inflammation. alpha9beta1, like the structurally related integrin alpha4beta1, mediates accelerated cell migration, an effect that depends on the alpha9 cytoplasmic domain. alpha4beta1 enhances migration through reversible binding to the adapter protein, paxillin, but alpha9beta1-dependent migration is paxillin independent. Using yeast two-hybrid screening, we identified the polyamine catabolizing enzyme spermidine/spermine N(1)-acetyltransferase (SSAT) as a specific binding partner of the alpha9 cytoplasmic domain. Overexpression of SSAT increased alpha9beta1-mediated migration, and small interfering RNA knockdown of SSAT inhibited this migration without affecting cell adhesion or migration that was mediated by other integrin cytoplasmic domains. The enzyme activity of SSAT is critical for this effect, because a catalytically inactive version did not enhance migration. We conclude that SSAT directly binds to the alpha9 cytoplasmic domain and mediates alpha9-dependent enhancement of cell migration, presumably by localized effects on acetylation of polyamines or of unidentified substrates.     

Reelin binds alpha3beta1 integrin and inhibits neuronal migration

Mice that are mutant for Reelin or Dab1, or doubly mutant for the VLDL receptor (VLDLR) and ApoE receptor 2 (ApoER2), show disorders of cerebral cortical lamination. How Reelin and its receptors regulate laminar organization of cerebral cortex is unknown. We show that Reelin inhibits migration of cortical neurons and enables detachment of neurons from radial glia. Recombinant and native Reelin associate with alpha3beta1 integrin, which regulates neuron-glia interactions and is required to achieve proper laminar organization. The effect of Reelin on cortical neuronal migration in vitro and in vivo depends on interactions between Reelin and alpha3beta1 integrin. Absence of alpha3beta1 leads to a reduction of Dab1, a signaling protein acting downstream of Reelin. Thus, Reelin may arrest neuronal migration and promote normal cortical lamination by binding alpha3beta1 integrin and modulating integrin-mediated cellular adhesion.     

Actin binds to the cytoplasmic tail of alpha 1 subunit of integrin in hepatocytes

alpha 1 beta 1-Integrin is a common receptor for laminin and collagen IV on hepatocytes. The interactions of intracellular domain of integrins with cytoplasmic elements are critical in the initiation and transduction of signals. In order to understand the nature of cytoplasmic components that can interact with cytoplasmic domain of alpha 1 integrin, cytoplasmic extracts of monolayers of rat hepatocytes were subjected to chromatography over an affinity column prepared by coupling a 60-mer synthetic cytoplasmic tail of alpha 1 subunit. SDS-PAGE analysis of the eluate showed the presence of a 47 kDa protein. Dot-Blot assay using radio-iodinated 47 kDa protein showed the binding of the protein to 60-mer C tail in a concentration dependent manner. Immunoblot analysis using specific antibodies showed that the 47 kDa protein is actin.     

The cytoplasmic domain of the integrin alpha9 subunit requires the adaptor protein paxillin to inhibit cell spreading but promotes cell migration in a paxillin-independent manner

The integrin alpha9 subunit forms a single heterodimer, alpha9beta1. The alpha9 subunit is most closely related to the alpha4 subunit, and like alpha4 integrins, alpha9beta1 plays an important role in leukocyte migration. The alpha4 cytoplasmic domain preferentially enhances cell migration and inhibits cell spreading, effects that depend on interaction with the adaptor protein, paxillin. To determine whether the alpha9 cytoplasmic domain has similar effects, a series of chimeric and deleted alpha9 constructs were expressed in Chinese hamster ovary cells and tested for their effects on migration and spreading on an alpha9beta1-specific ligand. Like alpha4, the alpha9 cytoplasmic domain enhanced cell migration and inhibited cell spreading. Paxillin also specifically bound the alpha9 cytoplasmic domain and to a similar level as alpha4. In paxillin(-/-) cells, alpha9 failed to inhibit cell spreading as expected but surprisingly still enhanced cell migration. Further, mutations that abolished the alpha9-paxillin interaction prevented alpha9 from inhibiting cell spreading but had no effect on alpha9-dependent cell migration. These findings suggest that the mechanisms by which the cytoplasmic domains of integrin alpha subunits enhance migration and inhibit cell spreading are distinct and that the alpha9 and alpha4 cytoplasmic domains, despite sequence and functional similarities, enhance cell migration by different intracellular signaling pathways.     

Integrin alpha 4 beta 1-dependent T cell migration requires both phosphorylation and dephosphorylation of the alpha 4 cytoplasmic domain to regulate the reversible binding of paxillin

alpha 4 integrins mediate increased cell migration and decreased cell spreading because the alpha 4 cytoplasmic domain (tail) binds tightly to paxillin, a signaling adaptor protein. Paxillin binding to the alpha 4 tail is blocked by alpha 4 phosphorylation at Ser988. To establish the biological role of alpha 4 phosphorylation, we reconstituted alpha 4-deficient Jurkat T cells with phosphorylation-mimicking (alpha 4(S988D)) or non-phosphorylatable (alpha 4(S988A)) mutants. alpha 4(S988D) disrupted paxillin binding and also inhibited cell migration and promoted cell spreading. In contrast, the non-phosphorylatable alpha 4(S988A) resulted in a further reduction in cell spreading; however, this mutation led to an unexpected suppression of cell migration. The suppression of cell migration by alpha 4(S988A) was ascribable to enhanced alpha 4-paxillin association, because enforced association by an alpha 4-paxillin fusion led to a phenotype similar to that of the non-phosphorylatable alpha 4(S988A) mutant. These data establish that optimal alpha 4-mediated cell migration requires both phosphorylation and dephosphorylation of the alpha 4 cytoplasmic domain to regulate the reversible binding of paxillin.     

Specialized functional properties of the integrin alpha 4 cytoplasmic domain.

For functional studies of the integrin alpha 4 cytoplasmic domain, we have expressed the following in K562 and Chinese hamster ovary (CHO) cells: 1) wild-type alpha 4 (called X4C4), 2) two chimeric forms of alpha 4 (called X4C2 and X4C5) that contain the cytoplasmic domains of alpha 2 and alpha 5, respectively, and 3) alpha 4 with no cytoplasmic domain (X4C0). Cytoplasmic domain exchange had no effect on VLA-4-dependent static cell adhesion or tethering to VCAM-1 in conditions of shear flow. However, the presence of the alpha 2 or alpha 5 tails markedly enhanced VLA-4-dependent K562 cells spreading (X4C2 > X4C5 > X4C4 > X4C0), increased localization of VLA-4 into focal adhesion-like complexes in CHO cells (X4C2 > X4C5 > X4C4), and strengthened CHO and K562 cell resistance to detachment from VCAM-1 in conditions of shear flow (X4C2 > X4C5 > X4C4 > X4C0). Conversely, the alpha 4 tail supported greater VLA-4-dependent haptotactic and chemotactic cell migration. In the absence of any alpha tail (i.e., X4C0), robust focal adhesions were observed, even though cell spreading and adhesion strengthening were minimal. Thus, such focal adhesions may have relatively little functional importance, and should not be compared with focal adhesions formed when alpha tails are present. Together, these results indicate that all three alpha-chain tails exert defined positive effects (compared with no tail at all), but suggest that the alpha 4 cytoplasmic domain may be specialized to engage in weaker cytoskeletal interactions, leading to diminished focal adhesion formation, cell spreading, and adhesion strengthening, while augmenting cell migration and facilitating rolling under shear flow. These properties of the alpha 4 tail are consistent with the role of alpha 4 integrins on highly motile lymphocytes, monocytes, and eosinophils.     

A membrane-distal segment of the integrin alpha IIb cytoplasmic domain regulates integrin activation

Previous evidence suggests that interactions between integrin cytoplasmic domains regulate integrin activation. We have constructed and validated recombinant structural mimics of the heterodimeric alpha(IIb)beta(3) cytoplasmic domain. The mimics elicited polyclonal antibodies that recognize a combinatorial epitope(s) formed in mixtures of the alpha(IIb) and beta(3) cytoplasmic domains but not present in either isolated tail. This epitope(s) is present within intact alpha(IIb)beta(3), indicating that interaction between the tails can occur in the native integrin. Furthermore, the combinatorial epitope(s) is also formed by introducing the activation-blocking beta(3)(Y747A) mutation into the beta(3) tail. A membrane-distal heptapeptide sequence in the alpha(IIb) tail ((997)RPPLEED) is responsible for this effect on beta(3). Membrane-permeant palmitoylated peptides, containing this alpha(IIb) sequence, specifically blocked alpha(IIb)beta(3) activation in platelets. Thus, this region of the alpha(IIb) tail causes the beta(3) tail to resemble that of beta(3)(Y747A) and suppresses activation of the integrin.     

Alpha1B-adrenoceptor signaling and cell motility: GTPase function of Gh/transglutaminase 2 inhibits cell migration through interaction with cytoplasmic tail of integrin alpha subunits

A multifunctional enzyme, G(h), is a GTP-binding protein that couples to the alpha(1B)-adrenoreceptor and stimulates phospholipase C-delta1 but also displays transglutaminase 2 (TG2) activity. G(h)/TG2 has been implicated to play a role in cell motility. In this study we have examined which function of G(h)/TG2 is involved in this cellular response and the molecular basis. Treatment of human aortic smooth muscle cell with epinephrine inhibits migration to fibronectin and vitronectin, and the inhibition is blocked by the alpha(1)-adrenoreceptor antagonist prazosin or chloroethylclonidine. Up-regulation or overexpression of G(h)/TG2 in human aortic smooth muscle cells, DDT1-MF2, or human embryonic kidney cells, HEK 293 cells, results in inhibition of the migratory activity, and stimulation of the alpha(1B)-adrenoreceptor with the alpha(1) agonist further augments the inhibition of migration of human aortic smooth muscle cells and DDT1-MF2. G(h)/TG2 is coimmunoprecipitated by an integrin alpha(5) antibody and binds to the cytoplasmic tail peptide of integrins alpha(5), alpha(v), and alpha(IIb) subunits in the presence of guanosine 5'-3-O-(thio)triphosphate (GTPgammaS). Mutation of Lys-Arg residues in the GFFKR motif, present in the alpha(5)-tail, significantly reduces the binding of GTPgammaS-G(h)/TG2. Moreover, the motif-containing integrin alpha(5)-tail peptides block G(h)/TG2 coimmunoprecipitation and reverse the inhibition of the migratory activity of HEK 293 cells caused by overexpression G(h)/TG2. These results provide evidence that G(h) function initiates the modulation of cell motility via association of GTP-bound G(h)/TG2 with the GFFKR motif located in integrin alpha subunits.     

Synectin, syndecan-4 cytoplasmic domain binding PDZ protein, inhibits cell migration

Syndecan-4, a member of the syndecan gene family of proteoglycans, is an important regulator of bFGF signaling. In particular, bFGF-dependent regulation of cell growth and migration has been linked to syndecan-4 cytoplasmic domain-mediated interactions. Screening of a yeast two-hybrid library with a cytoplasmic domain of rat syndecan-4 identified a novel binding partner, here termed synectin. Synectin is highly homologous to semaphorin F binding protein semcap1, glucose 1 transporter binding protein glut1cbp, and RGS-GAIP/neuropilin-1 binding protein GIPC. Overexpression of synectin in ECV304 cells in culture led to a dose-dependent inhibition of migration while not affecting cell adhesion or growth rate. We conclude that synectin is involved in syndecan-4-dependent interactions and may play a role in the assembly of syndecan-4 signaling complex.     

Role of the alpha(1) integrin cytoplasmic tail in the formation of focal complexes, actin organization, and in the control of cell migration

Integrins play a key role in cellular motility; an essential process for embryonic development and tissue morphogenesis, and also for pathological processes such as tumor cell invasion and metastasis. Recently, we showed that the cytoplasmic tail of integrin alpha(1) regulates the formation of focal complexes, F-actin cytoskeleton reorganization, and migration. We now report that the alpha(1) tail directly engages in collagen IV-mediated migration by regulation of the small GTPase Rac1. Deletion variants of the alpha(1) integrin differ in their ability to activate Rac1. Constitutively active Rac1 rescues motility in otherwise immotile cells expressing a truncated alpha(1) integrin without any cytoplasmic tail. In these cells, levels of GTP-Rac1 are constitutively elevated, but kept non-functional in the cytoplasm. The conserved GFFKR motif is sufficient to convey Rac1 activation, but downregulates the amount of GTP-Rac1 in the absence of the alpha(1)-specific sequence PLKKKMEK. This sequence is also required for the recruitment of PI3K to focal adhesions following Rac1 activation. Our results demonstrate that the short alpha(1) cytoplasmic tail is crucial for Rac1 activation and PI3K localization, which in turn results in cytoskeletal rearrangement and subsequent migration.     

Regulation of integrin alpha vbeta 3-mediated endothelial cell migration and angiogenesis by integrin alpha5beta1 and protein kinase A

Recent studies indicate that angiogenesis depends, in part, on ligation of integrin alpha(5)beta(1) by fibronectin. Evidence is now provided that integrin alpha(5)beta(1) regulates the function of integrin alpha(v)beta(3) on endothelial cells during their migration in vitro or angiogenesis in vivo. Secretion of fibronectin by endothelial cells leads to the ligation of integrin alpha(5)beta(1), which potentiates alpha(v)beta(3)-mediated migration on vitronectin without influencing alpha(v)beta(3)-mediated cell adhesion. Endothelial cell attachment to vitronectin suppresses protein kinase A (PKA) activity, while addition of soluble anti-alpha(5)beta(1) restores this activity. Moreover, agents that activate intracellular PKA, such as forskolin, dibutyryl cAMP or alpha(5)beta(1) antagonists, suppress endothelial cell migration on vitronectin in vitro or angiogenesis in vivo. In contrast, inhibitors of PKA reverse the anti-migratory or anti-angiogenic effects mediated by alpha(5)beta(1) antagonists. Therefore, alpha(v)beta(3)-mediated endothelial cell migration and angiogenesis can be regulated by PKA activity, which depends on the ligation state of integrin alpha(5)beta(1).     

Semaphorin 4D/Plexin-B1-mediated R-Ras GAP activity inhibits cell migration by regulating beta(1) integrin activity

Plexins are cell surface receptors for semaphorins and regulate cell migration in many cell types. We recently reported that the semaphorin 4D (Sema4D) receptor Plexin-B1 functions as a GTPase-activating protein (GAP) for R-Ras, a member of Ras family GTPases implicated in regulation of integrin activity and cell migration. We characterized the role of R-Ras downstream of Sema4D/Plexin-B1 in cell migration. Activation of Plexin-B1 by Sema4D suppressed the ECM-dependent R-Ras activation, R-Ras-mediated phosphatydylinositol 3-kinase activation, and beta(1) integrin activation through its R-Ras GAP domain, leading to inhibition of cell migration. In addition, inactivation of R-Ras by overexpression of the R-Ras-specific GAP or knockdown of R-Ras by RNA interference was sufficient for suppressing beta(1) integrin activation and cell migration in response to the ECM stimulation. Thus, we conclude that R-Ras activity is critical for ECM-mediated beta(1) integrin activation and cell migration and that inactivation of R-Ras by Sema4D/Plexin-B1-mediated R-Ras GAP activity controls cell migration by modulating the activity of beta(1) integrins.     

Extracellular matrix-derived peptide binds to alpha(v)beta(3) integrin and inhibits angiogenesis

Angiogenesis is associated with several pathological disorders as well as with normal physiological maintenance. Components of vascular basement membrane are speculated to regulate angiogenesis in both positive and negative manner. Recently, we reported that tumstatin (the NC1 domain of alpha 3 chain of type IV collagen) and its deletion mutant tum-5 possess anti-angiogenic activity. In the present study, we confirm that the anti-angiogenic activity of tumstatin and tum-5 is independent of disulfide bond requirement. This property of tum-5 allowed us to use overlapping synthetic peptide strategy to identify peptide sequence(s) which possess anti-angiogenic activity. Among these peptides, only the T3 peptide (69-88 amino acids) and T7 peptide (74-98 amino acids) inhibited proliferation and induced apoptosis specifically in endothelial cells. The peptides, similar to tumstatin and the tum-5 domain, bind and function via alpha(v)beta(3) in an RGD-independent manner. Restoration of a disulfide bond between two cysteines within the peptide did not alter the anti-angiogenic activity. Additionally, these studies show that tumstatin peptides can inhibit proliferation of endothelial cells in the presence of vitronectin, fibronectin, and collagen I. Anti-angiogenic effect of the peptides was further confirmed in vivo using a Matrigel plug assay in C57BL/6 mice. Collectively, these experiments suggest that the anti-angiogenic activity of tumstatin is localized to a 25-amino acid region of tumstatin and it is independent of disulfide bond linkage. Structural features and potency of the tumstatin peptide make it highly feasible as a potential anti-cancer drug.     

Paxillin binding to a conserved sequence motif in the alpha 4 integrin cytoplasmic domain

alpha(4)beta(1) integrin-mediated cell adhesion results in increased cell migration, reduced cell spreading, and focal adhesion formation relative to other beta(1) integrins. Paxillin, a signaling adapter protein, binds tightly to the alpha(4) cytoplasmic domain and is implicated in alpha(4) integrin signaling. We now report the mapping of a paxillin-binding site in the alpha(4) cytoplasmic domain and an assessment of its role in the alpha(4) tail-specific integrin functions. By using truncation mutants and a peptide competition assay, we found that a region of 9 amino acid residues (Glu(983)-Tyr(991)) within the alpha(4) cytoplasmic domain contains a minimal sequence sufficient for paxillin binding. Alanine scanning of this region implicated Tyr(991) and Glu(983) as critical residues. The role of these residues was confirmed by introducing these Ala substitutions into the full-length alpha(4) tail sequence. Y991A or E983A substitution disrupted the interaction of alpha(4) integrins with paxillin. These same two point mutations reversed the effects of the alpha(4) tail on cell spreading. The key features of the identified paxillin-binding sequence are present in all alpha(4) integrins sequenced to date, including that from Xenopus laevis. The maintenance of this sequence motif suggests that paxillin binding is an evolutionarily conserved function of alpha(4) integrins.     

Specific residues within the alpha 2 integrin subunit cytoplasmic domain regulate migration and cell cycle progression via distinct MAPK pathways.

The alpha(2) integrin subunit cytoplasmic domain is necessary for epidermal growth factor (EGF)-stimulated chemotactic migration and insulin-dependent entry into S-phase of mammary epithelial cells adherent to type I collagen. Truncation mutants revealed that the seven amino acids, KYEKMTK, in addition to the GFFKR motif were sufficient for these functions. Mutation of tyrosine 1134 to alanine inhibited the ability of the cells to phosphorylate p38 MAPK and to migrate in response to EGF but had only a modest effect on the ability of the cells to induce sustained phosphorylation of the ERK MAPK, to up-regulate cyclin E and cdk2 expression, and to enter S-phase when adherent to type I collagen. Conversely, mutation of the lysine 1136 inhibited the ability of the cells to increase cyclin E and cdk2 expression, to maintain long term phosphorylation of the ERK MAPK, and to enter S-phase but had no effect on the ability of the cells to phosphorylate the p38 MAPK or to migrate on type I collagen in response to EGF. Methionine 1137 was essential for both migration and entry into S-phase. Thus, distinctly different structural elements of the alpha(2) integrin cytoplasmic domain are required to engage the signaling pathways leading to cell migration or cell cycle progression.     

The integrin alpha 7 cytoplasmic domain regulates cell migration, lamellipodia formation, and p130CAS/Crk coupling

The integrin alpha(7)beta(1) is the major laminin-binding integrin in skeletal, heart, and smooth muscle and is a receptor for laminin-1 and -2. It mediates myoblast migration on laminin-1 and -2 and thus might be involved in muscle development and repair. Previously we have shown that alpha(7)B as well as the alpha(7)A and -C splice variants induce cell motility on laminin when transfected into nonmotile HEK293 cells. In this study we have investigated the role of the cytoplasmic domain of alpha(7) in the laminin-induced signal transduction of alpha(7)beta(1) integrin regulating cell adhesion and migration. Deletion of the cytoplasmic domain did not affect assembly of the mutated alpha(7)Deltacyt/beta(1) heterodimer on the cell surface or adhesion of alpha(7)Deltacyt-transfected cells to laminin. The motility of these cells on the laminin-1/E8 fragment, however, was significantly reduced to the level of mock-transfected cells; lamellipodia formation and polarization of the cells were also impaired. Adhesion to the laminin-1/E8 fragment induced tyrosine phosphorylation of the focal adhesion kinase, paxillin, and p130(CAS) as well as the formation of a p130(CAS)-Crk complex in wild-type alpha(7)B-transfected cells. In alpha(7)BDeltacyt cells, however, the extent of p130(CAS) tyrosine formation was reduced and formation of the p130(CAS)-Crk complex was impaired, with unaltered levels of p130(CAS) and Crk protein levels. These findings indicate adhesion-dependent regulation of p130(CAS)/Crk complex formation by the cytoplasmic domain of alpha(7)B integrin after cell adhesion to laminin-1/E8 and imply alpha(7)B-controlled lamellipodia formation and cell migration through the p130(CAS)/Crk protein complex.     

P21-activated kinase 4 interacts with integrin alpha v beta 5 and regulates alpha v beta 5-mediated cell migration

p21-activated kinase 1 (PAK1) can affect cell migration (Price et al., 1998; del Pozo et al., 2000) and modulate myosin light chain kinase and LIM kinase, which are components of the cellular motility machinery (Edwards, D.C., L.C. Sanders, G.M. Bokoch, and G.N. Gill. 1999. Nature Cell Biol. 1:253-259; Sanders, L.C., F. Matsumura, G.M. Bokoch, and P. de Lanerolle. 1999. SCIENCE: 283:2083-2085). We here present a novel cell motility pathway by demonstrating that PAK4 directly interacts with an integrin intracellular domain and regulates carcinoma cell motility in an integrin-specific manner. Yeast two-hybrid screening identified PAK4 binding to the cytoplasmic domain of the integrin beta 5 subunit, an association that was also found in mammalian cells between endogenous PAK4 and integrin alpha v beta 5. Furthermore, we mapped the PAK4 binding to the membrane-proximal region of integrin beta 5, and identified an integrin-binding domain at aa 505-530 in the COOH terminus of PAK4. Importantly, engagement of integrin alpha v beta 5 by cell attachment to vitronectin led to a redistribution of PAK4 from the cytosol to dynamic lamellipodial structures where PAK4 colocalized with integrin alpha v beta 5. Functionally, PAK4 induced integrin alpha v beta 5-mediated, but not beta1-mediated, human breast carcinoma cell migration, while no changes in integrin cell surface expression levels were observed. In conclusion, our results demonstrate that PAK4 interacts with integrin alpha v beta 5 and selectively promotes integrin alpha v beta 5-mediated cell migration.     

The alpha 6 beta 4 integrin and epithelial cell migration.

Although the involvement of alpha 6 beta 4, an integrin laminin receptor, in hemidesmosome organization has dominated the study of this integrin, recent studies are revealing novel functions for alpha 6 beta 4 in the migration of epithelial and carcinoma cells. The engagement of laminin by alpha 6 beta 4 can stabilize actin-rich protrusions and mediate traction forces necessary for cell movement. This integrin also has a significant impact on signaling molecules that stimulate migration and invasion, especially PI3-K and Rho GTPases. Activation of PI3-K by alpha 6 beta 4 enhances the formation of actin protrusions, and it may stimulate the function of other integrins, such as alpha 3 beta 1, that are also important for epithelial migration. Signaling through alpha 6 beta 4 may not always depend on the adhesive functions of this integrin, a possibility that has profound implications for migration and invasion because it implies that the ability of alpha 6 beta 4 to stimulate these processes is not limited to specific matrix environments.     

Cytoskeletal protein radixin activates integrin alpha(M)beta(2) by binding to its cytoplasmic tail

Talin binding of integrins, via its band 4.1, ezrin, radixin, and moesin (FERM)-homologous domain, directly activates the integrin receptor. However, it is not known whether other FERM-containing proteins also possess such an integrin activating capability. We report here that radixin, one of the original FERM-domain proteins, binds to the membrane-proximal region of the integrin beta(2) but not alpha(M) cytoplasmic tail. Importantly, we show that radixin binding significantly enhances the adhesive activity of integrin alpha(M)beta(2). Given the distinct biological activities of radixin and talin, radixin may represent a novel talin-independent pathway for integrin activation under specific settings.