Actin Filament Bundles Research Articles

Fimbrin phosphorylation by metaphase Cdk1 regulates actin cable dynamics in budding yeast.

Actin cables, composed of actin filament bundles nucleated by formins, mediate intracellular transport for cell polarity establishment and maintenance. We previously observed that metaphase cells preferentially promote actin cable assembly through cyclin-dependent kinase 1 (Cdk1) activity. However, the relevant metaphase Cdk1 targets were not known. Here we show that the highly conserved actin filament crosslinking protein fimbrin is a critical Cdk1 target for actin cable assembly regulation in budding yeast. Fimbrin is specifically phosphorylated on threonine 103 by the metaphase cyclin–Cdk1 complex, in vivo and in vitro. On the basis of conformational simulations, we suggest that this phosphorylation stabilizes fimbrin's N-terminal domain, and modulates actin filament binding to regulate actin cable assembly and stability in cells. Overall, this work identifies fimbrin as a key target for cell cycle regulation of actin cable assembly in budding yeast, and suggests an underlying mechanism.

SiRNA Screen Identifies Trafficking Host Factors that Modulate Alphavirus Infection.

Little is known about the repertoire of cellular factors involved in the replication of pathogenic alphaviruses. To uncover molecular regulators of alphavirus infection, and to identify candidate drug targets, we performed a high-content imaging-based siRNA screen. We revealed an actin-remodeling pathway involving Rac1, PIP5K1- α, and Arp3, as essential for infection by pathogenic alphaviruses. Infection causes cellular actin rearrangements into large bundles of actin filaments termed actin foci. Actin foci are generated late in infection concomitantly with alphavirus envelope (E2) expression and are dependent on the activities of Rac1 and Arp3. E2 associates with actin in alphavirus-infected cells and co-localizes with Rac1–PIP5K1-α along actin filaments in the context of actin foci. Finally, Rac1, Arp3, and actin polymerization inhibitors interfere with E2 trafficking from the trans-Golgi network to the cell surface, suggesting a plausible model in which transport of E2 to the cell surface is mediated via Rac1- and Arp3-dependent actin remodeling.

A Processive Arabidopsis Formin Modulates Actin Filament Dynamics in Association with Profilin

Formins are conserved regulators of actin cytoskeletal organization and dynamics that have been implicated to be important for cell division and cell polarity. The mechanism by which diverse formins regulate actin dynamics in plants is still not well understood. Using in vitro single-molecule imaging technology, we directly observed that the FH1-FH2 domain of an Arabidopsis thaliana formin, AtFH14, processively attaches to the barbed end of actin filaments as a dimer and slows their elongation rate by 90%. The attachment persistence of FH1-FH2 is concentration dependent. Furthermore, by use of the triple-color total internal reflection fluorescence microscopy, we found that ABP29, a barbed-end capping protein, competes with FH1-FH2 at the filament barbed end, where its binding is mutually exclusive with AtFH14. In the presence of different plant profilin isoforms, FH1-FH2 enhances filament elongation rates from about 10 to 42 times. Filaments buckle when FH1-FH2 is anchored specifically to cover slides, further indicating that AtFH14 moves processively on the elongating barbed end. At high concentration, AtFH14 bundles actin filaments randomly into antiparallel or parallel spindle-like structures; however, the FH1-FH2-mediated bundles become thinner and longer in the presence of plant profilins. This is the direct demonstration of a processive formin from plants. Our results also illuminate the molecular mechanism of AtFH14 in regulating actin dynamics via association with profilin.

The transcriptional repressor Sum1p counteracts Sir2p in regulation of the actin cytoskeleton, mitochondrial quality control and replicative lifespan in Saccharomyces cerevisiae.

Increasing the stability or dynamics of the actin cytoskeleton can extend lifespan in C. elegans and S. cerevisiae. Actin cables of budding yeast, bundles of actin filaments that mediate cargo transport, affect lifespan control through effects on mitochondrial quality control. Sir2p, the founding member of the Sirtuin family of lifespan regulators, also affects actin cable dynamics, assembly, and function in mitochondrial quality control. Here, we obtained evidence for novel interactions between Sir2p and Sum1p, a transcriptional repressor that was originally identified through mutations that genetically suppress sir2∆ phenotypes unrelated to lifespan. We find that deletion of SUM1 in wild-type cells results in increased mitochondrial function and actin cable abundance. Furthermore, deletion of SUM1 suppresses defects in actin cables and mitochondria of sir2∆ yeast, and extends the replicative lifespan and cellular health span of sir2∆ cells. Thus, Sum1p suppresses Sir2p function in control of specific aging determinants and lifespan in budding yeast.

Simulation of the Effect of Confinement in Actin Ring Formation

Actin filaments are vital for different network structures in living cells. During cytokinesis, they form a contractile ring containing myosin motor proteins and actin filament cross-linkers to separate one cell into two cells. Recent experimental studies have quantified the bundle, ring, and network structures that form when actin filaments polymerize in confined environments in vitro, in the presence of varying concentrations of cross-linkers and motor proteins. In this study, we performed numerical simulations to investigate the effect of actin spherical confinement and cross-linking in ring formation. We used a spring-bead model and Brownian dynamics to simulate semiflexible actin filaments that polymerize in a confining sphere with a rate proportional to the monomer concentration. We simulate cross-linking implicitly as an attractive short-range potential between filament beads. Applying the model for different size of the confining spheres shows that the probability of ring formation decreases by increasing the radius (at fixed initial monomer concentration), in agreement with prior experimental data. We observed and quantified other forms of networks for larger radii. We describe the effect of persistence length, orientation-dependent cross-linking, and initial actin monomer concentration. Simulations show that equilibrium configurations can be reached through zipping and unzipping of actin filaments in bundles and transient ring formation.

Structure and Dynamics of Filopodia Studied by Electron Cryo-Tomography and Single Molecule Fluorescence Imaging

Filopodia are cellular projections around 100nm in diameter and usually several micrometres in length. They are composed of fascin-bundled actin filaments and the molecular motor myosin‑10 localises at the extreme tip. Filopodia are important in cell migration and in making cell-cell contacts and have been linked with roles in diverse areas of biology including angiogenesis, and path-finding properties of neuronal growth cones. Filopodia can be induced in model cell lines by overexpresion of myosin‑10 and live-cell imaging shows that they undergo stochastic cycles of extension and retraction. Myosin‑10 transports cargo along the actin filament bundle towards the filopodial tip, forming a complex of proteins which are responsible for governing growth, retraction and adhesion. We have used a combination of optical and electron microscopy to study the dynamics and ultra-structure of myosin‑10 induced filopodia. Electron cryo-tomography of flash-frozen vitrified mammalian cells revealed the arrangement of actin filaments within the filopodia. Single molecule imaging by total internal reflection fluorescence microscopy allowed movement of myosin‑10 with the filopodia to be quantified and super-resolution localisation gave structural and mechanical information about the filopodium that could be correlated to our electron tomographic images.

Two Levels of Myosin-IIA Dynamics in Cells: Turnover of Filaments and Self-Organization of Filament Stacks

Dynamics and self-organization of myosin-II filaments in non-muscle cells are poorly understood. Here, using structured illumination microscopy (SIM), we visualized with high resolution bipolar myosin-II filaments in REF52 fibroblasts by marking myosin-II light chain (MLC), or myosin-IIA heavy chain (MHC-IIA). The filaments demonstrated a uniform length of 300 nm and formed characteristic periodic striations along actin filament bundles, such as ventral stress-fibers, or transverse arcs in spreading cells. Myosin IIA striations were alternating with domains of variable length enriched by actin cross-linking protein alpha-actinin. Free barbed ends of actin filaments (visualized by incorporation of labeled G-actin) were localized between myosin-II striations, while pointed ends of actin filaments (visualized by labeling tropomodulin-3) were enriched in the regions corresponding to myosin-II striations. The most striking feature of the myosin-IIA filament organization was in registry alignment of the myosin-IIA filaments with subsequent formation of several microns long filament “stacks” that apparently formed bridges between parallel stress-fibers or arcs. FRAP experiments were performed in SIM showed that the recovery time for both GFP-MLC and GFP-MHC-IIA in tens of seconds and did not depend on actin filament turnover. However, the process of self-organization of the individual filaments into stacks required much longer time (tens of minutes) and depended on both myosin II contractility and actin filament dynamics. Our observations reveal basic modes of such self-organization. A physical model considering myosin filaments as active force-dipoles embedded in a elastic medium shows that myosin filaments bound to neighboring actin bundles can interact mechanically through deformations of the intervening disordered cytoskeletal network leading to the observed stacking (registry) of these myosin filaments. Thus, myosin-IIA filament stacks are novel cytoskeletal elements that could maintain integrity and drive self-organization of the actin cytoskeleton.

Comparing the Motility of Myosin X with Parallel and Anti-Parallel Dimerization Domains

Myosin X is an actin-based molecular motor involved in filopodial formation, cell migration, adhesion and mitotic spindle orientation. It is upregulated in some cancers and may play a role in metastasis. Myosin X induces filopodial formation and localizes to filopodial tips by an unclear targeting mechanism. Studies using various truncated, forced-dimers have indicated that processivity of myosin X is greater on fascin-bundled actin filaments than single filaments, whereas other studies have not found different processivity on bundles relative to single filaments. Recently, the native dimerization domain has been proposed to form anti-parallel dimers, which might influence aspects of motility such as preference for actin bundles. To test this idea, we compared processive motility of forced parallel and anti-parallel myosin X constructs on single and bundled actin filaments. On single actin filaments parallel constructs are robustly processive, while processivity of the anti-parallel construct is much weaker. Run lengths for anti-parallel myosin X decreased as [MgATP] was raised from 0.5 to 2.5 micromolar. At 5 micromolar MgATP, run lengths are shorter than our detection limit, ∼100 nm. The velocity of the anti-parallel construct on single actin filaments is slower than that of the parallel construct, due to a substantial proportion of short (e.g. +18 nm) and backward steps along with +36 nm steps. On fascin-actin bundles, however, the processivity, velocity, step-size, and backward stepping frequency are similar to those characteristics of parallel myosin X. These results suggest that in cells, the anti-parallel coiled-coil domain may help myosin X select structures containing parallel actin bundles, such as filopodia. Supported by NIH grant R01GM086352 and T32HL07954 and TEXAS STAR Plus award.

Coordination of the axonal cytoskeleton during the emergence of axon collateral branches.

The formation of branches during development allows a single axon to make synaptic contacts with numerous target neurons, often in different parts of the nervous system, thereby allowing for the establishment of complex patterns of neuronal connectivity. Following injury, the formation of collateral branches contributes to the endogenous neuroplasticity which depending on the neuronal population can have beneficial (e.g., formation of compensatory relay circuitry) or adverse effects (e.g., development of autonomic dysreflexia). The formation of axon collateral branches from the axon shaft involves a complex series of signaling events and cytoskeletal reorganization. The initial step in the formation of a collateral branch is the emergence of axonal filopodia, supported by a bundle of actin filaments (Figure 1A). However, axonal filopodia are generated from precursor structures consisting of small meshworks of actin filaments, termed actin patches (Figure 1A inset). Actin patches are dynamic structures which form along the axon shaft, grow in size and subsequently are disassembled (for reviews on actin patches see Gallo, 2011, 2013). Although in live imaging studies using fluorescently labeled actin most axonal filopodia arise from a detectable actin patch, only a subset of patches gives rise to filopodia before dissipating. As the duration of filopodia is longer than that of patches, they are often not detected at the base of longer pre-existing filopodia, although they often re-emerge at the base of existing filopodia. In order for an axonal filopodium to mature into a collateral branch it must first become invaded by axonal microtubules (Figure 1A). Following the targeting and retention of axonal microtubules into the filopodium, the filopodium then begins to reorganize its actin filament cytoskeleton transitioning from a linear bundle of actin filaments to a distal accumulation of filaments giving rise to a nascent branch (Figure 1A), ultimately giving rise to a small growth cone structure at the tip of the branch. The transformation of a filopodium into a nascent branch is referred to as maturation of the branch which then grows in length (Figure 1A).

Impaired cytoskeletal arrangements and failure of ventral body wall closure in chick embryos treated with rock inhibitor (Y-27632).

Rho-associated kinase (ROCK) signaling regulates numerous fundamental developmental processes during embryogenesis, primarily by controlling actin-cytoskeleton assembly and cell contractility. ROCK knockout mice exhibit a ventral body wall defect (VBWD) phenotype due to disorganization of actin filaments at the umbilical ring. However, the exact molecular mechanisms leading to VBWD still remain unclear. Improper somitogenesis has been hypothesized to contribute to failure of VBW closure. We designed this study to investigate the hypothesis that administration of ROCK inhibitor (Y-27632) disrupts cytoskeletal arrangements in morphology during early chick embryogenesis, which may contribute to the development of VBWD. At 60h incubation, chick embryos were explanted into shell-less culture and treated with 50µL of vehicle for controls (n=33) or 50µL of 500µM of Y-27632 for the experimental group (Y-27, n=56). At 8h post-treatment, RT-PCR was performed to evaluate mRNA levels of N-cadherin, E-cadherin and connexin43. Immunofluorescence confocal microscopy was performed to analyze the expression and distribution of actin, vinculin and microtubules in the neural tube and somites. A further cohort of embryos was treated in ovo by dropping 50µL of vehicle or 50µL of different concentrations of Y-27632 onto the embryo and allowing development to 12 and 14days for further assessment. Gene expression levels of N-cadherin, E-cadherin and connexin43 were significantly decreased in treated embryos compared with controls (p<0.05). Thickened actin filament bundles were recorded in the neural tube of Y-27 embryos. In somites, cells were dissociated with reduced actin distribution in affected embryos. Clumping of vinculin expression was found in the neural tube and somites, whereas reduced expression of microtubules was observed in Y-27 embryos compared with controls. At 12 and 14days of development, affected embryos presented with an enlarged umbilical ring and herniation of abdominal contents through the defect. ROCK inhibition alters cytoskeletal arrangement during early chick embryogenesis, which may contribute to failure of anterior body wall closure causing VBWD at later stages of development.

Hypoxia alters the recruitment of tropomyosins into the actin stress fibres of neuroblastoma cells.

BackgroundNeuroblastoma is the most common extracranial solid tumor of childhood. The heterogeneous microenvironment of solid tumors contains hypoxic regions associated with poor prognosis and chemoresistance. Hypoxia implicates the actin cytoskeleton through its essential roles in motility, invasion and proliferation. However, hypoxia-induced changes in the actin cytoskeleton have only recently been observed in human cells. Tropomyosins are key regulators of the actin cytoskeleton and we hypothesized that tropomyosins may mediate hypoxic phenotypes.MethodsNeuroblastoma (SH-EP) cells were incubated ± hypoxia (1 % O2, 5 % CO2) for up to 144 h, before examining the cytoskeleton by confocal microscopy and Western blotting.ResultsHypoxic cells were characterized by a more organized actin cytoskeleton and a reduced ability to degrade gelatin substrates. Hypoxia significantly increased mean actin filament bundle width (72 h) and actin filament length (72–96 h). This correlated with increased hypoxic expression and filamentous organization of stabilizing tropomyosins Tm1 and Tm2. However, isoform specific changes in tropomyosin expression were more evident at 96 h.ConclusionsThis study demonstrates hypoxia-induced changes in the recruitment of high molecular weight tropomyosins into the actin stress fibres of a human cancer. While hypoxia induced clear changes in actin organization compared with parallel normoxic cultures of neuroblastoma, the precise role of tropomyosins in this hypoxic actin reorganization remains to be determined.Electronic supplementary materialThe online version of this article (doi:10.1186/s12885-015-1741-8) contains supplementary material, which is available to authorized users.

Assessment of a Protease Inhibitor Peptide for Anti-Ageing.

Ageing and skin exposure to UV radiation induces production and activation of matrix metalloproteinases (MMPs) and human neutrophil elastase (HNE). These enzymes are known to break down the extracellular matrix (ECM) which leads to wrinkle formation. Here, we demonstrated the potential of a solid-in-oil nanodispersion containing a competitive inhibitor peptide of HNE mixed with hyaluronic acid (HA), displaying 158 nm of mean diameter, to protect the skin against the ageing effects. Western blot analysis demonstrated that activation of MMP-1 in fibroblasts by HNE treatment is inhibited by the solid-in-oil nanodispersion containing the peptide and HA. The results clearly demonstrate that solid-in-oil nanodispersion containing the HNE inhibitor peptide is a promising strategy for anti-ageing effects. This effect can be seen particularly by ECM regulation by affecting fibroblasts. The formulation also enhances the formation of thicker bundles of actin filaments.

Metavinculin Tunes the Flexibility and the Architecture of Vinculin-Induced Bundles of Actin Filaments

Vinculin is an abundant protein found at cell–cell and cell–extracellular matrix junctions. In muscles, a longer splice isoform of vinculin, metavinculin, is also expressed. The metavinculin-specific insert is part of the C-terminal tail domain, the actin-binding site of both isoforms. Mutations in the metavinculin-specific insert are linked to heart disease such as dilated cardiomyopathies. Vinculin tail domain (VT) both binds and bundles actin filaments. Metavinculin tail domain (MVT) binds actin filaments in a similar orientation but does not bundle filaments. Recently, MVT was reported to sever actin filaments. In this work, we asked how MVT influences F-actin alone or in combination with VT. Cosedimentation and limited proteolysis experiments indicated a similar actin binding affinity and mode for both VT and MVT. In real-time total internal reflection fluorescence microscopy experiments, MVT's severing activity was negligible. Instead, we found that MVT binding caused a 2-fold reduction in F-actin's bending persistence length and increased susceptibility to breakage. Using mutagenesis and site-directed labeling with fluorescence probes, we determined that MVT alters actin interprotomer contacts and dynamics, which presumably reflect the observed changes in bending persistence length. Finally, we found that MVT decreases the density and thickness of actin filament bundles generated by VT. Altogether, our data suggest that MVT alters actin filament flexibility and tunes filament organization in the presence of VT. Both of these activities are potentially important for muscle cell function. Perhaps MVT allows the load of muscle contraction to act as a signal to reorganize actin filaments.

Reorganization of actin and microtubule systems in human vein endothelial cells during intercellular contact formation

Endothelial cells tightly joined to each other line the interior surface of all vessels in living organisms and ensure the regulation of vascular permeability and exchange between the blood circulating in vessels and tissue fluids of those organs in which these vessels are located. In vitro, the endothelial cell monolayer retains the basic barrier function of the native endothelium. We use endothelial cells growing in vitro as a model system to study the interaction between cytoskeleton and adhesion structures in endotheliocytes. In the present paper, cultured human vein endothelial cells were used to quantify cytoskeleton alterations from cell spreading and formation of the first cell–cell contacts to confluent monolayer formation. The actin filament system in vein endothelial cells consists of a cortical network and stress fibers arranged parallel to the substrate. This cells express two isoforms of nonmuscle actin: β- and γ-cytoplasmic actins. Antibodies to β-actin stain actin filament bundles, whereas antibodies to γ-actin visualize cortical and lamellar networks. To evaluate the state of actin structures we analyzed their fluorescence intensity near the free edge and zone of cell–cell contacts. Fluorescence intensity of β-actin structures was higher in the areas of cell–cell contacts. Fluorescence of γ-actin was the highest at the leading edges of lamellae and the weakest in the stable edges of the cells with formed cell–cell contacts. The endothelial monolayer formation was accompanied by microtubule system alteration: the number of microtubules increased at the cell edge, and what was more, the quantity of microtubules in the area of the already-formed cell–cell contact always being higher than in the leading edge region.

A microfluidic cell culture system for monitoring of sequential changes in endothelial cells after heat stress

Endothelial damage induced by a highly elevated body temperature is crucial in some diseases including viral hemorrhagic fevers. Here, we report the heat-induced sequential changes of endothelial cells under shear stress, which were determined with a microfluidic culture system. Although live cell imaging showed only minor changes in the appearance of heat-treated cells, Hsp70 mRNA expression analysis demonstrated that the endothelial cells in channels of the system responded well to heat treatment. F-actin staining also revealed clear changes in the bundles of actin filaments after heat treatment. Well-organized bundles of actin filaments in control cells disappeared in heat-treated cells cultured in the channel. Furthermore, the system enabled detection of sequential changes in plasminogen activator inhibitor-1 (PAI-1) secretion from endothelial cells. PAI-1 concentration in the effluent solution was significantly elevated for the first 15min after initiation of heat treatment, and then decreased subsequently. This study provides fundamental information on heat-induced endothelial changes under shear stress and introduces a potent tool for analyzing endothelial secretions.

Tau co-organizes dynamic microtubule and actin networks.

The crosstalk between microtubules and actin is essential for cellular functions. However, mechanisms underlying the microtubule-actin organization by cross-linkers remain largely unexplored. Here, we report that tau, a neuronal microtubule-associated protein, binds to microtubules and actin simultaneously, promoting in vitro co-organization and coupled growth of both networks. By developing an original assay to visualize concomitant microtubule and actin assembly, we show that tau can induce guided polymerization of actin filaments along microtubule tracks and growth of single microtubules along actin filament bundles. Importantly, tau mediates microtubule-actin co-alignment without changing polymer growth properties. Mutagenesis studies further reveal that at least two of the four tau repeated motifs, primarily identified as tubulin-binding sites, are required to connect microtubules and actin. Tau thus represents a molecular linker between microtubule and actin networks, enabling a coordination of the two cytoskeletons that might be essential in various neuronal contexts.

Morphological and immunohistochemical analyses of soluble proteins in mucous membranes of living mouse intestines by cryotechniques

We have performed immunohistochemical or ultrastructural analyses of living mouse small intestines using Epon blocks prepared by 'in vivo cryotechnique' (IVCT). By electron microscopy, intracellular ultrastructures of epithelial cells were well preserved in tissue areas 5-10 μm away from cryogen-contact surface tissues. Their microvilli contained dynamically waving actin filaments, and highly electron-dense organelles, such as mitochondria, were seen under the widely organized terminal web. By quick-freezing of fresh resected tissues (FT-QF), many erythrocytes were congested within blood vessels due to loss of blood pressure. By immersion-fixation (IM-DH) and perfusion-fixation (PF-DH), small vacuoles were often seen in the cytoplasm of epithelial cells, and their intercellular spaces were also dilated. Moreover, actin filament bundles were irregular in cross sections of microvilli, compared with those with IVCT. Epon-embedded thick sections were treated with sodium ethoxide, followed by antigen retrieval and immunostained for immunoglobulin A (IgA), Ig kappa light chain (Igκ), J-chain and albumin. By cryotechniques, IgA immunoreactivity was detected as tiny dot-like patterns in cytoplasm of some epithelial cells. Both J-chain and Igκ immunoreactivities were detected in the same local areas as those of IgA. By FT-QF, however, the IgA immunoreactivity was more weakly detected, compared with that with IVCT. In thick sections prepared by IM-DH and PF-DH, it was rarely observed in both plasma and epithelial cells. Another albumin was diffusely immunolocalized in extracellular matrices of mucous membranes and also within blood vessels. Thus, IVCT was useful for preservation of soluble proteins and ultrastructural analyses of dynamically changing epithelial cells of living mouse small intestines.

A Dynamin-Actin Interaction Is Required for Vesicle Scission during Endocytosis in Yeast

SummaryActin is critical for endocytosis in yeast cells, and also in mammalian cells under tension. However, questions remain as to how force generated through actin polymerization is transmitted to the plasma membrane to drive invagination and scission. Here, we reveal that the yeast dynamin Vps1 binds and bundles filamentous actin. Mutational analysis of Vps1 in a helix of the stalk domain identifies a mutant RR457-458EE that binds actin more weakly. In vivo analysis of Vps1 function demonstrates that the mutation disrupts endocytosis but not other functions of Vps1 such as vacuolar trafficking or peroxisome fission. The mutant Vps1 is stably expressed in cells and co-localizes with the endocytic reporters Abp1 and the amphiphysin Rvs167. Detailed analysis of individual endocytic patch behavior indicates that the mutation causes aberrant movements in later stages of endocytosis, consistent with a scission defect. Ultrastructural analysis of yeast cells using electron microscopy reveals a significant increase in invagination depth, further supporting a role for the Vps1-actin interaction during scission. In vitro analysis of the mutant protein demonstrates that—like wild-type Vps1—it is able to form oligomeric rings, but, critically, it has lost its ability to bundle actin filaments into higher-order structures. A model is proposed in which actin filaments bind Vps1 during invagination, and this interaction is important to transduce the force of actin polymerization to the membrane to drive successful scission.

SCAB3 Is Required for Reorganization of Actin Filaments during Light Quality Changes

The STOMATAL CLOSURE-RELATED ACTIN BINDING PROTEIN (SCAB) family is plant-specific, and its members all contain a novel actin binding domain. Here, we report that SCAB3, a homolog of SCAB1, binds, stabilizes and bundles actin filaments. The SCAB3 promoter contains a cis-element which could be bound by the FHY3/FAR1 transcription factors. Consistently, the expression of SCAB3 is induced when plants were transferred from white light to far red light (T-Far Red) conditions. The scab3 mutants show defects in the control of hypocotyl elongation under T-Far Red condition, which may result from an impaired reorganization of actin filaments. Together, our results suggest that SCAB3 plays an important role in plant growth under changes of light conditions possibly by regulating actin filament dynamics.

Fascin - An actin binding and bundling protein in the testis and its role in ectoplasmic specialization dynamics.

In the mammalian testis such as in rats, a unique actin-rich cell-cell adherens junction (AJ) known as ectoplasmic specialization (ES) is found in the seminiferous epithelium. ES is conspicuously found between Sertoli cells near the basement membrane known as the basal ES, which together with tight junction (TJ), gap junction, and desmosome constitute the blood-testis barrier (BTB). The BTB, in turn, anatomically divides the seminiferous epithelium into the basal and the adluminal (apical) compartment. On the other hand, ES is also found at the Sertoli-spermatid interface known as apical ES which is the only anchoring device for developing step 8-19 spermatids during spermiogenesis. One of the most typical features of the ES is the array of actin microfilament bundles that lie perpendicular to the Sertoli cell plasma membrane and are sandwiched in-between the cisternae of endoplasmic reticulum and the Sertoli cell plasma membrane. While these actin filament bundles confer the adhesive strength of Sertoli cells at the BTB and also spermatids in the adluminal compartment, they must be rapidly re-organized from their bundled to unbundled/branched configuration and vice versa to provide plasticity to the ES so that preleptotene spermatocytes and spermatids can be transported across the immunological barrier and the adluminal compartment, respectively, during the epithelial cycle of spermatogenesis. Fascin is a family of actin microfilament cross-linking and bundling proteins that is known to confer bundling of parallel actin microfilaments in mammalian cells. A recent report has illustrated the significance of a fascin protein called fascin 1 in actin microfilaments at the ES, pertinent to its role in spermatogenesis (Gungor-Ordueri etal. Am J Physiol Endocrinol Metab 307, E738-753, 2004 (DOI:10.1152/ajpendo.00113.2014). In this Commentary, we critically evaluate these findings in light of the role of fascin in other mammalian cells, providing some insightful information for future investigations.