Structural basis of the cytoplasmic tail of adhesion molecule CD43 and its binding to ERM proteins

A bipartite NLS motif mediates the nuclear import of Drosophila moesin

The ERM protein family, which consists of three closely related proteins in vertebrates, ezrin, radixin, and moesin (ERM), is an ancient and important group of cytoplasmic actin-binding and organizing proteins. With their FERM domain, ERMs bind various transmembrane proteins and anchor them to the actin cortex through their C-terminal F-actin binding domain, thus they are major regulators of actin dynamics in the cell. ERMs participate in many fundamental cellular processes, such as phagocytosis, microvilli formation, T-cell activation and tumor metastasis. We have previously shown that, besides its cytoplasmic activities, the single ERM protein of Drosophila melanogaster, moesin, is also present in the cell nucleus, where it participates in gene expression and mRNA export. Here we study the mechanism by which moesin enters the nucleus. We show that the nuclear import of moesin is an NLS-mediated, active process. The nuclear localization sequence of the moesin protein is an evolutionarily highly conserved, conventional bipartite motif located on the surface of the FERM domain. Our experiments also reveal that the nuclear import of moesin does not require PIP2 binding or protein activation, and occurs in monomeric form. We propose, that the balance between the phosphorylated and non-phosphorylated protein pools determines the degree of nuclear import of moesin.

Elucidation of Short Linear Motif-Based Interactions of the FERM Domains of Ezrin, Radixin, Moesin, and Merlin

The ERM (ezrin, radixin, and moesin) family of proteins and the related protein merlin participate in scaffolding and signaling events at the cell cortex. The proteins share an N-terminal FERM [band four-point-one (4.1) ERM] domain composed of three subdomains (F1, F2, and F3) with binding sites for short linear peptide motifs. By screening the FERM domains of the ERMs and merlin against a phage library that displays peptides representing the intrinsically disordered regions of the human proteome, we identified a large number of novel ligands. We determined the affinities for the ERM and merlin FERM domains interacting with 18 peptides and validated interactions with full-length proteins through pull-down experiments. The majority of the peptides contained an apparent Yx[FILV] motif; others show alternative motifs. We defined distinct binding sites for two types of similar but distinct binding motifs (YxV and FYDF) using a combination of Rosetta FlexPepDock computational peptide docking protocols and mutational analysis. We provide a detailed molecular understanding of how the two types of peptides with distinct motifs bind to different sites on the moesin FERM phosphotyrosine binding-like subdomain and uncover interdependencies between the different types of ligands. The study expands the motif-based interactomes of the ERMs and merlin and suggests that the FERM domain acts as a switchable interaction hub.

The genetic landscape and possible therapeutics of neurofibromatosis type 2

Neurofibromatosis type 2 (NF2) is a genetic condition marked by the development of multiple benign tumors in the nervous system. The most common tumors associated with NF2 are bilateral vestibular schwannoma, meningioma, and ependymoma. The clinical manifestations of NF2 depend on the site of involvement. Vestibular schwannoma can present with hearing loss, dizziness, and tinnitus, while spinal tumor leads to debilitating pain, muscle weakness, or paresthesias. Clinical diagnosis of NF2 is based on the Manchester criteria, which have been updated in the last decade. NF2 is caused by loss-of-function mutations in the NF2 gene on chromosome 22, leading the merlin protein to malfunction. Over half of NF2 patients have de novo mutations, and half of this group are mosaic. NF2 can be managed by surgery, stereotactic radiosurgery, monoclonal antibody bevacizumab, and close observation. However, the nature of multiple tumors and the necessity of multiple surgeries over the lifetime, inoperable tumors like meningiomatosis with infiltration of the sinus or in the area of the lower cranial nerves, the complications caused by the operation, the malignancies induced by radiotherapy, and inefficiency of cytotoxic chemotherapy due to the benign nature of NF-related tumors have led a march toward exploring targeted therapies. Recent advances in genetics and molecular biology have allowed identifying and targeting of underlying pathways in the pathogenesis of NF2. In this review, we explain the clinicopathological characteristics of NF2, its genetic and molecular background, and the current knowledge and challenges of implementing genetics to develop efficient therapies.

Structure of the FERM domain of a neural scaffold protein FRMPD4 implicated in X-linked intellectual disability

Scaffold proteins play crucial roles in orchestrating synaptic signaling and plasticity in the excitatory synapses by providing a structural link between glutamatergic receptors, signaling molecules, and neuronal cytoskeletons. FRMPD4 is a neural scaffold protein that binds to metabotropic glutamate receptors via its FERM domain. Here, we determine the crystal structure of the FERM domain of FRMPD4 at 2.49 Å resolution. The structure reveals that the canonical target binding groove of FRMPD4 FERM is occupied by a conserved fragment C-terminal to the FERM domain, suggesting that the FRMPD4-mGluR interaction may adopt a distinct binding mode. In addition, FRMPD4 FERM does not contain a typical phosphoinositide binding site at the F1/F3 cleft found in ERM family FERM domains, but it possesses a conserved basic residue cluster on the F2 lobe which could bind to lipid effectively. Finally, analysis of mutations that are associated with X-linked intellectual disability suggests that they may compromise the biological function of FRMPD4 by destabilizing the FERM structure.

Structural analysis of the LDL receptor-interacting FERM domain in the E3 ubiquitin ligase IDOL reveals an obscured substrate-binding site

Hepatic abundance of the low-density lipoprotein receptor (LDLR) is a critical determinant of circulating plasma LDL cholesterol levels and hence development of coronary artery disease. The sterol-responsive E3 ubiquitin ligase inducible degrader of the LDLR (IDOL) specifically promotes ubiquitination and subsequent lysosomal degradation of the LDLR and thus controls cellular LDL uptake. IDOL contains an extended N-terminal FERM (4.1 protein, ezrin, radixin, and moesin) domain, responsible for substrate recognition and plasma membrane association, and a second C-terminal RING domain, responsible for the E3 ligase activity and homodimerization. As IDOL is a putative lipid-lowering drug target, we investigated the molecular details of its substrate recognition. We produced and isolated full-length IDOL protein, which displayed high autoubiquitination activity. However, in vitro ubiquitination of its substrate, the intracellular tail of the LDLR, was low. To investigate the structural basis for this, we determined crystal structures of the extended FERM domain of IDOL and multiple conformations of its F3ab subdomain. These reveal the archetypal F1-F2-F3 trilobed FERM domain structure but show that the F3c subdomain orientation obscures the target-binding site. To substantiate this finding, we analyzed the full-length FERM domain and a series of truncated FERM constructs by small-angle X-ray scattering (SAXS). The scattering data support a compact and globular core FERM domain with a more flexible and extended C-terminal region. This flexibility may explain the low activity in vitro and suggests that IDOL may require activation for recognition of the LDLR.

ERM Proteins at the Crossroad of Leukocyte Polarization, Migration and Intercellular Adhesion

Ezrin, radixin and moesin proteins (ERMs) are plasma membrane (PM) organizers that link the actin cytoskeleton to the cytoplasmic tail of transmembrane proteins, many of which are adhesion receptors, in order to regulate the formation of F-actin-based structures (e.g., microspikes and microvilli). ERMs also effect transmission of signals from the PM into the cell, an action mainly exerted through the compartmentalized activation of the small Rho GTPases Rho, Rac and Cdc42. Ezrin and moesin are the ERMs more highly expressed in leukocytes, and although they do not always share functions, both are mainly regulated through phosphatidylinositol 4,5-bisphosphate (PIP₂) binding to the N-terminal band 4.1 protein-ERM (FERM) domain and phosphorylation of a conserved Thr in the C-terminal ERM association domain (C-ERMAD), exerting their functions through a wide assortment of mechanisms. In this review we will discuss some of these mechanisms, focusing on how they regulate polarization and migration in leukocytes, and formation of actin-based cellular structures like the phagocytic cup-endosome and the immune synapse in macrophages/neutrophils and lymphocytes, respectively, which represent essential aspects of the effector immune response.

Two Sides of the Coin: Ezrin/Radixin/Moesin and Merlin Control Membrane Structure and Contact Inhibition

The merlin-ERM (ezrin, radixin, moesin) family of proteins plays a central role in linking the cellular membranes to the cortical actin cytoskeleton. Merlin regulates contact inhibition and is an integral part of cell–cell junctions, while ERM proteins, ezrin, radixin and moesin, assist in the formation and maintenance of specialized plasma membrane structures and membrane vesicle structures. These two protein families share a common evolutionary history, having arisen and separated via gene duplication near the origin of metazoa. During approximately 0.5 billion years of evolution, the merlin and ERM family proteins have maintained both sequence and structural conservation to an extraordinary level. Comparing crystal structures of merlin-ERM proteins and their complexes, a picture emerges of the merlin-ERM proteins acting as switchable interaction hubs, assembling protein complexes on cellular membranes and linking them to the actin cytoskeleton. Given the high level of structural conservation between the merlin and ERM family proteins we speculate that they may function together.

The Janus Kinase (JAK) FERM and SH2 Domains: Bringing Specificity to JAK-Receptor Interactions

The Janus kinases (JAKs) are non-receptor tyrosine kinases essential for signaling in response to cytokines and interferons and thereby control many essential functions in growth, development, and immune regulation. JAKs are unique among tyrosine kinases for their constitutive yet non-covalent association with class I and II cytokine receptors, which upon cytokine binding bring together two JAKs to create an active signaling complex. JAK association with cytokine receptors is facilitated by N-terminal FERM and SH2 domains, both of which are classical mediators of peptide interactions. Together, the JAK FERM and SH2 domains mediate a bipartite interaction with two distinct receptor peptide motifs, the proline-rich "Box1" and hydrophobic "Box2," which are present in the intracellular domain of cytokine receptors. While the general sidechain chemistry of Box1 and Box2 peptides is conserved between receptors, they share very weak primary sequence homology, making it impossible to posit why certain JAKs preferentially interact with and signal through specific subsets of cytokine receptors. Here, we review the structure and function of the JAK FERM and SH2 domains in light of several recent studies that reveal their atomic structure and elucidate interaction mechanisms with both the Box1 and Box2 receptor motifs. These crystal structures demonstrate how evolution has repurposed the JAK FERM and SH2 domains into a receptor-binding module that facilitates interactions with multiple receptors possessing diverse primary sequences.

Structural characterization suggests models for monomeric and dimeric forms of full-length ezrin

Ezrin is a member of the ERM (ezrin–radixin–moesin) family of proteins that have been conserved through metazoan evolution. These proteins have dormant and active forms, where the latter links the actin cytoskeleton to membranes. ERM proteins have three domains: an N-terminal FERM [band Four-point-one (4.1) ERM] domain comprising three subdomains (F1, F2, and F3); a helical domain; and a C-terminal actin-binding domain. In the dormant form, FERM and C-terminal domains form a stable complex. We have determined crystal structures of the active FERM domain and the dormant FERM:C-terminal domain complex of human ezrin. We observe a bistable array of phenylalanine residues in the core of subdomain F3 that is mobile in the active form and locked in the dormant form. As subdomain F3 is pivotal in binding membrane proteins and phospholipids, these transitions may facilitate activation and signaling. Full-length ezrin forms stable monomers and dimers. We used small-angle X-ray scattering to determine the solution structures of these species. As expected, the monomer shows a globular domain with a protruding helical coiled coil. The dimer shows an elongated dumbbell structure that is twice as long as the monomer. By aligning ERM sequences spanning metazoan evolution, we show that the central helical region is conserved, preserving the heptad repeat. Using this, we have built a dimer model where each monomer forms half of an elongated antiparallel coiled coil with domain-swapped FERM:C-terminal domain complexes at each end. The model suggests that ERM dimers may bind to actin in a parallel fashion.

Myosin MyTH4-FERM structures highlight important principles of convergent evolution

Myosins containing MyTH4-FERM (myosin tail homology 4-band 4.1, ezrin, radixin, moesin, or MF) domains in their tails are found in a wide range of phylogenetically divergent organisms, such as humans and the social amoeba Dictyostelium (Dd). Interestingly, evolutionarily distant MF myosins have similar roles in the extension of actin-filled membrane protrusions such as filopodia and bind to microtubules (MT), suggesting that the core functions of these MF myosins have been highly conserved over evolution. The structures of two DdMyo7 signature MF domains have been determined and comparison with mammalian MF structures reveals that characteristic features of MF domains are conserved. However, across millions of years of evolution conserved class-specific insertions are seen to alter the surfaces and the orientation of subdomains with respect to each other, likely resulting in new sites for binding partners. The MyTH4 domains of Myo10 and DdMyo7 bind to MT with micromolar affinity but, surprisingly, their MT binding sites are on opposite surfaces of the MyTH4 domain. The structural analysis in combination with comparison of diverse MF myosin sequences provides evidence that myosin tail domain features can be maintained without strict conservation of motifs. The results illustrate how tuning of existing features can give rise to new structures while preserving the general properties necessary for myosin tails. Thus, tinkering with the MF domain enables it to serve as a multifunctional platform for cooperative recruitment of various partners, allowing common properties such as autoinhibition of the motor and microtubule binding to arise through convergent evolution.

Structural basis for the phosphorylation-regulated interaction between the cytoplasmic tail of cell polarity protein crumbs and the actin-binding protein moesin

The type I transmembrane protein crumbs (Crb) plays critical roles in the establishment and maintenance of cell polarities in diverse tissues. As such, mutations of Crb can cause different forms of cancers. The cell intrinsic role of Crb in cell polarity is governed by its conserved, 37-residue cytoplasmic tail (Crb-CT) via binding to moesin and protein associated with Lin7-1 (PALS1). However, the detailed mechanism governing the Crb·moesin interaction and the balance of Crb in binding to moesin and PALS1 are not well understood. Here we report the 1.5 Å resolution crystal structure of the moesin protein 4.1/ezrin/radixin/moesin (FERM)·Crb-CT complex, revealing that both the canonical FERM binding motif and the postsynaptic density protein-95/Disc large-1/Zonula occludens-1 (PDZ) binding motif of Crb contribute to the Crb·moesin interaction. We further demonstrate that phosphorylation of Crb-CT by atypical protein kinase C (aPKC) disrupts the Crb·moesin association but has no impact on the Crb·PALS1 interaction. The above results indicate that, upon the establishment of the apical-basal polarity in epithelia, apical-localized aPKC can actively prevent the Crb·moesin complex formation and thereby shift Crb to form complex with PALS1 at apical junctions. Therefore, Crb may serve as an aPKC-mediated sensor in coordinating contact-dependent cell growth inhibition in epithelial tissues.

Structural basis of DDB1-and-Cullin 4-associated Factor 1 (DCAF1) recognition by merlin/NF2 and its implication in tumorigenesis by CD44-mediated inhibition of merlin suppression of DCAF1 function

Merlin, a tumor suppressor encoded by the neurofibromatosis type 2 gene, has been shown to suppress tumorigenesis by inhibiting the Cullin 4-RING E3 ubiquitin ligase CRL4(DCAF) (1) in the nucleus. This inhibition is mediated by direct binding of merlin to DDB1-and-Cullin 4-associated Factor 1 (DCAF1), yet the binding mode of merlin to DCAF1 is not well defined. Here, we report structural and biophysical studies of the merlin binding to DCAF1 and its interference with CD44 binding. The crystal structure of the merlin FERM domain bound to the DCAF1 C-terminal acidic tail reveals that the hydrophobic IILXLN motif located at the C-terminal end of DCAF1 binds subdomain C of the FERM domain by forming a β-strand. The binding site and mode resemble that of merlin binding to the CD44 cytoplasmic tail. Competition binding assay showed that CD44 and DCAF1 compete for binding to the merlin FERM domain in solution. The CD44 cytoplasmic tail is known to be cleaved for nuclear translocation by regulated intra-membrane proteolysis (RIP). Our structure implies that, in the nucleus, the CD44 cytoplasmic tail cleaved by RIP could release DCAF1 from merlin by competing for binding to the merlin FERM domain, which results in the inhibition of merlin-mediated suppression of tumorigenesis.

Structural basis of the binding of Merlin FERM domain to the E3 ubiquitin ligase substrate adaptor DCAF1

The tumor suppressor gene Nf2 product, Merlin, plays vital roles in controlling proper development of organ sizes by specifically binding to a large number of target proteins localized both in cytoplasm and nuclei. The FERM domain of Merlin is chiefly responsible for its binding to target proteins, although the molecular basis governing these interactions are poorly understood due to lack of structural information. Here, we report the crystal structure of the Merlin FERM domain in complex with its binding domain derived from the E3 ubiquitin ligase substrate adaptor DCAF1 (also known as VPRBP). Unlike target binding modes found in ERM proteins, the Merlin-FERM binding domain of DCAF1 folds as a β-hairpin and binds to the α1/β5-groove of the F3 lobe of Merlin-FERM via extensive hydrophobic interactions. In addition to providing the first structural glimpse of a Merlin-FERM·target complex, the structure of the Merlin·DCAF1 complex is likely to be valuable for understanding the interactions of Merlin with its binding partners other than DCAF1.

Conserved sequence repeats of IQGAP1 mediate binding to Ezrin

Mammalian IQGAP proteins all feature multiple ∼50 amino acid sequence repeats near their N-termini, and little is known about the function of these "Repeats". We have expressed and purified the Repeats from human IQGAP1 to identify binding partners. We used mass spectrometry to identify 42 mouse kidney proteins that associate with the IQGAP1 Repeats including the ERM proteins ezrin, radixin, and moesin. ERM proteins have an N-terminal FERM domain (4.1, ezrin, radixin, moesin) through which they bind to protein targets and phosphatidylinositol 4,5-bisphosphate (PIP2) and a C-terminal actin-binding domain and function to link the actin cytoskeleton to distinct locations on the cell cortex. Isothermal titration calorimetry (ITC) reveals that the IQGAP1 Repeats directly bind to the ezrin FERM domain, while no binding is seen for full-length "autoinhibited" ezrin or a version of full-length ezrin intended to mimic the activated protein. ITC also indicates that the ezrin FERM domain binds to the Repeats from IQGAP2 but not the Repeats from IQGAP3. We conclude that IQGAP1 and IQGAP2 are positioned at the cell cortex by ERM proteins. We propose that the IQGAP3 Repeats may likewise bind to FERM domains for signaling purposes.

A Novel Interaction between Pyk2 and MAP4K4 Is Integrated with Glioma Cell Migration

Glioma cell migration correlates with Pyk2 activity, but the intrinsic mechanism that regulates the activity of Pyk2 is not fully understood. Previous studies have supported a role for the N-terminal FERM domain in the regulation of Pyk2 activity as mutations in the FERM domain inhibit Pyk2 phosphorylation. To search for novel protein-protein interactions mediated by the Pyk2 FERM domain, we utilized a yeast two-hybrid genetic selection to identify the mammalian Ste20 homolog MAP4K4 as a binding partner for the Pyk2 FERM domain. MAP4K4 coimmunoprecipitated with Pyk2 and was a substrate for Pyk2 but did not coimmunoprecipitate with the closely related focal adhesion kinase FAK. Knockdown of MAP4K4 expression inhibited glioma cell migration and effectively blocked Pyk2 stimulation of glioma cell. Increased expression of MAP4K4 stimulated glioma cell migration; however, this stimulation was blocked by knockdown of Pyk2 expression. These data support that the interaction of MAP4K4 and Pyk2 is integrated with glioma cell migration and suggest that inhibition of this interaction may represent a potential therapeutic strategy to limit glioblastoma tumor dispersion.

CLIC proteins, ezrin, radixin, moesin and the coupling of membranes to the actin cytoskeleton: a smoking gun?

The CLIC proteins are a highly conserved family of metazoan proteins with the unusual ability to adopt both soluble and integral membrane forms. The physiological functions of CLIC proteins may include enzymatic activity in the soluble form and anion channel activity in the integral membrane form. CLIC proteins are associated with the ERM proteins: ezrin, radixin and moesin. ERM proteins act as cross-linkers between membranes and the cortical actin cytoskeleton. Both CLIC and ERM proteins are controlled by Rho family small GTPases. CLIC proteins, ERM and Rho GTPases act in a concerted manner to control active membrane processes including the maintenance of microvillar structures, phagocytosis and vesicle trafficking. All of these processes involve the interaction of membranes with the underlying cortical actin cytoskeleton. The relationships between Rho GTPases, CLIC proteins, ERM proteins and the membrane:actin cytoskeleton interface are reviewed. Speculative models are proposed involving the formation of localised multi-protein complexes on the membrane surface that assemble via multiple weak interactions. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Proteomic analysis in giant axonal neuropathy: new insights into disease mechanisms

INTRODUCTION

Giant axonal neuropathy (GAN) is a progressive hereditary disease that affects the peripheral and central nervous systems. It is characterized morphologically by aggregates of intermediate filaments in different tissues. Mutations have been reported in the gene that codes for gigaxonin. Nevertheless, the underlying molecular mechanism remains obscure.

METHODS

Cell lines from 4 GAN patients and 4 controls were analyzed by iTRAQ.

RESULTS

Among the dysregulated proteins were ribosomal protein L29, ribosomal protein L37, galectin-1, glia-derived nexin, and aminopeptidase N. Also, nuclear proteins linked to formin-binding proteins were found to be dysregulated. Although the major role of gigaxonin is reported to be degradation of cytoskeleton-associated proteins, the amount of 76 structural cytoskeletal proteins was unaltered.

CONCLUSIONS

Several of the dysregulated proteins play a role in cytoskeletal reorganization. Based on these findings, we speculate that disturbed cytoskeletal regulation is responsible for formation of aggregates of intermediate filaments.

Ezrin/radixin/moesin are required for the purinergic P2X7 receptor (P2X7R)-dependent processing of the amyloid precursor protein

The amyloid precursor protein (APP) can be cleaved by α-secretases in neural cells to produce the soluble APP ectodomain (sAPPα), which is neuroprotective. We have shown previously that activation of the purinergic P2X7 receptor (P2X7R) triggers sAPPα shedding from neural cells. Here, we demonstrate that the activation of ezrin, radixin, and moesin (ERM) proteins is required for the P2X7R-dependent proteolytic processing of APP leading to sAPPα release. Indeed, the down-regulation of ERM by siRNA blocked the P2X7R-dependent shedding of sAPPα. We also show that P2X7R stimulation triggered the phosphorylation of ERM. Thus, ezrin translocates to the plasma membrane to interact with P2X7R. Using specific pharmacological inhibitors, we established the order in which several enzymes trigger the P2X7R-dependent release of sAPPα. Thus, a Rho kinase and the MAPK modules ERK1/2 and JNK act upstream of ERM, whereas a PI3K activity is triggered downstream. For the first time, this work identifies ERM as major partners in the regulated non-amyloidogenic processing of APP.

Restraint stress and stress hormones significantly impact T lymphocyte migration and function through specific alterations of the actin cytoskeleton

Stress triggers complex response mechanisms designed to recognize and adapt to perturbations in homeostasis. The immune system is highly responsive to stress, although the complete mechanisms linking stress and immune mediators including T lymphocytes, are not fully understood. Stress exerts its effects on immune effectors through two primary pathways: the sympathetic-adrenal-medullary pathway, and the hypothalamic-pituitary-adrenal pathway which modulate adaptive immunity and lymphocyte migration. In this report we show that stress via release of stress hormones induces early T cell activation and greatly impacts the cytoskeleton by modulating numerous actin-regulating proteins. In particular, proteomic profiling revealed significant decreases in numerous key actin-binding proteins including moesin. Although confocal microscopy showed that moesin and actin were uniformly distributed on the surface of resting T cells, a remarkable polarization and redistribution of moesin and actin was observed following treatment with stress hormones with moesin localizing at the distal pole complex. In addition, the alteration in moesin localization and eventual decrease in expression were accompanied by a loss of CD43; a receptor involved in negatively regulating T cell activation. In conclusion, we have defined a novel molecular mechanism whereby stress hormones negatively impact T cell activation and migration through regulation of key cytoskeletal and plasma membrane factors.

Structural basis of cargo recognition by the myosin-X MyTH4-FERM domain

Myosin-X is implicated in spindle assembly and filopodial transport. A detailed structure function analysis of the myosin-X tail domain in association with a cargo involved in axonal pathfinding, the netrin receptor DCC and competitive binding to integrin β5 and microtubules is presented.

Myosin-X is an important unconventional myosin that is critical for cargo transportation to filopodia tips and is also utilized in spindle assembly by interacting with microtubules. We present a series of structural and biochemical studies of the myosin-X tail domain cassette, consisting of myosin tail homology 4 (MyTH4) and FERM domains in complex with its specific cargo, a netrin receptor DCC (deleted in colorectal cancer). The MyTH4 domain is folded into a helical VHS-like structure and is associated with the FERM domain. We found an unexpected binding mode of the DCC peptide to the subdomain C groove of the FERM domain, which is distinct from previously reported β–β associations found in radixin–adhesion molecule complexes. We also revealed direct interactions between the MyTH4–FERM cassette and tubulin C-terminal acidic tails, and identified a positively charged patch of the MyTH4 domain, which is involved in tubulin binding. We demonstrated that both DCC and integrin bindings interfere with microtubule binding and that DCC binding interferes with integrin binding. Our results provide the molecular basis by which myosin-X facilitates alternative dual binding to cargos and microtubules.

Cargo recognition mechanism of myosin X revealed by the structure of its tail MyTH4-FERM tandem in complex with the DCC P3 domain

Myosin X (MyoX), encoded by Myo10, is a representative member of the MyTH4-FERM domain-containing myosins, and this family of unconventional myosins shares common functions in promoting formation of filopodia/stereocilia structures in many cell types with unknown mechanisms. Here, we present the structure of the MyoX MyTH4-FERM tandem in complex with the cytoplasmic tail P3 domain of the netrin receptor DCC. The structure, together with biochemical studies, reveals that the MyoX MyTH4 and FERM domains interact with each other, forming a structural and functional supramodule. Instead of forming an extended β-strand structure in other FERM binding targets, DCC_P3 forms a single α-helix and binds to the αβ-groove formed by β5 and α1 of the MyoX FERM F3 lobe. Structure-based amino acid sequence analysis reveals that the key polar residues forming the inter-MyTH4/FERM interface are absolutely conserved in all MyTH4-FERM tandem-containing proteins, suggesting that the supramodular nature of the MyTH4-FERM tandem is likely a general property for all MyTH4-FERM proteins.

Structural basis of tumor suppressor in lung cancer 1 (TSLC1) binding to differentially expressed in adenocarcinoma of the lung (DAL-1/4.1B)

Perturbed cell adhesion mechanisms are crucial for tumor invasion and metastasis. A cell adhesion protein, TSLC1 (tumor suppressor in lung cancer 1), is inactivated in a majority of metastatic cancers. DAL-1 (differentially expressed in adenocarcinoma of the lung protein), another tumor suppressor, binds through its FERM domain to the TSLC1 C-terminal, 4.1 glycophorin C-like, cytoplasmic domain. However, the molecular basis for this interaction is unknown. Here, we describe the crystal structure of a complex between the DAL-1 FERM domain and a portion of the TSLC1 cytoplasmic domain. DAL-1 binds to TSLC1 through conserved residues in a well defined hydrophobic pocket in the structural C-lobe of the DAL-1 FERM domain. From the crystal structure, it is apparent that Tyr(406) and Thr(408) in the TSLC1 cytoplasmic domain form the most important interactions with DAL-1, and this was also confirmed by surface plasmon resonance studies. Our results refute earlier exon deletion experiments that indicated that glycophorin C interacts with the α-lobe of 4.1 FERM domains.

CD43 interaction with ezrin-radixin-moesin (ERM) proteins regulates T-cell trafficking and CD43 phosphorylation

CD43 interaction with ERM proteins regulates CD43 phosphorylation and T-cell migration. CD43 phosphorylation can also drive CD43 localization in T-cells independently of ERM association.

Cell polarization is a key feature of cell motility, driving cell migration to tissues. CD43 is an abundantly expressed molecule on the T-cell surface that shows distinct localization to the migrating T-cell uropod and the distal pole complex (DPC) opposite the immunological synapse via association with the ezrin-radixin-moesin (ERM) family of actin regulatory proteins. CD43 regulates multiple T-cell functions, including T-cell activation, proliferation, apoptosis, and migration. We recently demonstrated that CD43 regulates T-cell trafficking through a phosphorylation site at Ser-76 (S76) within its cytoplasmic tail. Using a phosphorylation-specific antibody, we now find that CD43 phosphorylation at S76 is enhanced by migration signals. We further show that CD43 phosphorylation and normal T-cell trafficking depend on CD43 association with ERM proteins. Interestingly, mutation of S76 to mimic phosphorylation enhances T-cell migration and CD43 movement to the DPC while blocking ERM association, showing that CD43 movement can occur in the absence of ERM binding. We also find that protein kinase Cθ can phosphorylate CD43. These results show that while CD43 binding to ERM proteins is crucial for S76 phosphorylation, CD43 movement and regulation of T-cell migration can occur through an ERM-independent, phosphorylation–dependent mechanism.

Organizing the cell cortex: the role of ERM proteins

Specialized membrane domains are an important feature of almost all cells. In particular, they are essential to tissues that have a highly organized cell cortex, such as the intestinal brush border epithelium. The ERM proteins (ezrin, radixin and moesin) have a crucial role in organizing membrane domains through their ability to interact with transmembrane proteins and the cytoskeleton. In doing so, they can provide structural links to strengthen the cell cortex and regulate the activities of signal transduction pathways. Recent studies examining the structure and in vivo functions of ERMs have greatly advanced our understanding of the importance of membrane-cytoskeleton interactions.

The PHCCEx domain of Tiam1/2 is a novel protein- and membrane-binding module

Tiam1 and Tiam2 (Tiam1/2) are guanine nucleotide-exchange factors that possess the PH-CC-Ex (pleckstrin homology, coiled coil and extra) region that mediates binding to plasma membranes and signalling proteins in the activation of Rac GTPases. Crystal structures of the PH-CC-Ex regions revealed a single globular domain, PHCCEx domain, comprising a conventional PH subdomain associated with an antiparallel coiled coil of CC subdomain and a novel three-helical globular Ex subdomain. The PH subdomain resembles the beta-spectrin PH domain, suggesting non-canonical phosphatidylinositol binding. Mutational and binding studies indicated that CC and Ex subdomains form a positively charged surface for protein binding. We identified two unique acidic sequence motifs in Tiam1/2-interacting proteins for binding to PHCCEx domain, Motif-I in CD44 and ephrinB's and the NMDA receptor, and Motif-II in Par3 and JIP2. Our results suggest the molecular basis by which the Tiam1/2 PHCCEx domain facilitates dual binding to membranes and signalling proteins.

In Vitro and in Vivo Characterization of Molecular Interactions between Calmodulin, Ezrin/Radixin/Moesin, and L-selectin

L-selectin is a cell adhesion molecule that tethers leukocytes to the luminal walls of venules during inflammation and enables them to roll under the force of blood flow. Clustering of L-selectin during rolling is thought to promote outside-in signals that lead to integrin activation and chemokine receptor expression, ultimately contributing to leukocyte arrest. Several studies have underscored the importance of the L-selectin cytoplasmic tail in functionally regulating adhesion and signaling. Interestingly, the L-selectin tail comprises only 17 amino acids, and yet it is thought to bind simultaneously to several proteins. For example, constitutive association of calmodulin (CaM) and ezrin/radixin/moesin (ERM) to L-selectin confers resistance to proteolysis and microvillar positioning, respectively. In this report we found that recombinant purified CaM and ERM bound non-competitively to the same tail of L-selectin. Furthermore, molecular modeling supported the possibility that CaM, L-selectin, and moesin could form a heterotrimeric complex. Finally, using fluorescence lifetime imaging microscopy to measure fluorescence resonance energy transfer, it was shown that CaM, L-selectin, and ERM could interact simultaneously in vivo. Moreover, L-selectin clustering promoted CaM/ERM interaction in cis (i.e. derived from neighboring L-selectin tails). These results highlight a novel intracellular event that occurs as a consequence of L-selectin clustering, which could participate in transducing signals that promote the transition from rolling to arrest.

Crystallographic characterization of the radixin FERM domain bound to the cytoplasmic tail of membrane-type 1 matrix metalloproteinase (MT1-MMP)

ERM proteins play a role in the cross-linking found between plasma membranes and actin filaments. The N-terminal FERM domains of ERM proteins are responsible for membrane association through direct interaction with the cytoplasmic tails of integral membrane proteins. During cell migration and movement, membrane-type 1 matrix metalloproteinase (MT1-MMP) on plasma membranes sheds adhesion molecule CD44 in addition to degrading the extracellular matrix. Here, the interaction between the radixin FERM domain and the MT1-MMP cytoplasmic tail is reported and preliminary crystallographic characterization of crystals of the radixin FERM domain bound to the cytoplasmic tail of MT1-MMP is presented. The crystals belong to space group P6(1)22, with unit-cell parameters a = b = 122.7, c = 128.3 A, and contain one complex in the crystallographic asymmetric unit. The diffraction data were collected to a resolution of 2.4 A.