Number Of Filaments Research Articles

Cofilin drives rapid turnover and fluidization of entangled F-actin

The shape of most animal cells is controlled by the actin cortex, a thin network of dynamic actin filaments (F-actin) situated just beneath the plasma membrane. The cortex is held far from equilibrium by both active stresses and polymer turnover: Molecular motors drive deformations required for cell morphogenesis, while actin-filament disassembly dynamics relax stress and facilitate cortical remodeling. While many aspects of actin-cortex mechanics are well characterized, a mechanistic understanding of how nonequilibrium actin turnover contributes to stress relaxation is still lacking. To address this, we developed a reconstituted in vitro system of entangled F-actin, wherein the steady-state length and turnover rate of F-actin are controlled by the actin regulatory proteins cofilin, profilin, and formin, which sever, recycle, and assemble filaments, respectively. Cofilin-mediated severing accelerates the turnover and spatial reorganization of F-actin, without significant changes to filament length. We demonstrate that cofilin-mediated severing is a single-timescale mode of stress relaxation that tunes the low-frequency viscosity over two orders of magnitude. These findings serve as the foundation for understanding the mechanics of more physiological F-actin networks with turnover and inform an updated microscopic model of single-filament turnover. They also demonstrate that polymer activity, in the form of ATP hydrolysis on F-actin coupled to nucleotide-dependent cofilin binding, is sufficient to generate a form of active matter wherein asymmetric filament disassembly preserves filament number despite sustained severing.

Anomalous intracellular transport phases depend on cytoskeletal network features.

Intracellular transport in eukaryotic cells consists of phases of passive, diffusion-based transport and active, motor-driven transport along filaments that make up the cell's cytoskeleton. The interplay between superdiffusive transport along cytoskeletal filaments and the anomalous nature of subdiffusion in the bulk can lead to novel effects in transport behavior at the cellular scale. Here we develop a computational model of the process with cargo being ballistically transported along explicitly modeled cytoskeletal filament networks and passively transported in the cytoplasm by a subdiffusive continuous-time random walk (CTRW). We show that, over a physiologically relevant range of filament lengths and numbers, the network introduces a filament-length sensitive superdiffusive phase at early times which crosses over to a phase where the CTRW is dominant and produces subdiffusion at late times. We apply our approach to the problem of insulin secretion from cells and show that the superdiffusive phase introduced by the filament network manifests as a peak in the secretion at early times followed by an extended sustained release phase that is dominated by the CTRW process at late times. Our results are consistent with in vivo observations of insulin transport in healthy cells and shed light on the potential for the cell to tune functionally important transport phases by altering its cytoskeletal network.

Properties of filaments in solar cycle 20-23 from McIntosh Archive

A filament is a cool, dense structure suspended in the solar corona. The eruption of a filament is often associated with a coronal mass ejection (CME), which has an adverse effect on space weather. Hence, research on filaments has attracted much attention in the recent past. The tilt angle of active region (AR) magnetic bipoles is a crucial parameter in the context of the solar dynamo, which governs the conversion efficiency of the toroidal magnetic field to poloidal magnetic field. Filaments always form over polarity inversion lines (PILs), so the study of tilt angles for these filaments can provide valuable information about generation of a magnetic field in the Sun. We investigate the tilt angles of filaments and other properties using McIntosh Archive data. We fit a straight line to each filament to estimate its tilt angle. We examine the variation of mean tilt angle with time. The latitude distribution of positive tilt angle filaments and negative tilt angle filaments reveals that there is a dominance of positive tilt angle filaments in the southern hemisphere and negative tilt angle filaments dominate in the northern hemisphere. We study the variation of the mean tilt angle for low and high latitudes separately. Investigations of temporal variation with filament number indicate that total filament number and low latitude filament number vary cyclically, in phase with the solar cycle. There are fewer filaments at high latitudes and they also show a cyclic pattern in temporal variation. We also study the north-south asymmetry of filaments with different latitude criteria.

Characterization of fabric properties knitted from multifilament polyester textured yarns with various number of filaments

The purpose of this study is to investigate the physical and mechanical properties of fabrics knitted from textured polyester multifilament yarns with various numbers of filaments in the yarn cross-section. In this regard, some of the characteristics of yarns and fabrics, knitted from the mentioned yarns, were measured and analysed. Analysis of results showed that increasing the number of filaments in the yarn cross-section leads to a decrease in yarn’s diameter and abrasion resistance. Moreover, examining the properties of the weft-knitted fabrics revealed that increasing the number of filaments causes a rising trend in the compressibility and bursting strength of fabric. On the contrary, the thickness, air permeability and abrasion resistance decreases due to a growth in the number of filaments. A decline in the strength and elongation of yarns and fabrics produced from them is also another result of increasing the number of filaments in the yarn cross section.

Effect of Heart Structure on Ventricular Fibrillation in the Rabbit: A Simulation Study.

Ventricular fibrillation (VF) is a lethal condition that affects millions worldwide. The mechanism underlying VF is unstable reentrant electrical waves rotating around lines called filaments. These complex spatio-temporal patterns can be studied using both experimental and numerical methods. Computer simulations provide unique insights including high resolution dynamics throughout the heart and systematic control of quantities such as fiber orientation and cellular kinetics that are not feasible experimentally. Here we study filament dynamics using two bi-ventricular 3-D high-resolution rabbit heart geometries, one with detailed fine structure and another without fine structure. We studied filament dynamics using anisotropic and isotropic conductivities, and with four cellular action potential models with different recovery kinetics. Spiral wave dynamics observed in isotropic two-dimensional sheets were not predictive of the behavior in the whole heart. In 2-D the four cell models exhibited stable reentry, meandering spiral waves, and spiral-wave breakup. In the whole heart with fine structure, all simulation results exhibited complex dynamics reminiscent of fibrillation observed experimentally. In the whole heart without fine structure, anisotropy acted to destabilize filament dynamics although the number of filaments was reduced compared to the heart with structure. In addition, in isotropic hearts without structure the two cell models that exhibited meandering spiral waves in 2-D, stabilized into figure-of-eight surface patterns. We also studied the sensitivity of filament dynamics to computer system configuration and initial conditions. After large simulation times, different macroscopic results sometimes occurred across different system configurations, likely due to a lack of bitwise reproducibility. The study conclusions were insensitive to initial condition perturbations, however, the exact number of filaments over time and their trends were altered by these changes. In summary, we present the following new results. First, we provide a new cell model that resembles the surface patterns of VF in the rabbit heart both qualitatively and quantitatively. Second, filament dynamics in the whole heart cannot be predicted from spiral wave dynamics in 2-D and we identified anisotropy as one destabilizing factor. Third, the exact dynamics of filaments are sensitive to a variety of factors, so we suggest caution in their interpretation and their quantitative analyses.

Leaves and stems of Capparis erythrocarpos, more sustainable than roots, show antiarthritic effects

Ethnopharmacological relevanceCapparis erythrocarpos is a medicinal plant used widely in many parts of Africa for the management of pain and inflammatory conditions such as rheumatoid arthritis. Its wide range of use, popularity and high value, make C. erythrocarpos containing products a target for economically driven adulteration. This is made worse, by the use of roots, which are unsustainable plant parts. In addition, the mechanism of anti-inflammation is not clearly understood. Aim of the studyTherefore, this study comparatively evaluated the anti-arthritic and analgesic effects of the leaves, stems and roots of C. erythrocarpos, while elucidating the mechanism of anti-inflammation. Materials and methodsUsing the complete Freund's adjuvant arthritis model, the antiathritic effects were evaluated. The analgesic effects were determined by measuring responses to Von Frey filament number 9. Effects of C. erythrocarpos extracts on the levels of Interleukin-6 (IL-6) and Tumor Necrosis Factor-alpha (TNF-α) were determined using ELISA. Haematological and serum biochemical assays were also carried out. ResultsThe leaf, stem and root extracts significantly reduced paw volumes with ED50 values (mg/kg) of 182.5, 181.5 and 36.4 respectively. The leaf extract at a dose of 100 mg/kg showed substantial analgesic activity with a decrease in the percentage response to Von Frey filament 9. However, there was no significant difference in activities of the leaf, stem and root extracts. Results from ELISA assays show that lower doses of the stem extracts reduce levels of IL-6. Lower doses of all extracts also reduce TNF-α levels. Haematological analysis showed extracts reversed elevated WBC and platelet levels. Toxicity evaluation with kidney and liver function tests indicated no significant differences between the treatment and control groups. ConclusionsThe leaves and stems, just as the roots, have antiarthritic and analgesic effects and can be used as more sustainable alternatives to the roots. This will support the continuous growth of the industry that has developed around C. erythrocarpos.

Oral guaifenesin for treatment of filamentary keratitis: A pilot study

PurposePilot study to evaluate the safety and efficacy of oral guaifenesin in reducing the signs and symptoms of filamentary keratitis. MethodsProspective, uncontrolled open-label pilot study. Twelve patients with non-Sjögren dry eye disease (DED) and secondary filamentary keratitis received treatment with oral guaifenesin 600 mg twice a day (total dose of 1.2 g/day) for 4 weeks. Adverse events, change in the number of corneal filaments, corneal fluorescein staining (CFS; NEI grading system), and symptoms (Ocular Surface Disease Index) were assessed. ResultsBefore starting oral guaifenesin, all patients were on topical medical therapy for their condition. At baseline, the mean number of filaments was 5.8 ± 2.9, CFS score 7.3 ± 3.2, and OSDI score 55.6 ± 25. After 4 weeks of treatment, the number of filaments was 2.1 ± 2.2 (p = 0.04 vs. baseline), CFS score 6.5 ± 3.1 (p = 0.5), and OSDI score 46.1 ± 30.9 (p = 0.2). One patient discontinued the medication due to gastrointestinal side effects. ConclusionsOral guaifenesin was safe and generally well tolerated, and demonstrated modest efficacy in reducing the severity of filamentary keratitis. These results should be considered preliminary; however, placebo-controlled investigations would be justified to evaluate the therapeutic efficacy of oral guaifenesin as a mucolytic in treatment of filamentary keratitis.

Creating of luminescent defects in crystalline media by a scanning laser beam

Line-by-line two-dimensional step small-scale scanning irradiation of a transparent cubic LiF crystal was carried out by intense linearly polarized femtosecond laser radiation in the multiple filamentation mode. As a result, isolated longitudinal extended tracks consisting of induced color centers were formed in the medium. It was found that no transverse periodicity associated with the scanning step is observed in the arrangement of tracks formed by laser filaments. This is because inhomogeneities that stimulate filamentation are not contained in the laser beam itself but are formed randomly when the medium interacts with the first laser pulses and are supported and amplified by subsequent pulses. The efficiency of color center formation in crystals at normal laser beam incidence on the cube face depends periodically on the azimuth angle θ between the electric vector and the cube edge on the face, with the period of π/2. It was found that azimuthal dependences for defect formation (maximum at θ = π/4) and for carrier photogeneration (maximum at θ = 0) are in the antiphase. Calculations showed that the processes of self-focusing and filamentation controlled by the components of the third-order nonlinear susceptibility tensor are most effective at the orientation where θ = π/4. The experiment showed that at such an orientation, the critical power and the length of self-focusing decrease, and therefore, the density of the number of filaments in the beam section increases and, as a result, the average concentration of the color centers created by laser filaments increases.

Lift-Factor Analysis of Multifilamentary Coated Conductor Produced Using Two Level Undercut-Profile Substrates

Here, we present a detailed characterization of the critical current density of YBa ${}_2$ Cu ${}_3$ O ${}_{7-x}$ two level undercut-profile substrate (2LUPS)-based samples made at the Technical University of Denmark using Hastelloy substrate. The CC architecture was produced at Bruker HTS by means of alternating beam assisted deposition and pulsed laser deposition. The magnetization of different samples having all the same size, but with various numbers of filaments, including a non-filamentary reference sample have been measured in applied magnetic fields up to 16 T and at temperatures ranging from $T = 2$ K to 77 K. A simple multiple rooftop model is proposed to estimate the critical current density of the filaments and it has been observed that filaments of 0.25 mm and 0.5 mm width are providing similar critical current densities in the order of $J_c \approx 2.3\,\text{MA}/\text{cm}^2$ at $T = 4.2$ K and $B = 10$ T as well as the reference samples without filaments. This result indicates that the filaments have been fabricated with no degradation of the superconductor and that the filaments are decoupled with non-superconducting material. Finally, the lift factor of the critical current $I_c$ with respect to $T = 77$ K and self field of the 2LUPS samples is estimated. This is reaching 10 at $T = 4.2$ K and $B = 10$ T.

Characteristics and mechanisms of transition from filament to homogeneous glow in atmospheric helium dielectric barrier discharges under variation of the applied voltage amplitude

In this paper, a 2D fluid model is employed to investigate the radial evolution of the discharge structures in a helium dielectric barrier discharge with 100 ppm nitrogen impurity. By elevating the applied voltage amplitude (Vam), the discharge exhibits some distinctive radial evolution features that comprise three aspects: (1) the lateral migration of the peripheral filament, i.e. outward filament migration nearby the electrode edge; (2) the reduced intervals between two successive filaments; (3) the growth in the number of filaments. It is revealed that the radial position of the peripheral filament is basically consistent with that of the initial local intense discharge, whose location is closely related to the surface charge distribution during the initial breakdown. An increase in Vam reduces the duration of the current pulse, and hence, the displacement of space charges is restrained. When more charges are restrained in the gap rather than being attached to the dielectric surfaces, the surface charge distribution becomes more uniform, which contributes to the lateral migration of the peripheral filament. Meanwhile, the lateral uniformity of Penning ionization rate is improved with Vam increasing, and the seed electron level in the intervals becomes comparable to that in the filamentary channels, leading to a more uniform radial seed electron profile that attenuates the electric field distortion. As a result, the intervals between two adjacent filaments are shortened. With the lateral migration and reduced intervals as Vam increases, we observe the growth in the number of filaments, and the improvement of the radial discharge uniformity.

TRPV2 suppresses Rac1 and RhoA activation and invasion in rheumatoid arthritis fibroblast-like synoviocytes

The TRPV2 cation channel has been recently implicated in the regulation of arthritis severity, joint damage, and in the invasive behavior of the fibroblast-like synoviocyte (FLS). However, its mechanism of action was unknown. In this study we characterize the cell signaling events mediating the TRPV2 suppressive activity in FLS invasiveness. Studies with FLS cell lines derived from patients with RA revealed that TRPV2-specific stimulation significantly reduced FLS adhesion to different extracellular matrices that shared binding to αν, β1 and β3 integrins. Localization of these integrins to the plasma membrane and numbers of thick and organized actin filaments were diminished by TRPV2 specific stimulation, and cells developed a round and non-polarized morphology. TRPV2 stimulation significantly reduced levels of activated RhoA, Rac1 and cofilin. RhoA activators were able to overcome the TRPV2-induced suppression on both RhoA activation and invasion. These new discoveries suggest that TRPV2 regulates key intracellular processes implicated in cell invasion in arthritis and other processes such as cancer, and has the potential to become a useful target for drug development.

Single-particle measurements reveal damage to filamentous influenza virions during laboratory handling

Most laboratory strains of influenza virus produce near-spherical virions, but clinical isolates also produce extended filaments whose biophysical properties are understudied. Most functional studies of filamentous influenza viruses do not include data on the concentration or lengths of the virions, making it hard to interpret their sometimes contradictory results. Furthermore, anecdotal reports suggest that filaments are damaged during routine laboratory handling. Therefore, to understand filament function we require a tool to assess the number and dimensions of filaments in a sample and an assessment of how filaments respond to standard handling procedures. We initially sought to analyse filament populations using negative stain particle counting, but found that this was low-throughput and could not detect particles longer than 10 µm. Instead, we used confocal microscopy with semi-automated image analysis. This allowed a high-throughput, quantitative analysis of length distributions in filament populations. Using this, we assessed the effects of pipetting, vortexing, sonicating, clarification and freezing on filaments. Most procedures did not appreciably alter filament dimensions. Pipetting and vortexing both slightly reduced filament numbers, but their effects were only appreciable after extended treatment. In contrast, freezing substantially reduced the number and median length of filaments, as well as creating ‘kinks’ in filaments which suggest damage to the capsid. We conclude that confocal microscopy can provide the basic measurements needed to interpret functional studies of filamentous strains. Using this approach, we found that freezing filaments causes previously unappreciated damage, which should be considered when planning further research.

Buckling of filamentous actin bundles in filopodial protrusions

The mechanical behavior of filamentous actin bundles plays an essential role in filopodial protrusions at the leading edge of crawling cells. These bundles consist of parallel actin filaments that are hexagonally packed and interconnected via cross-linking proteins including α-actinin, filamin, and fascin. When pushing against the plasma membrane and/or external barriers, actin bundles in filopodial protrusions inevitably encounter a compressive load. The bending stiffness and buckling stability of actin bundles are therefore important in determining the filopodial architecture and subsequent cell morphology. In this work, we employ a coarse-grained molecular dynamics model to investigate the buckling behavior of cross-linked actin bundles under compression, explicitly accounting for the properties of the constituent filaments and the mechanical description of the cross-linkers. The bending stiffness of actin bundles exhibits a generic size effect depending on the number of filaments in the bundle, explicitly depending on the degree of interfilament coupling. The distinct buckling modes are analyzed for bundles with different coupling states and crosslinker strengths. This study could clarify the stability and buckling mechanisms of parallel packed actin bundles and the structure–function relations of mechanical components in filopodial protrusions.

Generation of multiple solitons using competing nonlocal nonlinearities.

We discuss the dynamics of fundamental Gaussian beams launched in saturable and nonlocal nonlinear media. Solely in the presence of a self-focusing saturable nonlinearity, the breathing solitons undergo strong deformation. The addition of a defocusing nonlinearity leads to the generation of couples of solitons. Experimentally, we demonstrate in nematic liquid crystals the formation of multiple spatial solitons starting from a bell-shaped input, with both direction and the number of filaments depending on the input power, confirming the theoretical predictions.

First passage of molecular motors on networks of cytoskeletal filaments.

Molecular motors facilitate intracellular transport through a combination of passive motion in the cytoplasm and active transport along cytoskeletal filaments. Although the motion of motors on individual filaments is often well characterized, it remains a challenge to understand their transport on networks of filaments. Here we use computer simulations of a stochastic jump process to determine first-passage times (FPTs) of a molecular motor traversing an interval containing randomly distributed filaments of fixed length. We characterize the mean first-passage time (MFPT) as a function of the number and length of filaments. Intervals containing moderate numbers of long filaments lead to the largest MFPTs with the largest relative standard deviation; in this regime, some filament configurations lead to anomalously large FPTs due to spatial regions where motors become trapped for long times. For specific filament configurations, we systematically reverse the directionality of single filaments and determine the MFPT of the perturbed configuration. Surprisingly, altering a single filament can dramatically impact the MFPT, and filaments leading to the largest changes are commonly found in different regions than the traps. We conclude by analyzing the mean square displacement of motors in unconfined systems with alargedensity of filaments and show that they behave diffusively at times substantially less than the MFPT to traverse the interval. However, the effective diffusion coefficient underestimates the MFPT across the bounded interval, emphasizing the importance of local configurations of filaments on first-passage properties.

Single Cell and Single-Molecule Assays Reveal Bacteria Regulate their Rate of Growth by Actively Reading Out the Level of Cell Wall Precursors

How cells regulate their growth rate in response to nutrient conditions remains an outstanding question in biology. Bacteria are surrounded by a peptidoglycan cell wall that holds their shape. This structure is built by MreB filaments that regulate the enzymes that synthesize the cell wall. Collectively termed the Rod Complex, these machines moving around the cell as they insert new material. To understand how cells grow, we used Structured Illumination Microscopy (SIM) and single-molecule tracking to quantitate the number of Rod complexes in Bacillus subtilis growing in different media. First, we found that cells regulate the filament density but not the speed in response to nutrient availability. The directional motion of enzymes is regulated by MreB filaments and their motions transitioning into a diffusive state when MreB filaments are disrupted. Second, we demonstrated that the number of MreB filaments is regulated by the cell wall precursor lipid II, as increased expression of key Mur enzymes that build lipid II increases the number of MreB filaments and the rate of growth. Furthermore, we showed that lipid II is sensed by the serine/theonine kinase PrkC, which phosphorylates RodZ, a MreB filament nucleator. Surprisingly we found that strains containing a phosphomimetic RodZ mutation or overexpression of PrkC causes cells to grow faster than they would in carbon limited media. Together these demonstrate that bacteria regulate their rate of growth by measuring the flux of peptidoglycan precursors through cell wall synthesis.

Study of the magnetization loss of CORC® cables using a 3D T-A formulation

A Conductor on Round Core (CORC®) cable wound with a high temperature superconductor is an important cable concept for high current density applications. The design of a CORC cable makes understanding its electromagnetic performance—for example its AC losses—challenging. This paper presents a thorough study of CORC cables by combining experimental and numerical methods. In particular, it focuses on understanding how the cable structure influences the magnetization losses and on how these can be reduced. A novelty of this paper lies in the use of a new T-A formulation, which, for the first time, is employed for three-dimensional modelling of a CORC cable with real geometry. The use of the new T-A formulation in finite element software enables the study of how the winding direction and multiple-layer structure affect the magnetization losses of CORC cables. Moreover, influence of striation in CORC cables is studied as an effective way to reduce their losses. A CORC cable with striated tapes shows a significant magnetization loss reduction at high magnetic fields, in comparison to its counterpart without striated tapes. At low magnetic fields, tape striation leads to an increase in loss when the number of filaments is low, then the loss drops with a further increase in the number of filaments, but this loss reduction is much weaker than that at high fields. This paper provides an efficient tool for investigating the electromagnetic behaviour of CORC cables, which can provide valuable guidance in designing CORC cables with minimized losses for high energy physics and energy conversion applications.

Ena/VASP processive elongation is modulated by avidity on actin filaments bundled by the filopodia cross-linker fascin.

Ena/VASP tetramers are processive actin elongation factors that localize to diverse F-actin networks composed of filaments bundled by different cross-linking proteins, such as filopodia (fascin), lamellipodia (fimbrin), and stress fibers (α-actinin). Previously, we found that Ena takes approximately threefold longer processive runs on trailing barbed ends of fascin-bundled F-actin. Here, we used single-molecule TIRFM (total internal reflection fluorescence microscopy) and developed a kinetic model to further dissect Ena/VASP’s processive mechanism on bundled filaments. We discovered that Ena’s enhanced processivity on trailing barbed ends is specific to fascin bundles, with no enhancement on fimbrin or α-actinin bundles. Notably, Ena/VASP’s processive run length increases with the number of both fascin-bundled filaments and Ena “arms,” revealing avidity facilitates enhanced processivity. Consistently, Ena tetramers form more filopodia than mutant dimer and trimers in Drosophila culture cells. Moreover, enhanced processivity on trailing barbed ends of fascin-bundled filaments is an evolutionarily conserved property of Ena/VASP homologues, including human VASP and Caenorhabditis elegans UNC-34. These results demonstrate that Ena tetramers are tailored for enhanced processivity on fascin bundles and that avidity of multiple arms associating with multiple filaments is critical for this process. Furthermore, we discovered a novel regulatory process whereby bundle size and bundling protein specificity control activities of a processive assembly factor.

Influence of different scattering medium on propagation characteristics to femtosecond laser pulses

During recent years, the filamentation of femtosecond laser in the atmosphere has contributed considerable interest to researchers. However, the actual atmosphere can result in different scattering medium, which are adverse to the application of filamentation in the atmosphere. In order to study the propagation of femtosecond laser in real scattering medium, the propagation of 800 nm femtosecond laser in ice cloud, water cloud, fog, aerosol and rainfall is simulated numerically. Combined with the theory of stratified medium model and Mie scattering theory, we constructed a scattering model with a changeable size distribution function in the nonlinear laser model. The results indicated that the different size distribution and phase state of particles have different influence on the propagation properties of the filaments. As the rainfall was dominated by large raindrops, the scattering on filament was the strongest, resulting in the lowest peak intensity and energy. In the case, the distribution of filament energy was extremely inhomogeneous, causing the shortest length of filament and generation of multi-filament. In the image of fluence distribution, a diffraction ring can be observed clearly in the rainfall but was blurred in other medium. The propagation properties of filaments in water cloud and fog were similar because of the same size distribution. However, due to the size of particle in fog was smaller than that in water cloud, the filaments had more higher energy and more concentrated distribution in fog. In addition, the scattering of ice particles was stronger than that of liquid droplets, so the energy of filament in ice cloud was lower than that in water cloud, resulting a reducing of the length and number of filaments in ice cloud. The size of aerosols was the smallest, which had the weakest influence on the filament. Accordingly, in the early of propagation, there had little perturbance on the filament and the beam was transmitting with a stable single filament, and results in the highest peak intensity and energy. With the propagation increasing, the accumulation of scattering attenuation produced the perturbation on filament at a position after the onset of filamentation.

YFR016c/Aip5 is part of an actin nucleation complex in yeast.

ABSTRACTThe polarisome comprises a network of proteins that organizes polar growth in yeast and filamentous fungi. The yeast formin Bni1 and the actin nucleation-promoting factor Bud6 are subunits of the polarisome that together catalyze the formation of actin cables below the tip of yeast cells. We identified YFR016c (Aip5) as an interaction partner of Bud6 and the polarisome scaffold Spa2. Yeast cells lacking Aip5 display a reduced number of actin cables. Aip5 binds with its N-terminal region to Spa2 and with its C-terminal region to Bud6. Both interactions collaborate to localize Aip5 at bud tip and neck, and are required to stimulate the formation of actin cables. Our experiments characterize Aip5 as a novel subunit of a complex that regulates the number of actin filaments at sites of polar growth.