The E3 ubiquitin ligase specificity subunit ASB2beta is a novel regulator of muscle differentiation that targets filamin B to proteasomal degradation

Ubiquitin-mediated protein degradation is the main mechanism for controlled proteolysis, which is crucial for muscle development and maintenance. The ankyrin repeat-containing protein with a suppressor of cytokine signaling box 2 gene (ASB2) encodes the specificity subunit of an E3 ubiquitin ligase complex involved in differentiation of hematopoietic cells. Here, we provide the first evidence that a novel ASB2 isoform, ASB2beta, is important for muscle differentiation. ASB2beta is expressed in muscle cells during embryogenesis and in adult tissues. ASB2beta is part of an active E3 ubiquitin ligase complex and targets the actin-binding protein filamin B (FLNb) for proteasomal degradation. Thus, ASB2beta regulates FLNb functions by controlling its degradation. Knockdown of endogenous ASB2beta by shRNAs during induced differentiation of C2C12 cells delayed FLNb degradation as well as myoblast fusion and expression of muscle contractile proteins. Finally, knockdown of FLNb in ASB2beta knockdown cells restores myogenic differentiation. Altogether, our results suggest that ASB2beta is involved in muscle differentiation through the targeting of FLNb to destruction by the proteasome.

Talin controls integrin activation

Tight, dynamic control of the affinity of integrin adhesion receptors for their extracellular ligands (integrin activation) is essential for the development and functioning of multicellular organisms. Integrin activation is controlled by intracellular signals that, through their action on integrin cytoplasmic domains, induce conformational changes in integrin extracellular domains, resulting in increased affinity for the ligand. Recent results indicate that the binding of talin, a major actin-binding protein, to integrin beta tails represents a final common step in integrin activation pathways. The major integrin-binding site lies within the talin FERM (four-point-one, ezrin, radixin, moesin) domain, and binding occurs via a variant of the classical PTB domain (phosphotyrosine-binding domain)-NPxY interaction. Formation of this talin-integrin complex plays a critical role in integrin activation, since mutations, in either talin or integrin beta tails, which disrupt complex formation, inhibit integrin activation. Furthermore, use of RNA interference to knockdown talin expression selectively reveals that talin is essential for integrin activation in response to physiological agonists. Thus the association of the cytoskeletal protein talin with integrin beta cytoplasmic domains is a critical step during integrin activation, and regulation of this step may be a final common element in the signalling pathways that control integrin activation.

Increased filamin binding to beta-integrin cytoplasmic domains inhibits cell migration

Multicellular animal development depends on integrins. These adhesion receptors link to the actin cytoskeleton, transmitting biochemical signals and force during cell migration and interactions with the extracellular matrix. Many integrin-cytoskeleton connections are formed by filamins and talin. The beta7 integrin tail binds strongly to filamin and supports less migration, fibronectin matrix assembly and focal adhesion formation than either the beta1D tail, which binds strongly to talin, or the beta1A tail, which binds modestly to both filamin and talin. To probe the role of filamin binding, we mapped the filamin-binding site of integrin tails and identified amino acid substitutions that led to selective loss of filamin binding to the beta7 tail and gain of filamin binding to the beta1A tail. These changes affected cell migration and membrane protrusions but not fibronectin matrix assembly or focal adhesion formation. Thus, tight filamin binding restricts integrin-dependent cell migration by inhibiting transient membrane protrusion and cell polarization.

PEA-15 mediates cytoplasmic sequestration of ERK MAP kinase

The ERK 1/2 MAP kinase pathway controls cell growth and survival and modulates integrin function. Here, we report that PEA-15, a protein variably expressed in multiple cell types, blocks ERK-dependent transcription and proliferation by binding ERKs and preventing their localization in the nucleus. PEA-15 contains a nuclear export sequence required for its capacity to anchor ERK in the cytoplasm. Genetic deletion of PEA-15 results in increased ERK nuclear localization with consequent increased cFos transcription and cell proliferation. Thus, PEA-15 can redirect the biological outcome of MAP kinase signaling by regulating the subcellular localization of ERK MAP kinase.

Calpain cleavage promotes talin binding to the beta 3 integrin cytoplasmic domain

Talin links integrin beta cytoplasmic domains to the actin cytoskeleton and is involved in the clustering and activation of these receptors. To understand how talin recognizes integrin beta cytoplasmic domains, we configured surface plasmon resonance methodology to measure the interaction of talin with the beta3 integrin cytoplasmic domain. Here we report that the N-terminal approximately 47-kDa talin head domain (talin-H) has a 6-fold higher binding affinity than intact talin for the beta3 tail. The affinity difference is mainly due to a difference in k(on). Calpain cleavage of intact talin released talin-H and resulted in a 16-fold increase in apparent K(a) and a 100-fold increase in apparent k(on). The increase in talin binding after cleavage was greater than predicted for stoichiometric liberation of free talin-H. This additional increase in binding was due to cooperative binding of talin-H and talin rod domain to the beta3 tail. Talin resembles ERM (ezrin, radixin, moesin) proteins in possessing an N-terminal FERM (band four-point-one, ezrin, radixin, moesin) domain. These data show that the talin FERM domain, like that in the ERM proteins, is masked in the intact molecule. Furthermore, they suggest that talin cleavage by calpain may contribute to the effects of the protease on the clustering and activation of integrins.

Distinct domains of CD98hc regulate integrins and amino acid transport

CD98 is a cell surface heterodimer formed by the covalent linkage of CD98 heavy chain (CD98hc) with several different light chains to form amino acid transporters. CD98hc also binds specifically to the integrin beta(1A) cytoplasmic domain and regulates integrin function. In this study, we examined the relationship between the ability of CD98hc to stimulate amino acid transport and to affect integrin function. By constructing chimeras with CD98hc and a type II transmembrane protein (CD69), we found that the cytoplasmic and transmembrane domains of CD98hc are required for its effects on integrin function, while the extracellular domain is required for stimulation of isoleucine transport. Consequently, the capacity to promote amino acid transport is not required for CD98hc's effect on integrin function. Furthermore, a mutant of CD98hc that lacks its integrin binding site can still promote increased isoleucine transport. Thus, these two functions of CD98hc are separable and require distinct domains of the protein.

Integrin cytoplasmic domain-binding proteins

Integrins are a large family of cell surface receptors that mediate cell adhesion and influence migration, signal transduction, and gene expression. The cytoplasmic domains of integrins play a pivotal role in these integrin-mediated cellular functions. Through interaction with the cytoskeleton, signaling molecules, and other cellular proteins, integrin cytoplasmic domains transduce signals from both the outside and inside of the cell and regulate integrin-mediated biological functions. Identification and functional analyses of integrin cytoplasmic domain-binding proteins have been pursued intensively. In recent years, more cellular proteins have been reported to directly interact with integrin cytoplasmic domains and some of these interactions may play important roles in integrin-mediated biological responses. Integrin (β) chains, for example, interact with actin-binding proteins (e.g. talin and filamin), which form mechanical links to the cytoskeleton. These and other proteins (e.g. FAK, ILK and novel proteins such as TAP20) might also link integrins to signaling mechanisms and, in some cases (e.g. JAB1) mediate integrin-dependent gene regulation.

Class- and splice variant-specific association of CD98 with integrin beta cytoplasmic domains

CD98 is a type II transmembrane protein involved in neutral and basic amino acid transport and in cell fusion events. CD98 was implicated in the function of integrin adhesion receptors by its capacity to reverse suppression of integrin activation by isolated integrin beta(1A) domains. Here we report that CD98 associates with integrin beta cytoplasmic domains with a unique integrin class and splice variant specificity. In particular, CD98 interacted with the ubiquitous beta(1A) but not the muscle-specific splice variant, beta(1D), or leukocyte-specific beta(7) cytoplasmic domains. The ability of CD98 to associate with integrin cytoplasmic domains correlated with its capacity to reverse suppression of integrin activation. The association of CD98 with integrin beta(1A) cytoplasmic domains may regulate the function and localization of these membrane proteins.

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.

Molecular characterisation of integrin-procollagen C-propeptide interactions

The carboxyl-terminal propeptide of type I procollagen (CPP-I) plays a key role in regulation of collagen fibrillogenesis, and may exert feedback control of collagen biosynthesis. We have previously shown that CPP-I is a ligand for the integrin alpha2beta1 [Weston, S. A., Hulmes, D. J. S., Mould, A. P., Watson, R. B. & Humphries, M. J. (1994) Identification of the integrin alpha2beta1 as a cell surface receptor for the C-propeptide of type I procollagen, J. Biol. Chem. 269, 20982-20986] suggesting that some of the phenotypic effects of C-propeptides may be mediated by adhesion receptors. Here we have extended this work to study the molecular basis of this interaction. We have broadened the ligand range by demonstrating that the C-terminal propeptide of type II procollagen supports alpha2beta1-mediated binding of NHS human fibroblasts in cell attachment assays. Also, we have used function-blocking antibodies in cell attachment and solid-phase binding assays with purified integrin to expand the CPP-I receptor family, showing that integrin alpha1beta1 is also a receptor for CPP-I. Integrin alpha-subunit A-domains are known to be major ligand-binding sites and recombinant alpha1 and alpha2 subunit A-domains were able to bind CPP-I. Finally we have shown that peptides corresponding to potential integrin-binding sequences in CPP-I do not mediate integrin-CPP-I adhesion. Taken together, these studies indicate that the interactions between C-propeptides and integrins are more numerous than previously reported, that C-propeptides are a new class of molecule which bind to A-domains, and that the integrin-C-propeptide interaction does not utilise established peptide motifs.

The integrin alpha1 A-domain is a ligand binding site for collagens and laminin

The integrin alpha1beta1 is a cell surface receptor for collagens and laminin. The alpha1 subunit contains an A-domain, and the A-domains of other integrins are known to mediate ligand binding. To determine the role of the alpha1 A-domain in ligand binding and the extent to which it reproduced the ligand binding activity and specificity of the parent molecule, we produced recombinant alpha1 A-domain and tested its ability to bind collagens and laminin. In solid phase assays, the A-domain from alpha1 was found to bind to collagen I, collagen IV, and laminin in a largely cation-dependent manner. The alpha2 A-domain, from the alpha2beta1 integrin, also bound to these ligands, but the binding hierarchy differed from that seen for alpha1. This is the first demonstration of laminin binding by A-domains. Specificity of A-domain-ligand binding was further investigated using the triple-helical proteolytic fragment of collagen IV, CB3, and its subfragments, F1 and F4. alpha1 A-domain bound to all three fragments, while the alpha2 A-domain bound CB3 less well and exhibited little binding to F1 and no binding to F4. These differences mirror previous reports of distinct integrin binding sites in collagen IV and for the first time identify a limited proteolytic fragment of a ligand that contains integrin A-domain binding activity. To gain insight into the contribution that the A-domain makes to ligand binding within the whole integrin heterodimer, we measured binding constants for A-domain-collagen interactions using surface plasmon resonance biosensor technology. The values obtained were similar to those reported for intact integrin binding, suggesting that the A-domain is the major collagen binding site in the alpha1beta1 and alpha2beta1 integrins.

Integrin alpha 2 I-domain is a binding site for collagens

Integrins alpha 1 beta 1 and alpha 2 beta 1 are major cellular receptors for collagens. The alpha 1 and alpha 2 subunits contain a approximately 200 amino acid inserted domain (I-domain) in their N-terminal region and, because of the homology between the I-domains and the collagen-binding A-domains of von Willebrand factor, it has been suggested that the I-domains might mediate the collagen-binding functions of alpha 1 beta 1 and alpha 2 beta 1. In order to fully investigate this hypothesis, we have generated recombinant human alpha 2 I-domain (r alpha 2I) by reverse transcriptase-polymerase chain reaction/bacterial expression and tested its ability to mediate the collagen-binding functions of alpha 2 beta 1. R alpha 2 I binds specifically to type I collagen in a concentration-dependent manner: binding is cation dependent and, like the complete receptor, is supported by magnesium and manganese ions but not by calcium ions. R alpha 2I is recognised by anti-functional anti-alpha 2 monoclonal antibodies 6F1, 5E8 and P1E6 in capture ELISAs, and anti-functional antibodies inhibited r alpha 2I-collagen binding. In addition, r alpha 2I inhibits cell spreading on collagen. R alpha 2I is therefore a collagen-binding domain and can account for many of the collagen-binding functions of integrin alpha 2 beta 1. We have also determined the collagen specificity of r alpha 2I and found that it binds types I, II and XI collagen.