Novel structural insights into F-actin-binding and novel functions of calponin homology domains

Methionine redox regulation of actin-interacting proteins primarily governs antioxidative signaling and response to the salvianolic acid B treatment in EA.hy926 cells

Actin-interacting proteins are important molecules for filament assembly and cytoskeletal signaling within vascular endothelium. Disruption in their interactions causes endothelial pathogenesis through redox imbalance. Actin filament redox regulation remains largely unexplored, in the context of pharmacological treatment. This work focused on the peptidyl methionine (M) redox regulation of actin-interacting proteins, aiming at elucidating its role on governing antioxidative signaling and response. Endothelial EA.hy926 cells were subjected to treatment with salvianolic acid B (Sal B) and tert-butyl-hydroperoxide (tBHP) stimulation. Mass spectrometry was employed to characterize redox status of proteins, including actin, myosin-9, kelch-like erythroid-derived cap-n-collar homology-associated protein 1 (Keap1), plastin-3, prelamin-A/C and vimentin. The protein redox landscape revealed distinct stoichiometric ratios or reaction site transitions mediated by M sulfoxide reductase and reactive oxygen species. In comparison with effects of tBHP stimulation, Sal B treatment prevented oxidation at actin M325, myosin-9 M1489/1565, Keap1 M120, plastin-3 M592, prelamin-A/C M187/371/540 and vimentin M344. For Keap1, reaction site was transitioned within its scaffolding region to the actin ring. These protein M oxidation regulations contributed to the Sal B cytoprotective effects on actin filament. Additionally, regarding the Keap1 homo-dimerization region, Sal B preventive roles against M120 oxidation acted as a primary signal driver to activate nuclear factor erythroid 2-related factor 2 (Nrf2). Transcriptional splicing of non-POU domain-containing octamer-binding protein was validated during the Sal B-mediated overexpression of NAD(P)H dehydrogenase [quinone] 1. This molecular redox regulation of actin-interacting proteins provided valuable insights into the phenolic structures of Sal B analogs, showing potential antioxidative effects on vascular endothelium.

Parvin: A hub of intracellular signalling pathways regulating cellular behaviour and disease progression

α-actinin superfamily houses the family of parvins, comprising α, β and γ isoforms in the vertebrates and a single orthologue in the invertebrates. Parvin as an adaptor protein is a member of the ternary IPP-complex including Integrin Linked Kinase (ILK) and particularly-interesting-Cys-His-rich protein (PINCH). Each of the complex proteins showed a conserved lineage and was principally used by the evolutionarily primitive integrin-adhesome machinery to regulate cellular behaviour and signalling pathways. Parvin facilitated integrin mediated integration of the extracellular matrix with cytoskeletal framework culminating in regulation of cellular adhesion and spreading, cytoskeleton reorganisation and cell survival. Studies have established role of parvin in pregnancy, lactation, matrix degradation, blood vessel formation and in several diseases such as cancer, NAFLD and cardiac diseases etc. This review narrates the history of parvin discovery, its elaborate gene structure and conservation across phyla including cellular expression, localisation and interacting partners in vertebrates as well as invertebrates. The review further discusses how parvin acts as an epicentre of signalling pathways, its associated mutants and diseased conditions.

Protein Interaction Domains: Structural Features and Drug Discovery Applications (Part 2)

BACKGROUND

Proteins present a modular organization made up of several domains. Apart from the domains playing catalytic functions, many others are crucial to recruit interactors. The latter domains can be defined as "PIDs" (Protein Interaction Domains) and are responsible for pivotal outcomes in signal transduction and a certain array of normal physiological and disease-related pathways. Targeting such PIDs with small molecules and peptides able to modulate their interaction networks, may represent a valuable route to discover novel therapeutics.

OBJECTIVE

This work represents a continuation of a very recent review describing PIDs able to recognize post-translationally modified peptide segments. On the contrary, the second part concerns with PIDs that interact with simple peptide sequences provided with standard amino acids.

METHODS

Crucial structural information on different domain subfamilies and their interactomes was gained by a wide search in different online available databases (including the PDB (Protein Data Bank), the Pfam (Protein family), and the SMART (Simple Modular Architecture Research Tool)). Pubmed was also searched to explore the most recent literature related to the topic.

RESULTS AND CONCLUSION

PIDs are multifaceted: they have all diverse structural features and can recognize several consensus sequences. PIDs can be linked to different diseases onset and progression, like cancer or viral infections and find applications in the personalized medicine field. Many efforts have been centered on peptide/peptidomimetic inhibitors of PIDs mediated interactions but much more work needs to be conducted to improve drug-likeness and interaction affinities of identified compounds.

EB1 Is Essential for Spindle Formation and Chromosome Alignment During Oocyte Meiotic Maturation in Mice

The cytoskeleton plays an orchestrating role in polarized cell growth. Microtubules (MTs) not only play critical roles in chromosome alignment and segregation but also control cell shape, division, and motility. A member of the plus-end tracking proteins, end-binding protein 1 (EB1), regulates MT dynamics and plays vital roles in maintaining spindle symmetry and chromosome alignment during mitosis. However, the role of EB1 in mouse oocyte meiosis remains unknown. Here, we examined the localization patterns and expression levels of EB1 at different stages. EB1 protein level was found to be stable during meiosis. EB1 mainly localized along the spindle and had a similar localization pattern as that of α-tubulin. The EB1 protein was degraded with a Trim-Away method, and the results were further confirmed with western blotting and immunofluorescence. At 12 h of culture after EB1 knockdown (KD), a reduced number of mature MII oocytes were observed. EB1 KD led to spindle disorganization, chromosome misalignment, and missegregation; β-catenin protein binds to actin via the adherens junctional complex, which was significantly reduced in the EB1 KD oocytes. Collectively, we propose that the impairment of EB1 function manipulates spindle formation, thereby promoting chromosomal loss, which is expected to fuel aneuploidy and possibly fertilization failure.

EH domain binding protein 1-like 1 (EHBP1L1), a protein with calponin homology domain, is expressed in the rat testis

In this paper, with the aim to find new genes involved in mammalian spermatogenesis, we isolated, for the first time in the rat testis, a partial cDNA clone that encoded EH domain binding protein 1-like 1 (Ehbp1l1), a protein that has a single calponin homology domain (CH). Bioinformatic analysis showed that EHBP1l1 contains three domains: the N-terminal C2-like, the CH and the C-terminal bivalent Mical/EHBP Rab binding (bMERB) domains, which are evolutionarily conserved in vertebrates. We found that Ehbp1l1 mRNA was expressed in several rat tissues, including the liver, intestine, kidney and also in the testis during its development, with a higher level in testis from 12-month-old animals. Interestingly, in situ hybridization experiments revealed that Ehbp1l1 is specifically expressed by types I and II spermatocytes, this result was validated by RT-PCR performed on total RNA obtained from enriched fractions of different testicular cell types. As EHBP1l1 has been described as linked to vesicular transport to the actin cytoskeleton and as an effector of the small GTPase Rab8, we hypothesized that it could participate both in cytoskeletal remodelling and in the regulation of vesicle sorting from the trans-Golgi network to the apical plasma membrane. Our findings provide a better understand of the molecular mechanisms of the differentiation process of spermatogenesis; Ehbp1l1 may also be used as a new marker of testicular activity.

Rapid, redox-mediated mechanical susceptibility of the cortical microtubule lattice in skeletal muscle

The highly ordered cortical microtubule lattice of skeletal muscle is disorganized in dystrophin-deficient mdx mice. Implicated mechanisms include loss of dystrophin binding, altered α-tubulin posttranslational modification, expression of a β-tubulin involved in regeneration, and reactive oxygen species (ROS). Here we show that the transverse microtubules in mdx muscle expressing miniaturized dystrophins are rapidly lost after eccentric contraction. Analysis of mdx lines expressing different dystrophin constructs demonstrate that spectrin-like repeats R4-15 and R20-23 were required for mechanically stable microtubules. Microtubule loss was prevented by the non-specific antioxidant N-acetylcysteine while inhibition of NADPH oxidase 2 had only a partial effect, suggesting that ROS from multiple sources mediate the rapid loss of transverse microtubules after eccentric contraction. Finally, ablation of α-dystrobrevin, β- or γ-cytoplasmic actin phenocopied the transverse microtubule instability of miniaturized dystrophins. Our data demonstrate that multiple dystrophin domains, α-dystrobrevin and cytoplasmic actins are necessary for mechanically stable microtubules.

The mechanism of activation of the actin binding protein EHBP1 by Rab8 family members

EHBP1 is an adaptor protein that regulates vesicular trafficking by recruiting Rab8 family members and Eps15-homology domain-containing proteins 1/2 (EHD1/2). It also links endosomes to the actin cytoskeleton. However, the underlying molecular mechanism of activation of EHBP1 actin-binding activity is unclear. Here, we show that both termini of EHBP1 have membrane targeting potential. EHBP1 associates with PI(3)P, PI(5)P, and phosphatidylserine via its N-terminal C2 domain. We show that in the absence of Rab8 family members, the C-terminal bivalent Mical/EHBP Rab binding (bMERB) domain forms an intramolecular complex with its central calponin homology (CH) domain and auto-inhibits actin binding. Rab8 binding to the bMERB domain relieves this inhibition. We have analyzed the CH:bMERB auto-inhibited complex and the active bMERB:Rab8 complex biochemically and structurally. Together with structure-based mutational studies, this explains how binding of Rab8 frees the CH domain and allows it to interact with the actin cytoskeleton, leading to membrane tubulation.

EHBP1 is an adaptor protein that regulates vesicular trafficking and links endosomes to the actin cytoskeleton. Here, authors show that both termini of EHBP1 have membrane targeting potential and that in the absence of its binding partner Rab8, the bMERB and CH domain of EHBP1 form an intramolecular complex which auto-inhibits actin binding.

Structural Characteristics, Binding Partners and Related Diseases of the Calponin Homology (CH) Domain

The calponin homology (CH) domain is one of the most common modules in various actin-binding proteins and is characterized by an α-helical fold. The CH domain plays important regulatory roles in both cytoskeletal dynamics and signaling. The CH domain is required for stability and organization of the actin cytoskeleton, calcium mobilization and activation of downstream pathways. The CH domain has recently garnered increased attention due to its importance in the onset of different diseases, such as cancers and asthma. However, many roles of the CH domain in various protein functions and corresponding diseases are still unclear. Here, we review current knowledge about the structural features, interactome and related diseases of the CH domain.

Hec1/Ndc80 Tail Domain Function at the Kinetochore-Microtubule Interface

Successful mitotic cell division is critically dependent on the formation of correct attachments between chromosomes and spindle microtubules. Microtubule attachments are mediated by kinetochores, which are large proteinaceous structures assembled on centromeric chromatin of mitotic chromosomes. These attachments must be sufficiently stable to transduce force; however, the strength of these attachments are also tightly regulated to ensure timely, error-free progression through mitosis. The highly conserved, kinetochore-associated NDC80 complex is a core component of the kinetochore-microtubule attachment machinery in eukaryotic cells. A small, disordered region within the Hec1 subunit of the NDC80 complex – the N-terminal “tail” domain – has been actively investigated during the last decade due to its roles in generating and regulating kinetochore-microtubule attachments. In this review, we discuss the role of the NDC80 complex, and specifically the Hec1 tail domain, at the kinetochore-microtubule interface, and how recent studies provide a more unified view of Hec1 tail domain function.

Tissue-Specificity of Dystrophin-Actin Interactions: Isoform-Specific Thermodynamic Stability and Actin-Binding Function of Tandem Calponin-Homology Domains

Genetic mutations in Duchenne muscular dystrophy (DMD) gene affecting the expression of dystrophin protein lead to a number of muscle disorders collectively called dystrophinopathies. In addition to muscle dystrophin, mutations in brain-specific dystrophin isoforms, in particular those that are expressed in the brain cortex and Purkinje neurons, result in cognitive impairment associated with DMD. These isoforms carry minor variations in the flanking region of the N-terminal actin-binding domain (ABD1) of dystrophin, which is composed of two calponin-homology (CH) domains in tandem. Determining the effect of these sequence variations is critical for understanding the mechanisms that govern varied symptoms of the disease. We studied the impact of differences in the N-terminal flanking region on the structure and function of dystrophin tandem CH domain isoforms. The amino acid changes did not affect the global structure of the protein but drastically affected the thermodynamic stability, with the muscle isoform more stable than the brain and Purkinje isoforms. Actin binding investigated with actin from different sources (skeletal muscle, smooth muscle, cardiac muscle, and platelets) revealed that the muscle isoform binds to filamentous actin (F-actin) with a lower affinity compared to the brain and Purkinje isoforms, and a similar trend was observed with actin from different sources. In addition, all isoforms showed a higher affinity to smooth muscle actin in comparison to actin from other sources. In conclusion, tandem CH domain isoforms might be using minor sequence variations in the N-terminal flanking regions to modulate their thermodynamic stability and actin-binding function, thus leading to specificity in dystrophin-actin interactions in various tissues.

Leucine-rich repeats and calponin homology containing 4 (Lrch4) regulates the innate immune response

Toll-like receptors (TLRs) are pathogen-recognition receptors that trigger the innate immune response. Recent reports have identified accessory proteins that provide essential support to TLR function through ligand delivery and receptor trafficking. Herein, we introduce leucine-rich repeats (LRRs) and calponin homology containing 4 (Lrch4) as a novel TLR accessory protein. Lrch4 is a membrane protein with nine LRRs in its predicted ectodomain. It is widely expressed across murine tissues and has two expression variants that are both regulated by lipopolysaccharide (LPS). Predictive modeling indicates that Lrch4 LRRs conform to the horseshoe-shaped structure typical of LRRs in pathogen-recognition receptors and that the best structural match in the protein database is to the variable lymphocyte receptor of the jawless vertebrate hagfish. Silencing Lrch4 attenuates cytokine induction by LPS and multiple other TLR ligands and dampens the in vivo innate immune response. Lrch4 promotes proper docking of LPS in lipid raft membrane microdomains. We provide evidence that this is through regulation of lipid rafts as Lrch4 silencing reduces cell surface gangliosides, a metric of raft abundance, as well as expression and surface display of CD14, a raft-resident LPS co-receptor. Taken together, we identify Lrch4 as a broad-spanning regulator of the innate immune response and a potential molecular target in inflammatory disease.

Structural basis of the filamin A actin-binding domain interaction with F-actin

Actin-cross-linking proteins assemble actin filaments into higher-order structures essential for orchestrating cell shape, adhesion, and motility. Missense mutations in the tandem calponin homology domains of their actin-binding domains (ABDs) underlie numerous genetic diseases, but a molecular understanding of these pathologies is hampered by the lack of high-resolution structures of any actin-cross-linking protein bound to F-actin. Here, taking advantage of a high-affinity, disease-associated mutant of the human filamin A (FLNa) ABD, we combine cryo-electron microscopy and functional studies to reveal at near-atomic resolution how the first calponin homology domain (CH1) and residues immediately N-terminal to it engage actin. We further show that reorientation of CH2 relative to CH1 is required to avoid clashes with actin and to expose F-actin-binding residues on CH1. Our data explain localization of disease-associated loss-of-function mutations to FLNaCH1 and gain-of-function mutations to the regulatory FLNaCH2. Sequence conservation argues that this provides a general model for ABD-F-actin binding.

Shaping neurodevelopment: distinct contributions of cytoskeletal proteins

Development of a neuron critically depends on the organization of its cytoskeleton. Cytoskeletal components, such as tubulins and actins, have the remarkable ability to organize themselves into filaments and networks to support specialized and compartmentalized functions. Alterations in cytoskeletal proteins have long been associated with a variety of neurodevelopmental disorders. This review focuses on recent findings, primarily from forward genetic screens in Caenorhabditis elegans that illustrate how different tubulin protein isotypes can play distinct roles in neuronal development and function. Additionally, we discuss studies revealing new regulators of the actin cytoskeleton, and highlight recent technological advances in in vivo imaging and functional dissection of the neuronal cytoskeleton.

Metabolic strategies for the degradation of the neuromodulator agmatine in mammals

Agmatine (1-amino-4-guanidinobutane), a precursor for polyamine biosynthesis, has been identified as an important neuromodulator with anticonvulsant, antineurotoxic and antidepressant actions in the brain. In this context it has emerged as an important mediator of addiction/satiety pathways associated with alcohol misuse. Consequently, the regulation of the activity of key enzymes in agmatine metabolism is an attractive strategy to combat alcoholism and related addiction disorders. Agmatine results from the decarboxylation of L-arginine in a reaction catalyzed by arginine decarboxylase (ADC), and can be converted to either guanidine butyraldehyde by diamine oxidase (DAO) or putrescine and urea by the enzyme agmatinase (AGM) or the more recently identified AGM-like protein (ALP). In rat brain, agmatine, AGM and ALP are predominantly localised in areas associated with roles in appetitive and craving (drug-reinstatement) behaviors. Thus, inhibitors of AGM or ALP are promising agents for the treatment of addictions. In this review, the properties of DAO, AGM and ALP are discussed with a view to their role in the agmatine metabolism in mammals.

Mammalian Plakins, Giant Cytolinkers: Versatile Biological Functions and Roles in Cancer

Cancer is a highly lethal disease that is characterized by aberrant cell proliferation, migration, and adhesion, which are closely related to the dynamic changes of cytoskeletons and cytoskeletal-adhesion. These will further result in cell invasion and metastasis. Plakins are a family of giant cytolinkers that connect cytoskeletal elements with each other and to junctional complexes. With various isoforms composed of different domain structures, mammalian plakins are broadly expressed in numerous tissues. They play critical roles in many cellular processes, including cell proliferation, migration, adhesion, and signaling transduction. As these cellular processes are key steps in cancer development, mammalian plakins have in recent years attracted more and more attention for their potential roles in cancer. Current evidence shows the importance of mammalian plakins in various human cancers and demonstrates mammalian plakins as potential biomarkers for cancer. Here, we introduce the basic characteristics of mammalian plakins, review the recent advances in understanding their biological functions, and highlight their roles in human cancers, based on studies performed by us and others. This will provide researchers with a comprehensive understanding of mammalian plakins, new insights into the development of cancer, and novel targets for cancer diagnosis and therapy.

Caldendrin Directly Couples Postsynaptic Calcium Signals to Actin Remodeling in Dendritic Spines

Compartmentalization of calcium-dependent plasticity allows for rapid actin remodeling in dendritic spines. However, molecular mechanisms for the spatio-temporal regulation of filamentous actin (F-actin) dynamics by spinous Ca²⁺-transients are still poorly defined. We show that the postsynaptic Ca²⁺ sensor caldendrin orchestrates nano-domain actin dynamics that are essential for actin remodeling in the early phase of long-term potentiation (LTP). Steep elevation in spinous [Ca²⁺]_i disrupts an intramolecular interaction of caldendrin and allows cortactin binding. The fast on and slow off rate of this interaction keeps cortactin in an active conformation, and protects F-actin at the spine base against cofilin-induced severing. Caldendrin gene knockout results in higher synaptic actin turnover, altered nanoscale organization of spinous F-actin, defects in structural spine plasticity, LTP, and hippocampus-dependent learning. Collectively, the data indicate that caldendrin-cortactin directly couple [Ca²⁺]_i to preserve a minimal F-actin pool that is required for actin remodeling in the early phase of LTP.

Quantitative phosphoproteomic analysis of acquired cancer drug resistance to pazopanib and dasatinib

Acquired drug resistance impacts the majority of patients being treated with tyrosine kinase inhibitors (TKIs) and remains a key challenge in modern anti-cancer therapy. The lack of clinically effective therapies to overcome resistance represents an unmet need. Understanding the signalling that drives drug resistance will facilitate the development of new salvage therapies to treat patients with secondary TKI resistance. In this study, we utilise mass spectrometry to characterise the global phosphoproteomic alterations that accompany the acquisition of resistance to two FDA-approved TKIs, pazopanib and dasatinib, in the A204 rhabdoid tumour cell line. Our analysis finds that only 6% and 9.7% of the quantified phosphoproteome is altered upon the acquisition of pazopanib and dasatinib resistance, respectively. Pazopanib resistant cells display elevated phosphorylation in cytoskeletal regulatory pathways while dasatinib resistant cells show an upregulation of the insulin receptor/IGF-1R signalling pathway. Drug response profiling rediscovers several previously reported vulnerabilities associated with pazopanib and dasatinib resistance and identifies a new dependency to the second generation HSP90 inhibitor NVP-AUY-922. This study provides a useful resource detailing the candidate signalling determinants of acquired TKI resistance; and reveals a therapeutic approach of inhibiting HSP90 function as a means of salvage therapy to overcome pazopanib and dasatinib resistance.

Significance

Pazopanib and dasatinib are tyrosine kinase inhibitors (TKIs) approved for the treatment of multiple cancer types. Patients who are treated with these drugs are prone to the development of drug resistance and consequently tumour relapse. Here we use quantitative phosphoproteomics to characterise the signalling pathways which are enriched in cells that have acquired resistance to these two drugs. Furthermore, targeted drug screens were used to identify salvage therapies capable of overcoming pazopanib and dasatinib resistance. This data advances our understanding of the mechanisms of TKI resistance and highlights candidate targets for cancer therapy.

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Pazopanib resistant cells display elevated phosphorylation in cytoskeletal regulatory pathways.

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Phosphoproteins in the insulin and IGF-1R pathways are upregulated in dasatinib resistant cells.

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Less than 10% of the phosphoproteome is altered in acquired drug-resistant A204 cells.

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Both dasatinib and pazopanib resistant A204 cells are vulnerable to HSP90 inhibition.

Spectraplakin family proteins - cytoskeletal crosslinkers with versatile roles

The different cytoskeletal networks in a cell are responsible for many fundamental cellular processes. Current studies have shown that spectraplakins, cytoskeletal crosslinkers that combine features of both the spectrin and plakin families of crosslinkers, have a critical role in integrating these different cytoskeletal networks. Spectraplakin genes give rise to a variety of isoforms that have distinct functions. Importantly, all spectraplakin isoforms are uniquely able to associate with all three elements of the cytoskeleton, namely, F-actin, microtubules and intermediate filaments. In this Review, we will highlight recent studies that have unraveled their function in a wide range of different processes, from regulating cell adhesion in skin keratinocytes to neuronal cell migration. Taken together, this work has revealed a diverse and indispensable role for orchestrating the function of different cytoskeletal elements in vivo.

The N-Terminal Flanking Region Modulates the Actin Binding Affinity of the Utrophin Tandem Calponin-Homology Domain

Despite sharing a high degree of sequence similarity, the tandem calponin-homology (CH) domain of utrophin binds to actin 30 times stronger than that of dystrophin. We have previously shown that this difference in actin binding affinity could not be ascribed to the differences in inter-CH-domain linkers [Bandi, S., et al. (2015) Biochemistry 54, 5480-5488]. Here, we examined the role of the N-terminal flanking region. The utrophin tandem CH domain contains a 27-residue flanking region before its CH1 domain. We examined its effect by comparing the structure and function of full-length utrophin tandem CH domain Utr(1-261) and its truncated Utr(28-261) construct. Both full-length and truncated constructs are monomers in solution, with no significant differences in their secondary or tertiary structures. Truncated construct Utr(28-261) binds to actin 30 times weaker than that of the full-length Utr(1-261), similar to that of the dystrophin tandem CH domain with a much shorter flanking region. Deletion of the N-terminal flanking region stabilizes the CH1 domain. The magnitude of the change in binding free energy upon truncation is similar to that of the change in thermodynamic stability. The isolated N-terminal peptide by itself is significantly random coil and does not bind to F-actin in the affinity range of Utr(1-261) and Utr(28-261). These results indicate that the N-terminal flanking region significantly affects the actin binding affinity of tandem CH domains. This observation further stresses that protein regions other than the three actin-binding surfaces identified earlier, irrespective of whether they directly bind to actin, also contribute to the actin binding affinity of tandem CH domains.

The microtubule end-binding protein FgEB1 regulates polar growth and fungicide sensitivity via different interactors in Fusarium graminearum

In yeasts, the end-binding protein 1 (EB1) homologs regulate microtubule dynamics, cell polarization, and chromosome stability. However, functions of EB1 orthologs in plant pathogenic fungi have not been characterized yet. Here, we observed that the FgEB1 deletion mutant (ΔFgEB1) of Fusarium graminearum exhibits twisted hyphae, increased hyphal branching and curved conidia, indicating that FgEB1 is involved in the regulation of cellular polarity. Microscopic examination further showed that the microtubules of ΔFgEB1 exhibited less organized in comparison with those of the wild type. In addition, the lack of FgEB1 also altered the distribution of polarity-related class I myosin via the interaction with the actin. On the other hand, we identified four core septins as FgEB1-interacting proteins, and found that FgEB1 and septins regulated conidial polar growth in the opposite orientation. Interestingly, FgEB1 and FgKar9 constituted another complex that modulated the response to carbendazim, a microtubule-damaging agent specifically. In addition, the deletion of FgEB1 led to dramatically decreased deoxynivalenol (DON) biosynthesis. Taken together, results of this study indicate that FgEB1 regulates cellular polarity, fungicide sensitivity and DON biosynthesis via different interactors in F. graminarum, which provides a novel insight into understanding of the biological functions of EB1 in filamentous fungi.

EHBP1L1 coordinates Rab8 and Bin1 to regulate apical-directed transport in polarized epithelial cells

The highly conserved Rab guanosine triphosphatase (GTPase) Rab8 plays a role in exocytosis toward the polarized plasma membrane in eukaryotic cells. In murine Rab8-deficient small intestine cells, apical proteins are missorted into lysosomes. In this study, we identified a novel Rab8-interacting protein complex containing an EH domain-binding protein 1-like 1 (EHBP1L1), Bin1/amphiphysin II, and dynamin. Biochemical analyses showed that EHBP1L1 directly bound to GTP-loaded Rab8 and Bin1. The spatial dependency of these complexes at the endocytic recycling compartment (ERC) was demonstrated through overexpression and knockdown experiments. EHBP1L1- or Bin1-depleted or dynamin-inhibited small intestine organoids significantly accumulated apical membrane proteins but not basolateral membrane proteins in lysosomes. Furthermore, in EHBP1L1-deficient mice, small intestine cells displayed truncated and sparse microvilli, suggesting that EHBP1L1 maintains the apical plasma membrane by regulating apical transport. In summary, our data demonstrate that EHBP1L1 links Rab8 and the Bin1-dynamin complex, which generates membrane curvature and excises the vesicle at the ERC for apical transport.

RAB-10 Promotes EHBP-1 Bridging of Filamentous Actin and Tubular Recycling Endosomes

EHBP-1 (Ehbp1) is a conserved regulator of endocytic recycling, acting as an effector of small GTPases including RAB-10 (Rab10). Here we present evidence that EHBP-1 associates with tubular endosomal phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] enriched membranes through an N-terminal C2-like (NT-C2) domain, and define residues within the NT-C2 domain that mediate membrane interaction. Furthermore, our results indicate that the EHBP-1 central calponin homology (CH) domain binds to actin microfilaments in a reaction that is stimulated by RAB-10(GTP). Loss of any aspect of this RAB-10/EHBP-1 system in the C. elegans intestinal epithelium leads to retention of basolateral recycling cargo in endosomes that have lost their normal tubular endosomal network (TEN) organization. We propose a mechanism whereby RAB-10 promotes the ability of endosome-bound EHBP-1 to also bind to the actin cytoskeleton, thereby promoting endosomal tubulation.

Endosomes are intracellular organelles that sort protein and lipid components integral to the membrane, as well as more loosely associated lumenal content, for delivery to distinct intracellular destinations. Endosomes associated with recycling cargo back to the plasma membrane are often tubular in morphology, and this morphology is thought to be essential for recycling function. Our previous work identified a particularly dramatic network of endosomal tubules involved in membrane protein recycling in the basolateral intestinal epithelial cells of C. elegans. Our subsequent genetic analysis of basolateral recycling in this system identified a number of key regulators of these endosomes, including the small GTPase RAB-10 and its effector EHBP-1. Our new work presented here shows that EHBP-1 promotes endosomal tubulation by linking the membrane lipid PI(4,5)P2 to the actin cytoskeleton, and that the linkage of EHBP-1 to actin is enhanced by the interaction of EHBP-1 with RAB-10. This work has broad implications for how endosomal tubulation occurs in all cells, and has specific implications for the role of EHBP-1 in related processes such as insulin-stimulated recycling of glucose transporters in human adipocytes, a process intimately linked to type II diabetes.

In vivo epidermal migration requires focal adhesion targeting of ACF7

Turnover of focal adhesions allows cell retraction, which is essential for cell migration. The mammalian spectraplakin protein, ACF7 (Actin-Crosslinking Factor 7), promotes focal adhesion dynamics by targeting of microtubule plus ends towards focal adhesions. However, it remains unclear how the activity of ACF7 is regulated spatiotemporally to achieve focal adhesion-specific guidance of microtubule. To explore the potential mechanisms, we resolve the crystal structure of ACF7’s NT (amino-terminal) domain, which mediates F-actin interactions. Structural analysis leads to identification of a key tyrosine residue at the calponin homology (CH) domain of ACF7, whose phosphorylation by Src/FAK (focal adhesion kinase) complex is essential for F-actin binding of ACF7. Using skin epidermis as a model system, we further demonstrate that the phosphorylation of ACF7 plays an indispensable role in focal adhesion dynamics and epidermal migration in vitro and in vivo. Together, our findings provide critical insights into the molecular mechanisms underlying coordinated cytoskeletal dynamics during cell movement.

The spectraplakin protein ACF7 binds to actin at focal adhesions and targets microtubule plus ends to focal adhesions, promoting their disassembly. Here the authors reveal that ACF7 is phosphorylated by Src/FAK, and this regulates actin binding and focal adhesion dynamics in vitro and in vivo.

Knockdown of Five Genes Encoding Uncharacterized Proteins Inhibits Entamoeba histolytica Phagocytosis of Dead Host Cells

Entamoeba histolytica is the protozoan parasite that causes invasive amebiasis, which is endemic to many developing countries and characterized by dysentery and liver abscesses. The virulence of E. histolytica correlates with the degree of host cell engulfment, or phagocytosis, and E. histolytica phagocytosis alters amebic gene expression in a feed-forward manner that results in an increased phagocytic ability. Here, we used a streamlined RNA interference screen to silence the expression of 15 genes whose expression was upregulated in phagocytic E. histolytica trophozoites to determine whether these genes actually function in the phagocytic process. When five of these genes were silenced, amebic strains with significant decreases in the ability to phagocytose apoptotic host cells were produced. Phagocytosis of live host cells, however, was largely unchanged, and the defects were surprisingly specific for phagocytosis. Two of the five encoded proteins, which we named E. histolytica ILWEQ (EhILWEQ) and E. histolytica BAR (EhBAR), were chosen for localization via SNAP tag labeling and localized to the site of partially formed phagosomes. Therefore, both EhILWEQ and EhBAR appear to contribute to E. histolytica virulence through their function in phagocytosis, and the large proportion (5/15 [33%]) of gene-silenced strains with a reduced ability to phagocytose host cells validates the previously published microarray data set demonstrating feed-forward control of E. histolytica phagocytosis. Finally, although only limited conclusions can be drawn from studies using the virulence-deficient G3 Entamoeba strain, the relative specificity of the defects induced for phagocytosis of apoptotic cells but not healthy cells suggests that cell killing may play a rate-limiting role in the process of Entamoeba histolytica host cell engulfment.

Modulation of MICAL Monooxygenase Activity by its Calponin Homology Domain: Structural and Mechanistic Insights

MICALs (Molecule Interacting with CasL) are conserved multidomain enzymes essential for cytoskeletal reorganization in nerve development, endocytosis, and apoptosis. In these enzymes, a type-2 calponin homology (CH) domain always follows an N-terminal monooxygenase (MO) domain. Although the CH domain is required for MICAL-1 cellular localization and actin-associated function, its contribution to the modulation of MICAL activity towards actin remains unclear. Here, we present the structure of a fragment of MICAL-1 containing the MO and the CH domains—determined by X-ray crystallography and small angle scattering—as well as kinetics experiments designed to probe the contribution of the CH domain to the actin-modification activity. Our results suggest that the CH domain, which is loosely connected to the MO domain by a flexible linker and is far away from the catalytic site, couples F-actin to the enhancement of redox activity of MICAL_MO-CH by a cooperative mechanism involving a trans interaction between adjacently bound molecules. Binding cooperativity is also observed in other proteins regulating actin assembly/disassembly dynamics, such as ADF/Cofilins.

Congenital macrothrombocytopenia-linked mutations in the actin-binding domain of α-actinin-1 enhance F-actin association

Mutations in the actin cross-linking protein actinin-1 were recently linked to dominantly inherited congenital macrothrombocytopenia. Here, we report that several disease-associated mutations that are located within the actinin-1 actin-binding domain cause increased binding of actinin-1 to actin filaments and enhance filament bundling in vitro. These actinin-1 mutants are also more stably associated with the cytoskeleton in cultured cells, as assessed by biochemical fractionation and fluorescence recovery after photobleaching experiments. For two mutations the disruption of contacts between the calponin homology domains within the actinin actin-binding domain may explain increased filament binding--providing mechanistic and structural insights into the basis of actinin-1 dysfunction in congenital macrothrombocytopenia.

Interactions between the Microtubule Binding Protein EB1 and F-Actin

Many cellular processes including cell division and cell migration require coordination between the actin and microtubule (MT) cytoskeletons. This coordination is as-yet poorly understood, but proteins such as formins and IQGAP1 are known to be involved. We show that the MT binding protein EB1 (end-binding protein 1), a key regulator of MT dynamics, can bind directly to filamentous actin (F-actin) F-actin. We determined that the EB1:F-actin interaction is salt sensitive and weak under physiological salt concentrations but might be relevant in contexts where the local concentration of actin is high. Using bioinformatics and mutagenesis, we found that the EB1:F-actin binding site partially overlaps the well-characterized EB1:MT binding interface. Congruently, competition experiments indicate that EB1 can bind to F-actin or MTs but not both simultaneously. These observations suggest that EB1:F-actin interactions may negatively regulate EB1:MT interactions, and we speculate that this interaction may assist cells in differentially regulating MT stability in the actin-rich cortex as opposed to the cell interior.

The Gas2 family protein Pigs is a microtubule +TIP that affects cytoskeleton organisation

Coordination between different cytoskeletal systems is crucial for many cell biological functions, including cell migration and mitosis, and also plays an important role during tissue morphogenesis. Proteins of the class of cytoskeletal crosslinkers, or cytolinkers, have the ability to interact with more than one cytoskeletal system at a time and are prime candidates to mediate any coordination. One such class comprises the Gas2-like proteins, combining a conserved calponin-homology-type actin-binding domain and a Gas2 domain predicted to bind microtubules (MTs). This domain combination is also found in spectraplakins, huge cytolinkers that play important roles in many tissues in both invertebrates and vertebrates. Here, we dissect the ability of the single Drosophila Gas2-like protein Pigs to interact with both actin and MT cytoskeletons, both in vitro and in vivo, and illustrate complex regulatory interactions that determine the localisation of Pigs to and its effects on the cytoskeleton.

Cloning of two LIMCH1 isoforms: characterization of their distribution in rat brain and their agmatinase activity

Agmatine, a precursor for polyamine biosynthesis, is also associated with neurotransmitter, anticonvulsant, antineurotoxic and antidepressant actions in the brain. This molecule results from the decarboxylation of L-arginine by arginine decarboxylase, and it is hydrolyzed to urea and putrescine by agmatinase. Recently, we have described a new protein that also hydrolyzes agmatine, agmatinase-like protein (ALP), which was identified through immunohistochemical analysis in the hypothalamus and hippocampus of rats. However, its sequence differs greatly from all known agmatinases and does not contain the typical Mn(2+) ligands associated with the urea hydrolase family of proteins. ALP has a LIM-like domain close to its carboxyl terminus, and the removal of which results in a truncated variant with a tenfold increased k cat value and a threefold decreased K m value for agmatine. Analysis of the gene database revealed several transcripts, denominated LIMCH1 isoforms, with extreme 3' sequences identical to ALP. Limch1 gene products have been described as members of a multi-domain family of proteins with the biggest isoform containing a calponin homology (CH) domain at its N-terminus. Here, we cloned two LIMCH1 transcripts, one of 3177 bp and the other of 2709 bp (ALP contains 1569 bp) and analyzed LIMCH1 expression and distribution in rat brain using RT-PCR, Western blot and immunohistochemical analyses. LIMCH1 was detected mainly in the hypothalamic and hippocampal regions, which is similar to the distribution of ALP and agmatine in brain. In addition, we cloned and expressed both isoforms in E. coli and confirmed that they were catalytically active on agmatine with kinetic parameters similar to ALP. LIM domain-truncated variants of both isoforms moderately increased the k cat and catalytic efficiency. Thus, we propose that LIMCH1 is useful to regulate the intracellular concentrations of the neurotransmitter/neuromodulator, agmatine.

The N- and C-Terminal Domains Differentially Contribute to the Structure and Function of Dystrophin and Utrophin Tandem Calponin-Homology Domains

Dystrophin and utrophin are two muscle proteins involved in Duchenne/Becker muscular dystrophy. Both proteins use tandem calponin-homology (CH) domains to bind to F-actin. We probed the role of N-terminal CH1 and C-terminal CH2 domains in the structure and function of dystrophin tandem CH domain and compared with our earlier results on utrophin to understand the unifying principles of how tandem CH domains work. Actin cosedimentation assays indicate that the isolated CH2 domain of dystrophin weakly binds to F-actin compared to the full-length tandem CH domain. In contrast, the isolated CH1 domain binds to F-actin with an affinity similar to that of the full-length tandem CH domain. Thus, the obvious question is why the dystrophin tandem CH domain requires CH2, when its actin binding is determined primarily by CH1. To answer, we probed the structural stabilities of CH domains. The isolated CH1 domain is very unstable and is prone to serious aggregation. The isolated CH2 domain is very stable, similar to the full-length tandem CH domain. These results indicate that the main role of CH2 is to stabilize the tandem CH domain structure. These conclusions from dystrophin agree with our earlier results on utrophin, indicating that this phenomenon of differential contribution of CH domains to the structure and function of tandem CH domains may be quite general. The N-terminal CH1 domains primarily determine the actin binding function whereas the C-terminal CH2 domains primarily determine the structural stability of tandem CH domains, and the extent of stabilization depends on the strength of inter-CH domain interactions.

F-actin clustering and cell dysmotility induced by the pathological W148R missense mutation of filamin B at the actin-binding domain

Filamin B (FLNB) is a dimeric actin-binding protein that orchestrates the reorganization of the actin cytoskeleton. Congenital mutations of FLNB at the actin-binding domain (ABD) are known to cause abnormalities of skeletal development, such as atelosteogenesis types I and III and Larsen's syndrome, although the underlying mechanisms are poorly understood. Here, using fluorescence microscopy, we characterized the reorganization of the actin cytoskeleton in cells expressing each of six pathological FLNB mutants that have been linked to skeletal abnormalities. The subfractionation assay showed a greater accumulation of the FLNB ABD mutants W148R and E227K than the wild-type protein to the cytoskeleton. Ectopic expression of FLNB-W148R and, to a lesser extent, FLNB-E227K induced prominent F-actin accumulations and the consequent rearrangement of focal adhesions, myosin II, and septin filaments and results in a delayed directional migration of the cells. The W148R protein-induced cytoskeletal rearrangement was partially attenuated by the inhibition of myosin II, p21-activated protein kinase, or Rho-associated protein kinase. The expression of a single-head ABD fragment with the mutations partially mimicked the rearrangement induced by the dimer. The F-actin clustering through the interaction with the mutant FLNB ABD may limit the cytoskeletal reorganization, preventing normal skeletal development.

Interdomain Linker Determines Primarily the Structural Stability of Dystrophin and Utrophin Tandem Calponin-Homology Domains Rather than Their Actin-Binding Affinity

Tandem calponin-homology (CH) domains are the most common actin-binding domains in proteins. However, structural principles underlying their function are poorly understood. These tandem domains exist in multiple conformations with varying degrees of inter-CH-domain interactions. Dystrophin and utrophin tandem CH domains share high sequence similarity (∼82%), yet differ in their structural stability and actin-binding affinity. We examined whether the conformational differences between the two tandem CH domains can explain differences in their stability and actin binding. Dystrophin tandem CH domain is more stable by ∼4 kcal/mol than that of utrophin. Individual CH domains of dystrophin and utrophin have identical structures but differ in their relative orientation around the interdomain linker. We swapped the linkers between dystrophin and utrophin tandem CH domains. Dystrophin tandem CH domain with utrophin linker (DUL) has similar stability as that of utrophin tandem CH domain. Utrophin tandem CH domain with dystrophin linker (UDL) has similar stability as that of dystrophin tandem CH domain. Dystrophin tandem CH domain binds to F-actin ∼30 times weaker than that of utrophin. After linker swapping, DUL has twice the binding affinity as that of dystrophin tandem CH domain. Similarly, UDL has half the binding affinity as that of utrophin tandem CH domain. However, changes in binding free energies due to linker swapping are much lower by an order of magnitude compared to the corresponding changes in unfolding free energies. These results indicate that the linker region determines primarily the structural stability of tandem CH domains rather than their actin-binding affinity.

Development of a Novel Green Fluorescent Protein-Based Binding Assay to Study the Association of Plakins with Intermediate Filament Proteins

Protein-protein interactions are fundamental for most biological processes, such as the formation of cellular structures and enzymatic complexes or in signaling pathways. The identification and characterization of protein-protein interactions are therefore essential for understanding the mechanisms and regulation of biological systems. The organization and dynamics of the cytoskeleton, as well as its anchorage to specific sites in the plasma membrane and organelles, are regulated by the plakins. These structurally related proteins anchor different cytoskeletal networks to each other and/or to other cellular structures. The association of several plakins with intermediate filaments (IFs) is critical for maintenance of the cytoarchitecture. Pathogenic mutations in the genes encoding different plakins can lead to dramatic manifestations, occurring principally in the skin, striated muscle, and/or nervous system, due to cytoskeletal disorganization resulting in abnormal cell fragility. Nevertheless, it is still unclear how plakins bind to IFs, although some general rules are slowly emerging. We here describe in detail a recently developed protein-protein fluorescence binding assay, based on the production of recombinant proteins tagged with green fluorescent protein (GFP) and their use as fluid-phase fluorescent ligands on immobilized IF proteins. Using this method, we have been able to assess the ability of C-terminal regions of GFP-tagged plakin proteins to bind to distinct IF proteins and IF domains. This simple and sensitive technique, which is expected to facilitate further studies in this area, can also be potentially employed for any kind of protein-protein interaction studies.

Spectrin binding motifs regulate Scribble cortical dynamics and polarity function

The tumor suppressor protein Scribble (SCRIB) plays an evolutionary conserved role in cell polarity. Despite being central for its function, the molecular basis of SCRIB recruitment and stabilization at the cell cortex is poorly understood. Here we show that SCRIB binds directly to the CH1 domain of β spectrins, a molecular scaffold that contributes to the cortical actin cytoskeleton and connects it to the plasma membrane. We have identified a short evolutionary conserved peptide motif named SADH motif (SCRIB ABLIMs DMTN Homology) which is necessary and sufficient to mediate protein interaction with β spectrins. The SADH domains contribute to SCRIB dynamics at the cell cortex and SCRIB polarity function. Furthermore, mutations in SCRIB SADH domains associated with spina bifida and cancer impact the stability of SCRIB at the plasma membrane, suggesting that SADH domain alterations may participate in human pathology.

DOI:http://dx.doi.org/10.7554/eLife.04726.001

Proteins found in cells often have more than one role. Scribble is one such multi-tasking protein that is found in a diverse range of species, including fruit flies and humans. Although Scribble commonly helps to ensure that the components of a cell are in their correct locations, its exact roles vary between species. To perform its role well, Scribble itself must localize to the cell cortex—the inside surface of the cell membrane—at the regions where cells connect to one another. How this localization occurs is not fully understood; and defects in the human form of Scribble have been linked to diseases including spina bifida and cancer.

Much of the Scribble protein is very similar across different species, but the fruit fly and human version of the protein have large differences in their ‘C-terminal region’ that makes up one end of each protein. Boëda and Etienne-Manneville now show that in humans and other animals with backbones—but not in fruit flies—the C-terminal region of Scribble contains three repeats of a sequence called the SADH motif. These motifs can bind to proteins called beta spectrins, which connect the cell's outer membrane to the scaffolding-like structure inside the cell that provides support.

Mutations that alter the SADH motif interfere with Scribble's ability to bind to the scaffolding, and alters Scribble localization at cell–cell contacts or the cell cortex. Boëda and Etienne-Manneville also found that some mutations linked to spina bifida and cancer affect the SADH motif, suggesting that this motif has a wider role in disease.

While the abnormal localization of Scribble inside cells is frequently observed in particularly difficult to survive cancers, the molecular mechanism that causes Scribble to fail to localize to the cell periphery is still poorly understood. Boëda and Etienne-Manneville's work establishes the beta spectrin family of proteins as regulators that stabilize Scribble at the cell cortex and suggests that Scribble-associated diseases might depend on the integrity of the spectrin network.

DOI:http://dx.doi.org/10.7554/eLife.04726.002

A divergent calponin homology (NN-CH) domain defines a novel family: implications for evolution of ciliary IFT complex B proteins

MOTIVATION

Microtubules are dynamic polymers of tubulin dimers that undergo continuous assembly and disassembly. A mounting number of microtubule-associated proteins (MAPs) regulate the dynamic behavior of microtubules and hence the assembly and disassembly of disparate microtubule structures within the cell. Despite recent advances in identification and functional characterization of MAPs, a substantial number of microtubule accessory factors have not been functionally annotated. Here, using profile-to-profile comparisons and structure modeling, we show that the yeast outer kinetochore components NDC80 and NUF2 share evolutionary ancestry with a novel protein family in mammals comprising, besides NDC80/HEC1 and NUF2, three Intraflagellar Transport (IFT) complex B subunits (IFT81, IFT57, CLUAP1) as well as six proteins with poorly defined function (FAM98A-C, CCDC22, CCDC93 and C14orf166). We show that these proteins consist of a divergent N-terminal calponin homology (CH)-like domain adjoined to an array of C-terminal heptad repeats predicted to form a coiled-coil arrangement. We have named the divergent CH-like domain NN-CH after the founding members NDC80 and NUF2.

The actin binding affinity of the utrophin tandem calponin-homology domain is primarily determined by its N-terminal domain

The structural determinants of the actin binding function of tandem calponin-homology (CH) domains are poorly understood, particularly the role of individual domains. We determined the actin binding affinity of isolated CH domains from human utrophin and compared them with the affinity of the full-length tandem CH domain. Traditional cosedimentation assays indicate that the C-terminal CH2 domain binds to F-actin much weaker than the full-length tandem CH domain. The N-terminal CH1 domain is less stable and undergoes severe protein aggregation; therefore, traditional actin cosedimentation assays could not be used. To address this, we have developed a folding-upon-binding method. We refolded the CH1 domain from its unfolded state in the presence of F-actin. This results in a competition between actin binding and aggregation. A differential centrifugation technique was used to distinguish actin binding from aggregation. Low-speed centrifugation pelleted CH1 aggregates, but not F-actin or its bound protein. Subsequent high-speed centrifugation resulted in the cosedimentation of bound CH1 along with F-actin. The CH1 domain binds to F-actin with an affinity similar to that of the full-length tandem CH domain, unlike the CH2 domain. The actin binding cooperativity between the two domains was quantitatively calculated from the association constants of the full-length tandem CH domain and its CH domains, and found to be much smaller than the association constant of the CH1 domain alone. These results indicate that the actin binding affinity of the utrophin tandem CH domain is primarily determined by its CH1 domain, when compared to that of its CH2 domain or the cooperativity between the two CH domains.

Unique ζ-chain motifs mediate a direct TCR-actin linkage critical for immunological synapse formation and T-cell activation

TCR-mediated activation induces receptor microclusters that evolve to a defined immune synapse (IS). Many studies showed that actin polymerization and remodeling, which create a scaffold critical to IS formation and stabilization, are TCR mediated. However, the mechanisms controlling simultaneous TCR and actin dynamic rearrangement in the IS are yet not fully understood. Herein, we identify two novel TCR ζ-chain motifs, mediating the TCR's direct interaction with actin and inducing actin bundling. While T cells expressing the ζ-chain mutated in these motifs lack cytoskeleton (actin) associated (cska)-TCRs, they express normal levels of non-cska and surface TCRs as cells expressing wild-type ζ-chain. However, such mutant cells are unable to display activation-dependent TCR clustering, IS formation, expression of CD25/CD69 activation markers, or produce/secrete cytokine, effects also seen in the corresponding APCs. We are the first to show a direct TCR-actin linkage, providing the missing gap linking between TCR-mediated Ag recognition, specific cytoskeleton orientation toward the T-cell-APC interacting pole and long-lived IS maintenance.

The N-terminal actin-binding tandem calponin-homology (CH) domain of dystrophin is in a closed conformation in solution and when bound to F-actin

Deficiency of the vital muscle protein dystrophin triggers Duchenne/Becker muscular dystrophy, but the structure-function relationship of dystrophin is poorly understood. To date, molecular structures of three dystrophin domains have been determined, of which the N-terminal actin-binding domain (N-ABD or ABD1) is of particular interest. This domain is composed of two calponin-homology (CH) domains, which form an important class of ABDs in muscle proteins. A previously determined x-ray structure indicates that the dystrophin N-ABD is a domain-swapped dimer, with each monomer adopting an extended, open conformation in which the two CH domains do not interact. This structure is controversial because it contradicts functional studies and known structures of similar ABDs from other muscle proteins. Here, we investigated the solution conformation of the dystrophin N-ABD using a very simple and elegant technique of pyrene excimer fluorescence. Using the wild-type protein, which contains two cysteines, and the corresponding single-cysteine mutants, we show that the protein is a monomer in solution and is in a closed conformation in which the two CH domains seem to interact, as observed from the excimer fluorescence of pyrene-labeled wild-type protein. Excimer fluorescence was also observed in its actin-bound form, indicating that the dystrophin N-ABD binds to F-actin in a closed conformation. Comparison of the dystrophin N-ABD conformation with other ABDs indicates that the tandem CH domains in general may be monomeric in solution and predominantly occur in closed conformation, whereas their actin-bound conformations may differ.

MICAL, the flavoenzyme participating in cytoskeleton dynamics

MICAL (from the Molecule Interacting with CasL) indicates a family of recently discovered cytosolic, multidomain proteins, which uniquely couple an N-terminal FAD-containing monooxygenase-like domain to typical calponine homology, LIM and coiled-coil protein-interaction modules. Genetic and cell biology approaches have demonstrated an essential role of the catalytic activity of the monooxygenase-like domain in transducing the signal initiated by semaphorins interaction with their plexin receptors, which results in local actin cytoskeleton disassembly as part of fundamental processes that include differentiation, migration and cell-cell contacts in neuronal and non-neuronal cell types. This review focuses on the structure-function relations of the MICAL monooxygenase-like domain as they are emerging from the available in vitro studies on mouse, human and Drosophila MICAL forms that demonstrated a NADPH-dependent actin depolymerizing activity of MICAL. With Drosophila MICAL forms, actin depolymerization was demonstrated to be associated to conversion of Met44 to methionine sulfone through a postulated hydroxylating reaction. Arguments supporting the concept that MICAL effect on F-actin may be reversible will be discussed.

End-binding proteins and Ase1/PRC1 define local functionality of structurally distinct parts of the microtubule cytoskeleton

The microtubule cytoskeleton is crucial for the intracellular organization of eukaryotic cells. It is a dynamic scaffold that has to perform a variety of very different functions. This multitasking is achieved through the activity of numerous microtubule-associated proteins. Two prominent classes of proteins are central to the selective recognition of distinct transiently existing structural features of the microtubule cytoskeleton. They define local functionality through tightly regulated protein recruitment. Here we summarize the recent developments in elucidating the molecular mechanism underlying the action of microtubule end-binding proteins (EBs) and antiparallel microtubule crosslinkers of the Ase1/PRC1 family that represent the core of these two recruitment modules. Despite their fundamentally different activities, these conserved families share several common features.

Myopodin is an F-actin bundling protein with multiple independent actin-binding regions

The assembly of striated muscle myofibrils is a multistep process in which a variety of proteins is involved. One of the first and most important steps in myofibrillogenesis is the arrangement of thin myofilaments into ordered I-Z-I brushes, requiring the coordinated activity of numerous actin binding proteins. The early expression of myopodin prior to sarcomeric α-actinin, as well as its binding to actin, α-actinin and filamin indicate an important role for this protein in actin cytoskeleton remodelling with the precise function of myopodin in this process yet remaining to be resolved. While myopodin was previously described as a protein capable of cross-linking actin filaments into thick bundles upon transient transfections, it has remained unclear whether myopodin alone is capable of bundling actin, or if additional proteins are involved. We have therefore investigated the in vitro actin binding properties of myopodin. High speed cosedimentation assays with skeletal muscle actin confirmed direct binding of myopodin to F-actin and showed that this interaction is mediated by at least two independent actin binding sites, found in all myopodin isoforms identified to date. Furthermore, low-speed cosedimentation assays revealed that not only full length myopodin, but also the fragment containing only the second binding site, bundles microfilaments in the absence of accessory proteins. Ultrastructural analysis demonstrated that this bundling activity resembled that of α-actinin. Biochemical experiments revealed that bundling was not achieved by myopodin's ability to dimerize, indicating the presence of two individual F-actin binding sites within the second binding segment. Thus full length myopodin contains at least three F-actin binding sites. These data provide further understanding of the mechanisms by which myopodin contributes to actin reorganization during myofibril assembly.

A calcium-dependent interaction between calmodulin and the calponin homology domain of human IQGAP1

IQGAPs are cytoskeletal scaffolding proteins which collect information from a variety of signalling pathways and pass it on to the microfilaments and microtubules. There is a well-characterised interaction between IQGAP and calmodulin through a series of IQ-motifs towards the middle of the primary sequence. However, it has been shown previously that the calponin homology domain (CHD), located at the N-terminus of the protein, can also interact weakly with calmodulin. Using a recombinant fragment of human IQGAP1 which encompasses the CHD, we have demonstrated that the CHD undergoes a calcium ion-dependent interaction with calmodulin. The CHD can also displace the hydrophobic fluorescent probe 1-anilinonaphthalene-8-sulphonate from calcium-calmodulin, suggesting that the interaction involves non-polar residues on the surface of calmodulin. Molecular modelling identified a possible site on the CHD for calmodulin interaction. The physiological significance of this interaction remains to be discovered.

Identification and characterization of myophilin-like protein: a life stage and tissue-specific antigen of Clonorchis sinensis

Clonorchiasis, caused by Clonorchis sinensis infection, has been an important public health problem in China. More messages about biology and pathogenicity of C. sinensis will be better for development of new strategies for clonorchiasis control. In the current study, using data from the published genomic and cDNA sequences for C. sinensis, we identify a gene encoding a myophilin-like protein (Cs myophilin-like). The cDNA sequence is 573 bp in length which encoded a protein of 190 amino acids. DNA sequence of Cs myophilin-like consists of four extrons and three introns. By mapping the draft genome of C. sinensis, single copy of myophilin-like gene is annotated. Bioinformatic analysis shows Cs myophilin-like comprises a calponin homology domain and three actin binding surfaces. It expresses at the life stage of adult worm and metacercaria but not in eggs. It intensively distributes in muscular locations such as oral sucker and pharynx of adult worm and sucker of metacercaria. The transcript level of Cs myophilin-like in oral sucker and pharynx is significantly higher than that in the rest of adult worm. Moreover, recombinant Cs myophilin-like is obtained and of antigenicity and immunoreactivity. It can be probed by sera from patients infected with trematode or cestode. Our data suggests that Cs myophilin-like may be involved in the host tissue invasion besides regulating the contraction of smooth muscle and movement of worm body. Taken together, the stable and soluble expression in Escherichia coli and immunogenicity of recombinant Cs myophilin-like proteins suggest that Cs myophilin-like may be a potential candidate for vaccine and drug target for clonorchiasis.

Filamins in mechanosensing and signaling

Filamins are essential, evolutionarily conserved, modular, multidomain, actin-binding proteins that organize the actin cytoskeleton and maintain extracellular matrix connections by anchoring actin filaments to transmembrane receptors. By cross-linking and anchoring actin filaments, filamins stabilize the plasma membrane, provide cellular cortical rigidity, and contribute to the mechanical stability of the plasma membrane and the cell cortex. In addition to binding actin, filamins interact with more than 90 other binding partners including intracellular signaling molecules, receptors, ion channels, transcription factors, and cytoskeletal and adhesion proteins. Thus, filamins scaffold a wide range of signaling pathways and are implicated in the regulation of a diverse array of cellular functions including motility, maintenance of cell shape, and differentiation. Here, we review emerging structural and functional evidence that filamins are mechanosensors and/or mechanotransducers playing essential roles in helping cells detect and respond to physical forces in their local environment.

Utrophin ABD binds to F-actin in an open conformation

Structural analyses of actin binding regions comprising tandem calponin homology domains alone and when bound to F-actin have revealed a number of different conformations with calponin homology domains in ‘open’ and ‘closed’ positions. In an attempt to resolve these issues we have examined the properties of the utrophin actin binding domain in open and closed conformations in order to verify the conformation when bound to F-actin. Locking the actin binding domain in a closed conformation using engineered cysteine residues in each calponin homology domain reduced the affinity for F-actin without affecting the stoichiometry furthermore differential scanning calorimetry experiments revealed a reduction in melting temperature on binding to actin. The data suggest the amino-terminal utrophin actin binding domain is in an open conformation in solution and when bound to F-actin.