Indirect association of ezrin with F-actin: isoform specificity and calcium sensitivity

Structural and functional mechanisms of actin isoforms

Actin is a highly conserved and fundamental protein in eukaryotes and participates in a broad spectrum of cellular functions. Cells maintain a conserved ratio of actin isoforms, with muscle and non-muscle actins representing the main actin isoforms in muscle and non-muscle cells, respectively. Actin isoforms have specific and redundant functional roles and display different biochemistries, cellular localization, and interactions with myosins and actin-binding proteins. Understanding the specific roles of actin isoforms from the structural and functional perspective is crucial for elucidating the intricacies of cytoskeletal dynamics and regulation and their implications in health and disease. Here, we review how the structure contributes to the functional mechanisms of actin isoforms with a special emphasis on the questions of how post-translational modifications and disease-linked mutations affect actin isoforms biochemistry, function, and interaction with actin-binding proteins and myosin motors.

Cytochalasans and Their Impact on Actin Filament Remodeling

The eukaryotic actin cytoskeleton comprises the protein itself in its monomeric and filamentous forms, G- and F-actin, as well as multiple interaction partners (actin-binding proteins, ABPs). This gives rise to a temporally and spatially controlled, dynamic network, eliciting a plethora of motility-associated processes. To interfere with the complex inter- and intracellular interactions the actin cytoskeleton confers, small molecular inhibitors have been used, foremost of all to study the relevance of actin filaments and their turnover for various cellular processes. The most prominent inhibitors act by, e.g., sequestering monomers or by interfering with the polymerization of new filaments and the elongation of existing filaments. Among these inhibitors used as tool compounds are the cytochalasans, fungal secondary metabolites known for decades and exploited for their F-actin polymerization inhibitory capabilities. In spite of their application as tool compounds for decades, comprehensive data are lacking that explain (i) how the structural deviances of the more than 400 cytochalasans described to date influence their bioactivity mechanistically and (ii) how the intricate network of ABPs reacts (or adapts) to cytochalasan binding. This review thus aims to summarize the information available concerning the structural features of cytochalasans and their influence on the described activities on cell morphology and actin cytoskeleton organization in eukaryotic cells.

Ez-Metastasizing: The Crucial Roles of Ezrin in Metastasis

Ezrin is the cytoskeletal organizer and functions in the modulation of membrane-cytoskeleton interaction, maintenance of cell shape and structure, and regulation of cell-cell adhesion and movement, as well as cell survival. Ezrin plays a critical role in regulating tumor metastasis through interaction with other binding proteins. Notably, Ezrin has been reported to interact with immune cells, allowing tumor cells to escape immune attack in metastasis. Here, we review the main functions of Ezrin, the mechanisms through which it acts, its role in tumor metastasis, and its potential as a therapeutic target.

Ezrin and Its Phosphorylated Thr567 Form Are Key Regulators of Human Extravillous Trophoblast Motility and Invasion

The protein ezrin has been shown to enhance cancer cell motility and invasion leading to malignant behaviours in solid tumours, but a similar regulatory function in the early physiological reproduction state is, however, much less clear. We speculated that ezrin may play a key role in promoting first-trimester extravillous trophoblast (EVT) migration/invasion. Ezrin, as well as its Thr567 phosphorylation, were found in all trophoblasts studied, whether primary cells or lines. Interestingly, the proteins were seen in a distinct cellular localisation in long, extended protrusions in specific regions of cells. Loss-of-function experiments were carried out in EVT HTR8/SVneo and Swan71, as well as primary cells, using either ezrin siRNAs or the phosphorylation Thr567 inhibitor NSC668394, resulting in significant reductions in both cell motility and cellular invasion, albeit with differences between the cells used. Our analysis further demonstrated that an increase in focal adhesion was, in part, able to explain some of the molecular mechanisms involved. Data collected using human placental sections and protein lysates further showed that ezrin expression was significantly higher during the early stage of placentation and, importantly, clearly seen in the EVT anchoring columns, further supporting the potential role of ezrin in regulating migration and invasion in vivo.

The RIT1 C-terminus associates with lipid bilayers via charge complementarity

RIT1 is a member of the Ras superfamily of small GTPases involved in regulation of cellular signaling. Mutations to RIT1 are involved in cancer and developmental disorders. Like many Ras subfamily members, RIT1 is localized to the plasma membrane. However, RIT1 lacks the C-terminal prenylation that helps many other subfamily members adhere to cellular membranes. We used molecular dynamics simulations to examine the mechanisms by which the C-terminal peptide (CTP) of RIT1 associates with lipid bilayers. We show that the CTP is unstructured and that its membrane interactions depend on lipid composition. While a 12-residue region of the CTP binds strongly to anionic bilayers containing phosphatidylserine lipids, the CTP termini fray from the membrane allowing for accommodation of the RIT1 globular domain at the membrane-water interface.

Regulation of actin isoforms in cellular and developmental processes

Actin is one of the most abundant and essential intracellular proteins that mediates nearly every form of cellular movement and underlies such key processes as embryogenesis, tissue integrity, cell division and contractility of all types of muscle and non-muscle cells. In mammals, actin is represented by six isoforms, which are encoded by different genes but produce proteins that are 95-99 % identical to each other. The six actin genes have vastly different functions in vivo, and the small amino acid differences between the proteins they encode are rigorously maintained through evolution, but the underlying differences behind this distinction, as well as the importance of specific amino acid sequences for each actin isoform, are not well understood. This review summarizes different levels of actin isoform-specific regulation in cellular and developmental processes, starting with the nuclear actin's role in transcription, and covering the gene-level, mRNA-level, and protein-level regulation, with a special focus on mammalian actins in non-muscle cells.

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.

Neutrophil Cell Shape Change: Mechanism and Signalling during Cell Spreading and Phagocytosis

Perhaps the most important feature of neutrophils is their ability to rapidly change shape. In the bloodstream, the neutrophils circulate as almost spherical cells, with the ability to deform in order to pass along narrower capillaries. Upon receiving the signal to extravasate, they are able to transform their morphology and flatten onto the endothelium surface. This transition, from a spherical to a flattened morphology, is the first key step which neutrophils undergo before moving out of the blood and into the extravascular tissue space. Once they have migrated through tissues towards sites of infection, neutrophils carry out their primary role-killing infecting microbes by performing phagocytosis and producing toxic reactive oxygen species within the microbe-containing phagosome. Phagocytosis involves the second key morphology change that neutrophils undergo, with the formation of pseudopodia which capture the microbe within an internal vesicle. Both the spherical to flattened stage and the phagocytic capture stage are rapid, each being completed within 100 s. Knowing how these rapid cell shape changes occur in neutrophils is thus fundamental to understanding neutrophil behaviour. This article will discuss advances in our current knowledge of this process, and also identify an important regulated molecular event which may represent an important target for anti-inflammatory therapy.

The makings of the 'actin code': regulation of actin's biological function at the amino acid and nucleotide level

The actin cytoskeleton plays key roles in every eukaryotic cell and is essential for cell adhesion, migration, mechanosensing, and contractility in muscle and non-muscle tissues. In higher vertebrates, from birds through to mammals, actin is represented by a family of six conserved genes. Although these genes have evolved independently for more than 100 million years, they encode proteins with ≥94% sequence identity, which are differentially expressed in different tissues, and tightly regulated throughout embryogenesis and adulthood. It has been previously suggested that the existence of such similar actin genes is a fail-safe mechanism to preserve the essential function of actin through redundancy. However, knockout studies in mice and other organisms demonstrate that the different actins have distinct biological roles. The mechanisms maintaining this distinction have been debated in the literature for decades. This Review summarizes data on the functional regulation of different actin isoforms, and the mechanisms that lead to their different biological roles in vivo We focus here on recent studies demonstrating that at least some actin functions are regulated beyond the amino acid level at the level of the actin nucleotide sequence.

Cytokinesis requires localized β-actin filament production by an actin isoform specific nucleator

Cytokinesis is initiated by the localized assembly of the contractile ring, a dynamic actomyosin structure that generates a membrane furrow between the segregating chromosomal masses to divide a cell into two. Here we show that the stabilization and organization of the cytokinetic furrow is specifically dependent on localized β-actin filament assembly at the site of cytokinesis. β-actin filaments are assembled directly at the furrow by an anillin-dependent pathway that enhances RhoA-dependent activation of the formin DIAPH3, an actin nucleator. DIAPH3 specifically generates homopolymeric filaments of β-actin in vitro. By employing enhancers and activators, cells can achieve acute spatio-temporal control over isoform-specific actin arrays that are required for distinct cellular functions.

Cytokinesis is initiated by the localized assembly of the contractile ring. Here the authors show that the stabilization and organization of the cytokinetic furrow requires localized β-actin filament assembly at the site of cytokinesis by an actin isoform specific nucleator.

Ezrin regulates focal adhesion and invadopodia dynamics by altering calpain activity to promote breast cancer cell invasion

Ezrin regulates proper focal adhesion and invadopodia turnover by regulating calpain-1, in part by directing its proteolytic activity toward key substrates talin, FAK, and cortactin. Ezrin-deficient tumor cells show reduced lung seeding and colonization in vivo but not primary tumor growth, thus implicating ezrin as a metastasis-associated protein.

Up-regulation of the cytoskeleton linker protein ezrin frequently occurs in aggressive cancer types and is closely linked with metastatic progression. However, the underlying molecular mechanisms detailing how ezrin is involved in the invasive and metastatic phenotype remain unclear. Here we report a novel function of ezrin in regulating focal adhesion (FA) and invadopodia dynamics, two key processes required for efficient invasion to occur. We show that depletion of ezrin expression in invasive breast cancer cells impairs both FA and invadopodia turnover. We also demonstrate that ezrin-depleted cells display reduced calpain-mediated cleavage of the FA and invadopodia-associated proteins talin, focal adhesion kinase (FAK), and cortactin and reduced calpain-1–specific membrane localization, suggesting a requirement for ezrin in maintaining proper localization and activity of calpain-1. Furthermore, we show that ezrin is required for cell directionality, early lung seeding, and distant organ colonization but not primary tumor growth. Collectively our results unveil a novel mechanism by which ezrin regulates breast cancer cell invasion and metastasis.

Loss of γ-cytoplasmic actin triggers myofibroblast transition of human epithelial cells

Transdifferentiation of epithelial cells into mesenchymal cells and myofibroblasts plays an important role in tumor progression and tissue fibrosis. Such epithelial plasticity is accompanied by dramatic reorganizations of the actin cytoskeleton, although mechanisms underlying cytoskeletal effects on epithelial transdifferentiation remain poorly understood. In the present study, we observed that selective siRNA-mediated knockdown of γ-cytoplasmic actin (γ-CYA), but not β-cytoplasmic actin, induced epithelial-to-myofibroblast transition (EMyT) of different epithelial cells. The EMyT manifested by increased expression of α-smooth muscle actin and other contractile proteins, along with inhibition of genes responsible for cell proliferation. Induction of EMyT in γ-CYA-depleted cells depended on activation of serum response factor and its cofactors, myocardial-related transcriptional factors A and B. Loss of γ-CYA stimulated formin-mediated actin polymerization and activation of Rho GTPase, which appear to be essential for EMyT induction. Our findings demonstrate a previously unanticipated, unique role of γ-CYA in regulating epithelial phenotype and suppression of EMyT that may be essential for cell differentiation and tissue fibrosis.

Pericyte contractility controls endothelial cell cycle progression and sprouting: insights into angiogenic switch mechanics

Microvascular stability and regulation of capillary tonus are regulated by pericytes and their interactions with endothelial cells (EC). While the RhoA/Rho kinase (ROCK) pathway has been implicated in modulation of pericyte contractility, in part via regulation of the myosin light chain phosphatase (MLCP), the mechanisms linking Rho GTPase activity with actomyosin-based contraction and the cytoskeleton are equivocal. Recently, the myosin phosphatase-RhoA-interacting protein (MRIP) was shown to mediate the RhoA/ROCK-directed MLCP inactivation in vascular smooth muscle. Here we report that MRIP directly interacts with the β-actin-specific capping protein βcap73. Furthermore, manipulation of MRIP expression influences pericyte contractility, with MRIP silencing inducing cytoskeletal remodeling and cellular hypertrophy. MRIP knockdown induces a repositioning of βcap73 from the leading edge to stress fibers; thus MRIP-silenced pericytes increase F-actin-driven cell spreading twofold. These hypertrophied and cytoskeleton-enriched pericytes demonstrate a 2.2-fold increase in contractility upon MRIP knockdown when cells are plated on a deformable substrate. In turn, silencing pericyte MRIP significantly affects EC cycle progression and angiogenic activation. When MRIP-silenced pericytes are cocultured with capillary EC, there is a 2.0-fold increase in EC cycle entry. Furthermore, in three-dimensional models of injury and repair, silencing pericyte MRIP results in a 1.6-fold elevation of total tube area due to EC network formation and increased angiogenic sprouting. The pivotal role of MRIP expression in governing pericyte contractile phenotype and endothelial growth should lend important new insights into how chemomechanical signaling pathways control the "angiogenic switch" and pathological angiogenic induction.

Human NK cell lytic granules and regulation of their exocytosis

Natural killer (NK) cells form a subset of lymphocytes that play a key role in immuno-surveillance and host defense against cancer and viral infections. They recognize stressed cells through a variety of germline-encoded activating cell surface receptors and utilize their cytotoxic ability to eliminate abnormal cells. Killing of target cells is a complex, multi-stage process that concludes in the directed secretion of lytic granules, containing perforin and granzymes, at the immunological synapse. Upon delivery to a target cell, perforin mediates generation of pores in membranes of target cells, allowing granzymes to access target cell cytoplasm and induce apoptosis. Therefore, lytic granules of NK cells are indispensable for normal NK cell cytolytic function. Indeed, defects in lytic granule secretion lead or are related to serious and often fatal diseases, such as familial hemophagocytic lymphohistiocytosis (FHL) type 2–5 or Griscelli syndrome type 2. A number of reports highlight the role of several proteins involved in lytic granule release and NK cell-mediated killing of tumor cells. This review focuses on lytic granules of human NK cells and the advancements in understanding the mechanisms controlling their exocytosis.

Advanced glycation end-products induce calpain-mediated degradation of ezrin

Advanced glycation end-products (AGEs) are important mediators of diabetic complications via incompletely understood pathways. AGEs bind to intracellular ERM proteins (ezrin, radixin and moesin) that modulate cell shape, motility, adhesion and signal transduction. AGEs bind to the N-terminal domain of ezrin but not full-length ezrin. The AGE binding site may be made accessible either by proteolysis releasing an N-terminal fragment or ezrin activation by phosphorylation. Increased intracellular calcium is a primary event in cell activation by high glucose or AGEs. Calpain activity is increased concomitantly, and ezrin is a calpain substrate. The present study assessed whether glycated proteins affect ezrin cleavage and activation in renal tubule epithelial cells. After 7 days, AGE-BSA decreased ezrin levels in MDCK renal tubular cells to 66 ± 4% of control. AGE-RNAse, ribosylated fetal bovine serum and methylglyoxal-BSA all had similar effects. The AGE-BSA-induced decrease in ezrin was abolished by calpastatin peptide, a specific calpain inhibitor, and 1,2-bis-aminophenoxyethane-tetraacetic acid acetoxymethyl ester (BAPTA-AM), a calcium chelator. Ezrin breakdown products were increased in AGE-BSA-treated cells, with a main fragment of ∼ 43 kDa. In vitro, calpain 1 cleaved recombinant human ezrin, generating breakdown fragments including an N-terminal fragment of ∼ 43 kDa. Studies with ezrin mutants showed that non-phosphorylated ezrin was more susceptible to calpain cleavage. AGE-BSA decreased phosphorylated ERM levels to 31 ± 12% in MDCK cells. Thus, AGE-BSA promotes calpain-mediated proteolysis of ezrin in MDCK cells by both increasing calpain activity and reducing phosphorylation. Therapies targeting both glycated proteins and calpain may provide protection against diabetic complications.

Nonredundant roles of cytoplasmic β- and γ-actin isoforms in regulation of epithelial apical junctions

The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific siRNAs and cell-permeable inhibitory peptides. Unique roles of cytoplasmic actin isoforms in regulating structure and remodeling of adherens and tight junctions are revealed.

Association with the actin cytoskeleton is critical for normal architecture and dynamics of epithelial tight junctions (TJs) and adherens junctions (AJs). Epithelial cells express β-cytoplasmic (β-CYA) and γ-cytoplasmic (γ-CYA) actins, which have different cellular localization and functions. This study elucidates the roles of cytoplasmic actins in regulating structure and remodeling of AJs and TJs in model intestinal epithelia. Immunofluorescence labeling and latrunculin B treatment reveal affiliation of dynamic β-CYA filaments with newly assembled and mature AJs, whereas an apical γ-CYA pool is composed of stable perijunctional bundles and rapidly turning-over nonjunctional filaments. The functional effects of cytoplasmic actins on epithelial junctions are examined by using isoform-specific small interfering RNAs and cell-permeable inhibitory peptides. These experiments demonstrate unique roles of β-CYA and γ-CYA in regulating the steady-state integrity of AJs and TJs, respectively. Furthermore, β-CYA is selectively involved in establishment of apicobasal cell polarity. Both actin isoforms are essential for normal barrier function of epithelial monolayers, rapid AJ/TJ reassembly, and formation of three-dimensional cysts. Cytoplasmic actin isoforms play unique roles in regulating structure and permeability of epithelial junctions.

Targeted gene inactivation reveals a functional role of calpain-1 in platelet spreading

BACKGROUND

Calpains are implicated in a wide range of cellular functions including the maintenance of hemostasis via the regulation of cytoskeletal modifications in platelets.

OBJECTIVES

Determine the functional role of calpain isoforms in platelet spreading.

METHODS AND RESULTS

Platelets from calpain-1(-/-) mice show enhanced spreading on collagen- and fibrinogen-coated surfaces as revealed by immunofluorescence, differential interference contrast (DIC) and scanning electron microscopy. The treatment of mouse platelets with MDL, a cell permeable inhibitor of calpains 1/2, resulted in increased spreading. The PTP1B-mediated enhanced tyrosine dephosphorylation in calpain-1(-/-) platelets did not fully account for the enhanced spreading as platelets from the double knockout mice lacking calpain-1 and PTP1B showed only a partial rescue of the spreading phenotype. In non-adherent platelets, proteolysis and GTPase activity of RhoA and Rac1 were indistinguishable between the wild-type (WT) and calpain-1(-/-) platelets. In contrast, the ECM-adherent calpain-1(-/-) platelets showed higher Rac1 activity at the beginning of spreading, whereas RhoA was more active at later time points. The ECM-adherent calpain-1(-/-) platelets showed an elevated level of RhoA protein but not Rac1 and Cdc42. Proteolysis of recombinant RhoA, but not Rac1 and Cdc42, indicates that RhoA is a calpain-1 substrate in vitro.

CONCLUSIONS

Potentiation of the platelet spreading phenotype in calpain-1(-/-) mice suggests a novel role of calpain-1 in hemostasis, and may explain the normal bleeding time observed in the calpain-1(-/-) mice.

Deoxyelephantopin impedes mammary adenocarcinoma cell motility by inhibiting calpain-mediated adhesion dynamics and inducing reactive oxygen species and aggresome formation

We previously showed that deoxyelephantopin (DET), a plant sesquiterpene lactone, exhibits more profound suppression than paclitaxel (PTX) of lung metastasis of mammary adenocarcinoma TS/A cells in mice. Proteomics studies suggest that DET affects actin cytoskeletal protein networks and downregulates calpain-mediated proteolysis of several actin-associated proteins, whereas PTX mainly interferes with microtubule proteins. Here, DET was observed to significantly deregulate adhesion formation in TS/A cells, probably through inhibition of m-calpain activity. Epithelial growth factor (EGF)-mediated activation of Rho GTPase Rac1 and formation of lamellipodia in TS/A cells were remarkably suppressed by DET treatment. Further, DET impaired vesicular trafficking of EGF and induced protein carbonylation and formation of centrosomal aggregates in TS/A cells. DET-induced reactive oxygen species were observed to be the upstream stimulus for the formation of centrosomal ubiquitinated protein aggregates that might subsequently restrict cancer cell motility. PTX, however, caused dramatic morphological changes, interfered with microtubule networking, and moderately inhibited calpain-mediated cytoskeletal and focal adhesion protein cleavage in TS/A cells. This study provides novel mechanistic insights into the pharmacological action of DET against metastatic mammary cell migration and suggests that modulation of oxidative stress might be a potential strategy for treatment of metastatic breast cancer.

How and why does β-actin mRNA target?

beta-Actin mRNA is localized near the leading edge in several cell types where actin polymerization is actively promoting forward protrusion. The localization of the beta-actin mRNA near the leading edge is facilitated by a short sequence in the 3'UTR (untranslated region), the 'zipcode'. Localization of the mRNA at this region is important physiologically. Treatment of chicken embryo fibroblasts with antisense oligonucleotides complementary to the localization sequence (zipcode) in the 3'UTR leads to delocalization of beta-actin mRNA, alteration of cell phenotype and a decrease in cell motility. The dynamic image analysis system (DIAS) used to quantify movement of cells in the presence of sense and antisense oligonucleotides to the zipcode showed that net pathlength and average speed of antisense-treated cells were significantly lower than in sense-treated cells. This suggests that a decrease in persistence of direction of movement and not in velocity results from treatment of cells with zipcode-directed antisense oligonucleotides. We postulate that delocalization of beta-actin mRNA results in delocalization of nucleation sites and beta-actin protein from the leading edge followed by loss of cell polarity and directional movement. Hence the physiological consequences of beta-actin mRNA delocalization affect the stability of the cell phenotype.

Calpain mediates epithelial cell microvillar effacement by enterohemorrhagic Escherichia coli

A member of the attaching and effacing (AE) family of pathogens, enterohemorrhagic Escherichia coli (EHEC) induces dramatic changes to the intestinal cell cytoskeleton, including effacement of microvilli. Effacement by the related pathogen enteropathogenic E. coli (EPEC) requires the activity of the Ca⁺²-dependent host protease, calpain, which participates in a variety of cellular processes, including cell adhesion and motility. We found that EHEC infection results in an increase in epithelial (CaCo-2a) cell calpain activity and that EHEC-induced microvillar effacement was blocked by ectopic expression of calpastatin, an endogenous calpain inhibitor, or by pretreatment of intestinal cells with a cell-penetrating version of calpastatin. In addition, ezrin, a known calpain substrate that links the plasma membrane to axial actin filaments in microvilli, was cleaved in a calpain-dependent manner during EHEC infection and lost from its normal locale within microvilli. Calpain may be a central conduit through which EHEC and other AE pathogens induce enterocyte cytoskeletal remodeling and exert their pathogenic effects.

The calpain system and cancer

The calpains are a conserved family of cysteine proteinases that catalyse the controlled proteolysis of many specific substrates. Calpain activity is implicated in several fundamental physiological processes, including cytoskeletal remodelling, cellular signalling, apoptosis and cell survival. Calpain expression is altered during tumorigenesis, and the proteolysis of numerous substrates, such as inhibitors of nuclear factor-κB (IκB), focal adhesion proteins (including, focal adhesion kinase and talin) and proto-oncogenes (for example, MYC), has been implicated in tumour pathogenesis. Recent evidence indicates that the increased expression of certain family members might influence the response to cancer therapies, providing justification for the development of novel calpain inhibitors.

Calpain- and talin-dependent control of microvascular pericyte contractility and cellular stiffness

Pericytes surround capillary endothelial cells and exert contractile forces modulating microvascular tone and endothelial growth. We previously described pericyte contractile phenotype to be Rho GTPase- and α-smooth muscle actin (αSMA)-dependent. However, mechanisms mediating adhesion-dependent shape changes and contractile force transduction remain largely equivocal. We now report that the neutral cysteine protease, calpain, modulates pericyte contractility and cellular stiffness via talin, an integrin-binding and F-actin associating protein. Digital imaging and quantitative analyses of living cells reveal significant perturbations in contractile force transduction detected via deformation of silicone substrata, as well as perturbations of mechanical stiffness in cellular contractile subdomains quantified via atomic force microscope (AFM)-enabled nanoindentation. Pericytes overexpressing GFP-tagged talin show significantly enhanced contractility (~two-fold), which is mitigated when either the calpain-cleavage resistant mutant talin L432G or vinculin are expressed. Moreover, the cell-penetrating, calpain-specific inhibitor termed CALPASTAT reverses talin-enhanced, but not Rho GTP-dependent, contractility. Interestingly, our analysis revealed that CALPASTAT, but not its inactive mutant, alters contractile cell-driven substrata deformations while increasing mechanical stiffness of subcellular contractile regions of these pericytes. Altogether, our results reveal that calpain-dependent cleavage of talin modulates cell contractile dynamics, which in pericytes may prove instrumental in controlling normal capillary function or microvascular pathophysiology.

Transcriptional alteration of cytoskeletal genes induced by microcystins in three organs of rats

This study explored the mechanisms of toxicity of microcystins by measuring the transcription levels of nine cytoskeletal genes (actin, tubulin, vimentin, ezrin, radixin, moesin, MAP1b, tau, stathmin) in the liver, kidney and spleen of male Wistar rats treated with microcystins at a dose of 80 microg MC-LReq kg(-1) bw. Microcystins disrupted the transcriptional homeostasis of cytoskeletal genes in these organs. Changes in the transcription of four genes (beta-actin, ezrin, radixin and tau) in liver, one gene (stathmin) in kidney, and one gene (radixin) in spleen were significantly correlated with the tissue concentration of microcystins. However, the influences on the transcription of most genes we studied were greater in the liver than in the kidney or spleen. The effects of microcystins on the transcription of cytoskeletal genes may explain some of the morphological and pathological changes observed in these organs and provide new information on the hepatotoxicity of these compounds. Additionally, transcriptional changes in tumor-associated cytoskeletal genes (ezrin, moesin and stathmin) that were observed in the present study provide a possible clue to the tumor-promoting potential of microcystins and their influences on the transcription of MAP1b and tau imply possible neurological toxicity of microcystins in vertebrates.

A single vegetative actin isovariant overexpressed under the control of multiple regulatory sequences is sufficient for normal Arabidopsis development

The relative significance of gene regulation and protein isovariant differences remains unexplored for most gene families, particularly those participating in multicellular development. Arabidopsis thaliana encodes three vegetative actins, ACT2, ACT7, and ACT8, in two ancient and highly divergent subclasses. Mutations in any of these differentially expressed actins revealed only mild phenotypes. However, double mutants were extremely dwarfed, with altered cell and organ morphology and an aberrant F-actin cytoskeleton (e.g., act2-1 act7-4 and act8-2 act7-4) or totally root-hairless (e.g., act2-1 act8-2). Our studies suggest that the three vegetative actin genes and protein isovariants play distinct subclass-specific roles during plant morphogenesis. For example, during root development, ACT7 was involved in root growth, epidermal cell specification, cell division, and root architecture, and ACT2 and ACT8 were essential for root hair tip growth. Also, genetic complementation revealed that the ACT2 and ACT8 isovariants, but not ACT7, fully rescued the root hair growth defects of single and double mutants. Moreover, we synthesized fully normal plants overexpressing the ACT8 isovariant from multiple actin regulatory sequences as the only vegetative actin in the act2-1 act7-4 background. In summary, it is evident that differences in vegetative actin gene regulation and the diversity in actin isovariant sequences are essential for normal plant development.

Inhibition of angiogenesis in vitro: a central role for beta-actin dependent cytoskeletal remodeling

Endothelial cell migration and proliferation, central steps in both physiologic and pathologic angiogenesis, require cytoskeletal-dependent remodeling, which is, in large part, achieved by the dynamic regulation of the beta-actin network. Specifically, the beta-actin network has previously been shown to be (i) enriched in regions of highly motile cytoplasm, and (ii) modulated by its isoactin-specific barbed-end capping protein, beta cap73. We hypothesize that regulated over-expression of beta cap73 could disrupt angiogenesis by capping beta-actin-filament assembly thus inhibiting the incipient cellular migration and microvascular morphogenesis that ensues. Indeed, upon infection of capillary endothelial cells (cEC) with an adenovirus encoding the full-length beta cap73 (Ad-beta cap73), there is a robust cellular rounding response that occurs concomitantly with cytoskeletal disruption, as visualized with immunofluorescence microscopy. Further, we demonstrate that over-expression of Ad-beta cap73 inhibits cEC migration in wound healing studies. Quantitative in vitro angiogenesis assays reveal that Ad-beta cap73 not only prevents endothelial cells from forming capillary-like networks, but also induces the collapse of preformed endothelial tubes. In testing whether Ad-beta cap73 impairs angiogenic events by inducing anoikis/apoptosis, we demonstrate that beta cap73 infection activates a caspase-3-mediated cell death response as observed by quantitative Western blotting and immunofluorescence analyses. Altogether, these findings suggest that endothelial-specific targeting and beta cap73 over-expression may represent an innovative therapeutic approach capable of abrogating pathologic angiogenesis.

Cell and molecular biology of nuclear actin

Actin is a highly conserved protein and one of the major components of the cytoplasm and the nucleus in eukaryotic cells. In the nucleus, actin is involved in a variety of nuclear processes that include transcription and transcription regulation, RNA processing and export, intranuclear movement, and structure maintenance. Recent advances in the field of nuclear actin have established that functions of actin in the nucleus are versatile, complex, and interconnected. It also has become increasingly evident that the cytoplasmic and nuclear pools of actin are functionally linked. However, while the biological significance of nuclear actin has become clear, we are only beginning to understand the mechanisms that lie behind the regulation of nuclear actin. This review provides an overview of our current understanding of the functions of actin in the nucleus.

Properties of actin from the fission yeast Schizosaccharomyces pombe and interaction with fission yeast profilin

The fission yeast Schizosaccharomyces pombe serves as a model system for studying role of actin cytoskeleton, since it has simple actin cytoskeletons and is genetically tractable. In contrast, biochemical approaches using this organism are still developing; fission yeast actin has so far not been isolated in its native form and characterized, and therefore, biochemical assays of fission yeast actin-binding proteins (ABPs) or myosin have been performed using rabbit skeletal muscle actin that may interact with the fission yeast ABPs in a manner different from fission yeast actin. Here, we report a novel method for isolating functionally active actin from fission yeast cells. The highly purified fission yeast actin polymerized with kinetics somewhat different from those of muscle actin and forms filaments that are structurally indistinguishable from skeletal muscle actin filaments. The fission yeast actin was a significantly weaker activator of Mg(2+)-ATPase of HMM of skeletal muscle myosin than muscle actin. The fission yeast profilin Cdc3 suppressed polymerization of fission yeast actin more effectively than that of muscle actin and showed an affinity for fission yeast actin higher than for muscle actin. The establishment of purification of fission yeast actin will enable reconstruction of physiologically relevant interactions between the actin and fission yeast ABPs or myosins and contribute to clarification of function of actin cytoskeleton in various cellular activities.

Visualization of mRNA translation in living cells

The role of mRNA localization is presumably to effect cell asymmetry by synthesizing proteins in specific cellular compartments. However, protein synthesis has never been directly demonstrated at the sites of mRNA localization. To address this, we developed a live cell method for imaging translation of β-actin mRNA. Constructs coding for β-actin, containing tetracysteine motifs, were transfected into C2C12 cells, and sites of nascent polypeptide chains were detected using the biarsenial dyes FlAsH and ReAsH, a technique we call translation site imaging. These sites colocalized with β-actin mRNA at the leading edge of motile myoblasts, confirming that they were translating. β-Actin mRNA lacking the sequence (zipcode) that localizes the mRNA to the cell periphery, eliminated the translation there. A pulse-chase experiment on living cells showed that the recently synthesized protein correlated spatially with the sites of its translation. Additionally, localization of β-actin mRNA and translation activity was enhanced at cell contacts and facilitated the formation of intercellular junctions.

An essential role for beta-actin mRNA localization and translation in Ca2+-dependent growth cone guidance

Axon pathfinding requires directional responses of growth cones to extracellular cues, which have been shown to involve local synthesis of protein. The identity and functions of the locally produced proteins remain, however, unclear. Here we report that Ca(2+)-dependent bidirectional turning of Xenopus laevis growth cones requires localized distribution and translation of beta-actin messenger RNA. Both beta-actin mRNA and its zipcode-binding protein, ZBP1, are localized at the growth cone and become asymmetrically distributed upon local exposure to brain-derived neurotrophic factor (BDNF). Inhibition of protein synthesis or antisense interference with beta-actin mRNA-ZBP1 binding abolishes both Ca(2+)-mediated attraction and repulsion. In addition, attraction involves a local increase in beta-actin, whereas repulsion is accompanied by a local decrease in beta-actin; thus, both produce a synthesis- and ZBP1 binding-dependent beta-actin asymmetry but with opposite polarities. Together with a similar asymmetry in Src activity during bidirectional responses, our findings indicate that Ca(2+)-dependent spatial regulation of beta-actin synthesis through Src contributes to the directional motility of growth cones during guidance.

Direct binding of SWAP-70 to non-muscle actin is required for membrane ruffling

Membrane ruffling induced by growth factor stimulation is caused by actin remodeling, which is mediated by various signaling molecules including Rac. We have shown that SWAP-70, which binds phosphatidylinositol trisphosphate, is one such molecule required for membrane ruffling in mouse kidney cells. Here, we show that SWAP-70 directly binds to F-actin. The bacterially expressed C-terminal region of SWAP-70 co-sedimented with non-muscle F-actin, suggesting direct binding of SWAP-70 to F-actin. The binding was much weaker in muscle F-actin. A truncated mutant of SWAP-70 containing only the C-terminal region readily colocalizes with F-actin, supporting this idea. Full-length SWAP-70 does not colocalize with F-actin unless cells are stimulated with growth factors, suggesting the presence of a stimuli-dependent regulatory mechanism for actin-binding activity in vivo. Overexpression of the mutant SWAP-70 lacking this binding domain inhibits the membrane ruffling induced by epidermal growth factor stimulation in COS7 cells. This dominant-negative effect is also observed in membrane ruffling induced by a dominant-active Rac, suggesting that SWAP-70 cooperates with Rac. These results suggest that the binding activity of SWAP-70 to non-muscle F-actin is required for membrane ruffling.

Arf6 modulates the beta-actin specific capping protein, betacap73

Recent work from our laboratory has revealed that isoactin cytoskeletal and membrane dynamics are coordinately regulated. In this chapter, we review some of the recent and relevant scientific literature focusing on key aspects of cytoskeletal and membrane-mediated signal transduction. Additionally, we highlight some of the strategic molecular, biochemical, and cell-based methodologies that we have either developed or implemented in our efforts aimed at revealing the pivotal role(s) that the actin isoforms play in controlling cell shape and motility during developmental and/or disease-associated events. Furthermore, we address the central position of beta-actin and its barbed end-specific capping protein, betacap73, in modulating nonmuscle cell membrane dynamics and cell migration. In studying the molecular mechanisms mediating these cytoskeletal protein interactions, we have recently recognized that cell motility and beta-actin dynamics are controlled by the direct association of betacap73 with the plasma membrane- and endosome-associated protein, ADP-ribosylation factor 6 (Arf6).

Epidermal growth factor activates m-calpain (calpain II), at least in part, by extracellular signal-regulated kinase-mediated phosphorylation

How m-calpain is activated in cells has challenged investigators because in vitro activation requires near-millimolar calcium. Previously, we demonstrated that m-calpain activation by growth factors requires extracellular signal-regulated kinase (ERK); this enables tail deadhesion and allows productive motility. We now show that ERK directly phosphorylates and activates m-calpain both in vitro and in vivo. We identified serine 50 as required for epidermal growth factor (EGF)-induced calpain activation in vitro and in vivo. Replacing the serine with alanine limits activation by EGF and subsequent cell deadhesion and motility. A construct with the serine converted to glutamic acid displays constitutive activity in vivo; expression of an estrogen receptor fusion construct produces a tamoxifen-sensitive enzyme. Interestingly, EGF-induced m-calpain activation occurs in the absence of increased intracellular calcium levels; EGF triggers calpain even in the presence of intracellular calcium chelators and in calcium-free media. These data provide evidence that m-calpain can be activated through the ERK cascade via direct phosphorylation and that this activation may occur in the absence of cytosolic calcium fluxes.

Myoblast migration is regulated by calpain through its involvement in cell attachment and cytoskeletal organization

Cell migration is a fundamental cellular function particularly during skeletal muscle development. Ubiquitous calpains are well known to play a pivotal role during muscle differentiation, especially at the onset of fusion. In this study, the possible positive regulation of myoblast migration by calpains, a crucial step required to align myoblasts to permit them to fuse, was investigated. Inhibition of calpain activity by different pharmacological inhibitors argues for the involvement of these proteinases during the migration of myoblasts. Moreover, a clonal cell line that fourfold overexpresses calpastatin, the endogenous inhibitor of calpains, and that exhibits deficient calpain activities was obtained. The results showed that the migratory capacity of C2C12 and fusion into multinucleated myotubes were completely prevented in these clonal cells. Calpastatin-overexpressing myoblasts unable to migrate were characterized by rounded morphology, the loss of membrane extensions, the disorganization of stress fibers and exhibited a major defect in new adhesion formation. Surprisingly, the proteolytic patterns of desmin, talin, vinculin, focal adhesion kinase (FAK) and ezrin, radixin, moesin (ERM) proteins are the same in calpastatin-overexpressing myoblasts as compared to control cells. However, an important accumulation of myristoylated alanine-rich C kinase substrate (MARCKS) was observed in cells showing a reduced calpain activity, suggesting that the proteolysis of this actin-binding protein is calpain-dependent and could be involved in both myoblast adhesion and migration.

Localization of a beta-actin messenger ribonucleoprotein complex with zipcode-binding protein modulates the density of dendritic filopodia and filopodial synapses

The dendritic transport and local translation of mRNA may be an essential mechanism to regulate synaptic growth and plasticity. We investigated the molecular mechanism and function of beta-actin mRNA localization in dendrites of cultured hippocampal neurons. Previous studies have shown that beta-actin mRNA localization to the leading edge of fibroblasts or the growth cones of developing neurites involved a specific interaction between a zipcode sequence in the 3' untranslated region and the mRNA-binding protein zipcode-binding protein-1 (ZBP1). Here, we show that ZBP1 is required for the localization of beta-actin mRNA to dendrites. Knock-down of ZBP1 using morpholino antisense oligonucleotides reduced dendritic levels of ZBP1 and beta-actin mRNA and impaired growth of dendritic filopodia in response to BDNF treatment. Transfection of an enhanced green fluorescent protein (EGFP)-beta-actin construct, which contained the zipcode, increased the density of dendritic filopodia and filopodial synapses. Transfection of an EGFP construct, also with the zipcode, resulted in recruitment of endogenous ZBP1 and beta-actin mRNA into dendrites and similarly increased the density of dendritic filopodia. However, the beta-actin zipcode did not affect filopodial length or the density of mature spines. These results reveal a novel function for an mRNA localization element and its binding protein in the regulation of dendritic morphology and synaptic growth via dendritic filopodia.

Localization of a beta-actin messenger ribonucleoprotein complex with zipcode-binding protein modulates the density of dendritic filopodia and filopodial synapses

The dendritic transport and local translation of mRNA may be an essential mechanism to regulate synaptic growth and plasticity. We investigated the molecular mechanism and function of beta-actin mRNA localization in dendrites of cultured hippocampal neurons. Previous studies have shown that beta-actin mRNA localization to the leading edge of fibroblasts or the growth cones of developing neurites involved a specific interaction between a zipcode sequence in the 3' untranslated region and the mRNA-binding protein zipcode-binding protein-1 (ZBP1). Here, we show that ZBP1 is required for the localization of beta-actin mRNA to dendrites. Knock-down of ZBP1 using morpholino antisense oligonucleotides reduced dendritic levels of ZBP1 and beta-actin mRNA and impaired growth of dendritic filopodia in response to BDNF treatment. Transfection of an enhanced green fluorescent protein (EGFP)-beta-actin construct, which contained the zipcode, increased the density of dendritic filopodia and filopodial synapses. Transfection of an EGFP construct, also with the zipcode, resulted in recruitment of endogenous ZBP1 and beta-actin mRNA into dendrites and similarly increased the density of dendritic filopodia. However, the beta-actin zipcode did not affect filopodial length or the density of mature spines. These results reveal a novel function for an mRNA localization element and its binding protein in the regulation of dendritic morphology and synaptic growth via dendritic filopodia.

Betacap73-ARF6 interactions modulate cell shape and motility after injury in vitro

To understand the role that ARF6 plays in regulating isoactin dynamics and cell motility, we transfected endothelial cells (EC) with HA-tagged ARF6: the wild-type form (WT), a constitutively-active form unable to hydrolyze GTP (Q67L), and two dominant-negative forms, which are either unable to release GDP (T27N) or fail to bind nucleotide (N122I). Motility was assessed by digital imaging microscopy before Western blot analysis, coimmunoprecipitation, or colocalization studies using ARF6, beta-actin, or beta-actin-binding protein-specific antibodies. EC expressing ARF6-Q67L spread and close in vitro wounds at twice the control rates. EC expressing dominant-negative ARF6 fail to develop a leading edge, are unable to ruffle their membranes (N122I), and possess arborized processes. Colocalization studies reveal that the Q67L and WT ARF6-HA are enriched at the leading edge with beta-actin; but T27N and N122I ARF6-HA are localized on endosomes together with the beta-actin capping protein, betacap73. Coimmunoprecipitation and Western blot analyses reveal the direct association of ARF6-HA with betacap73, defining a role for ARF6 in signaling cytoskeletal remodeling during motility. Knowledge of the role that ARF6 plays in orchestrating membrane and beta-actin dynamics will help to reveal molecular mechanisms regulating actin-based motility during development and disease.

Calpain regulates enterocyte brush border actin assembly and pathogenic Escherichia coli-mediated effacement

This study identifies calpain as being instrumental for brush border (BB) microvillus assembly during differentiation and effacement during bacterial pathogenesis. Calpain activity is decreased by 25-80% in Caco 2 lines stably overexpressing calpastatin, the physiological inhibitor of calpain, and the effect is proportional to the calpastatin/calpain ratio. These lines exhibit a 2.5-fold reduction in the rate of microvillus extension. Apical microvillus assembly is reduced by up to 50%, as measured by quantitative fluorometric microscopy (QFM) of ezrin, indicating that calpain recruits ezrin to BB microvilli. Calpain inhibitors ZLLYCHN2, MDL 28170, and PD 150606 block BB assembly and ezrin recruitment to the BB. The HIV protease inhibitor ritonavir, which inhibits calpain at clinically relevant concentrations, also blocks BB assembly, whereas cathepsin and proteasome inhibitors do not. Microvillus effacement is inhibited after exposure of calpastatin-overexpressing cells to enteropathogenic Escherichia coli. These results suggest that calpain regulates BB assembly as well as pathological effacement, and indicate that it is an important regulator involved in HIV protease inhibitor toxicity and host-microbial pathogen interactions.

The amino-terminal domains of the ezrin, radixin, and moesin (ERM) proteins bind advanced glycation end products, an interaction that may play a role in the development of diabetic complications

The presence of advanced glycation end products (AGEs) formed because of hyperglycemia in diabetic patients has been strongly linked to the development of diabetic complications and disturbances in cellular function. In this report, we describe the isolation and identification of novel AGE-binding proteins from diabetic rat kidneys. The proteins were purified by cation exchange and AGE-modified bovine serum albumin (AGE-BSA) affinity chromatography. NH2-terminal and internal sequencing identified the proteins as the NH2-terminal domains of ezrin, radixin, and moesin (ERM proteins). Using BIAcore biosensor analysis, human N-ezrin-(1-324) bound to immobilized AGE-BSA with a KD of 5.3 +/- 2.1 x 10 -7 m, whereas full-length ezrin-(1-586) and C-ezrin-(323-586) did not bind. Other glycated proteins such as AGE-RNase, N in -carboxymethyllysine (CML)-BSA, and glycated human serum albumin isolated from hyperglycemic diabetic sera competed with the immobilized AGE-BSA for binding to N-ezrin, but non-glycated BSA and RNase did not. Thus N-ezrin binds to AGEs in a glycation- and concentration-dependent manner. Phosphorylated ezrin plays a crucial role in cell shape changes, cell attachment, and cell adhesion. The effect of AGE-BSA on ezrin function was studied in a tubulogenesis model in which LLC-PK1 cell tubule formation is dependent on phosphorylated ezrin. Addition of AGE-BSA completely inhibited the ability of the cells to produce tubules. Furthermore, in vitro tyrosine phosphorylation of N-ezrin and ezrin was also inhibited by AGE-BSA. These proteins represent a novel family of intracellular binding molecules for glycated proteins and provide a potential new target for therapeutic intervention in the prevention or treatment of diabetic complications.

The role of the CD44/ezrin complex in cancer metastasis

CD44 is a cell adhesion molecule that was traditionally known as 'homing receptor'. This molecule is known to interact with the ezrin family (ERM family) members and form a complex that plays diverse roles within both normal and abnormal cells, particularly cancer cells. CD44 and ezrin and their respective complex have properties suggesting that they may be important in the process of tumour-endothelium interactions, cell migrations, cell adhesion, tumour progression and metastasis. This article reviews the role of CD44, ezrin family and the CD44/ezrin complex in cancer cells and their clinical impact in patients with cancer.

Inhibition of calpain is a component of nitric oxide-induced down-regulation of human mast cell adhesion

Nitric oxide is an important messenger that regulates mast cell activity by modifications to gene expression and intracellular pathways associated with exocytosis and adhesion. Integrin interactions with extracellular matrix components modulate an array of cell activities, including mediator production and secretion. To investigate the molecular mechanisms underlying NO regulation of mast cell function, we studied its effects on adhesion of a human mast cell line (HMC-1) to fibronectin (FN). The NO donors S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine strongly down-regulated the adhesion of HMC-1 to FN. Inhibitors of soluble guanylate cyclase and protein kinase G did not alter the response of cells to NO. A peroxynitrite scavenger did not affect modulation of adhesion by NO, nor could the effect of NO be mimicked by the peroxynitrite-producing compound 3-morpholinosydnonimine. NO donors inhibited the cysteine protease, calpain, while calpain inhibitors mimicked the effect of NO and led to a decrease in the ability of HMC-1 cells to adhere to FN. Thus, NO is an effective down-regulator of human mast cell adhesion. The mechanism for this action does not involve peroxynitrite or activation of soluble guanylate cyclase. Instead, a portion of NO-induced down-regulation of adhesion may be attributed to inhibition of the cysteine protease, calpain, an enzyme that has been associated with control of integrin activation in other cell types. The inhibition of calpain is most likely mediated via nitrosylation of its active site thiol group. Calpain may represent a novel therapeutic target for the regulation of mast cell activity in inflammatory disorders.

Spatiotemporal regulation of moesin phosphorylation and rear release by Rho and serine/threonine phosphatase during neutrophil migration

Neutrophil motility is crucial to effective host defenses against microorganisms. While uropod retraction is a critical step in the migration of neutrophils, the underlying molecular mechanism is not well understood. Here, we show that inhibition of the Rho small GTPase with C3 exoenzyme prevented the retraction of trailing uropods, indicating that the process of rear release is mediated by a Rho signaling pathway. C3 exoenzyme caused marked elongation of directionally migrating neutrophils, suggesting an additional role for Rho in the maintenance of functional polarized cell shape. We also show that phosphorylation and dephosphorylation of the plasma membrane-actin filament cross-linker moesin are spatiotemporally controlled in migrating neutrophils. In particular, phosphorylation of moesin at threonine 558 depended on Rho activity. Videomicroscopy showed that dephosphorylation of this carboxy-terminal threonine preceded uropod retraction. Calyculin A, an inhibitor of type 1 and type 2A serine/threonine phosphatases, suppressed the moesin dephosphorylation and impaired uropod retraction in a dose-dependent manner. Cypermethrin, an inhibitor of type 2B serine/threonine phosphatase, had no such effects. The finding that Rho small GTPase and type 1/type 2A phosphatases are involved in rear release yields novel insights into the biochemical mechanisms of neutrophil migration.

Calpain II colocalizes with detergent-insoluble rafts on human and Jurkat T-cells

Calpain, a calcium-dependent cysteine protease, is known to associate with the T-cell plasma membrane and subsequently cleave a number of cytoskeletal-associated proteins. In this study, we report the novel observation that calpain II, but not calpain I, associates with membrane lipid rafts on human peripheral blood T-cells and Jurkat cells. Raft-associated calpain activity is enhanced with exogenous calcium and inhibited with calpeptin, a specific inhibitor of calpain activity. In addition, we demonstrate that calpain cleaves the cytoskeletal-associated protein, talin, during the first 30-min after cell stimulation. We propose that lipid raft associated-calpain II could function in early TCR signaling to facilitate immune synapse formation through cytoskeletal remodeling mechanisms. Hence, we demonstrate that the positioning of calpain II within T-cell lipid rafts strategically places it in close proximity to known calpain substrates that are cleaved during Ag-specific T-cell signaling and immune synapse formation.

Cloning and characterization of betaCAP73, a novel regulator of beta-actin assembly

In non-muscle cells, the isoactins are differentially localized, with beta-actin specifically enriched at the cell cortex within motile structures, such as lamellae, while gamma-actin shows no specific localization. To understand the sorting and regulation of beta-actin within moving cells, we previously isolated betaCAP73, a novel beta-actin-specific binding protein (Cell Motil. Cytoskel. 35 (1996) 175). Here, we have cloned and characterized the 4718 nucleotide betaCAP73 cDNA from an endothelial cell library. betaCAP73 cDNA contains six predicted ankyrin-like repeats at the amino terminus and is partially homologous to three previously reported sequences of unknown function. Northern analysis reveals betaCAP73 expression in all tissues tested, with highest levels in skeletal muscle. Consistent with previously demonstrated interactions between native betaCAP73 and beta-actin filament barbed-ends, recombinant betaCAP73 inhibits pyrene-actin assembly in an isoactin-specific manner. Compared to stationary cells betaCAP73 mRNA is down regulated in crawling cells. Similarly, motility-defective cells have increased betaCAP73 protein. Overexpression of full-length betaCAP73 induces the formation of novel membrane protrusions that are enriched in betaCAP73, while overexpression of betaCAP73 domains alters cell morphology. Combined, these results indicate that betaCAP73 modulates isoactin dynamics to regulate the morphological alterations required for cell growth and motility.

One of the two cytoplasmic actin isoforms in Drosophila is essential

Actin is a highly conserved protein found in all eukaryotic organisms. Most organisms have multiple cytoplasmic actin genes that encode isoforms with slightly different amino acid sequences. These different isoforms are coexpressed in many cell types. Why organisms have multiple very similar cytoplasmic actin genes is unclear. We have addressed this question with the cytoplasmic actins in Drosophila, Act5C, and Act42A. These isoforms differ by only two amino acids and both genes are expressed in all cells at all times during development. We identified P element insertions in the Act5C gene that resulted in a lethal phenotype. The lethal phenotype is rescued by a transgene with a genomic fragment that includes Act5C regulatory and amino acid coding sequences. A hybrid transgene containing the protein coding sequence for the Act42A isoform, under the control of the regulatory regions of the Act5C gene, also rescues the lethality of the Act5C mutants. Furthermore, flies that carry only one copy each of Act5C and Act42A are viable. These results suggest the amino acid differences between these two cytoplasmic actin isoforms are not important for function and the need for increased gene dosage to provide more actin is not likely to explain the existence of multiple genes. Instead, our results suggest that regulated expression of Act5C is essential to the fly.