Analysis of unregulated formin activity reveals how yeast can balance F-actin assembly between different microfilament-based organizations

Dynamic remodeling of actin networks by cyclase-associated protein and CAP-Abp1 complexes

How actin filaments are spatially organized and remodeled into diverse higher-order networks in vivo is still not well understood. Here, we report an unexpected F-actin "coalescence" activity driven by cyclase-associated protein (CAP) and enhanced by its interactions with actin-binding protein 1 (Abp1). We directly observe S. cerevisiae CAP and Abp1 rapidly transforming branched or linear actin networks by bundling and sliding filaments past each other, maximizing filament overlap, and promoting compaction into bundles. This activity does not require ATP and is conserved, as similar behaviors are observed for the mammalian homologs of CAP and Abp1. Coalescence depends on the CAP oligomerization domain but not the helical folded domain (HFD) that mediates its functions in F-actin severing and depolymerization. Coalescence by CAP-Abp1 further depends on interactions between CAP and Abp1 and interactions between Abp1 and F-actin. Our results are consistent with a mechanism in which the formation of energetically favorable sliding CAP and CAP-Abp1 crosslinks drives F-actin bundle compaction. Roles for CAP and CAP-Abp1 in actin remodeling in vivo are supported by strong phenotypes arising from deletion of the CAP oligomerization domain and by genetic interactions between sac6Δ and an srv2-301 mutant that does not bind Abp1. Together, these observations identify a new actin filament remodeling function for CAP, which is further enhanced by its direct interactions with Abp1.

F-Actin Cytoskeleton Network Self-Organization Through Competition and Cooperation

Many fundamental cellular processes such as division, polarization, endocytosis, and motility require the assembly, maintenance, and disassembly of filamentous actin (F-actin) networks at specific locations and times within the cell. The particular function of each network is governed by F-actin organization, size, and density as well as by its dynamics. The distinct characteristics of different F-actin networks are determined through the coordinated actions of specific sets of actin-binding proteins (ABPs). Furthermore, a cell typically assembles and uses multiple F-actin networks simultaneously within a common cytoplasm, so these networks must self-organize from a common pool of shared globular actin (G-actin) monomers and overlapping sets of ABPs. Recent advances in multicolor imaging and analysis of ABPs and their associated F-actin networks in cells, as well as the development of sophisticated in vitro reconstitutions of networks with ensembles of ABPs, have allowed the field to start uncovering the underlying principles by which cells self-organize diverse F-actin networks to execute basic cellular functions.

Bil2 Is a Novel Inhibitor of the Yeast Formin Bnr1 Required for Proper Actin Cable Organization and Polarized Secretion

Cell growth in budding yeast depends on rapid and on-going assembly and turnover of polarized actin cables, which direct intracellular transport of post-Golgi vesicles to the bud tip. Saccharomyces cerevisiae actin cables are polymerized by two formins, Bni1 and Bnr1. Bni1 assembles cables in the bud, while Bnr1 is anchored to the bud neck and assembles cables that specifically extend filling the mother cell. Here, we report a formin regulatory role for YGL015c, a previously uncharacterized open reading frame, which we have named Bud6 Interacting Ligand 2 (BIL2). bil2Δ cells display defects in actin cable architecture and partially-impaired secretory vesicle transport. Bil2 inhibits Bnr1-mediated actin filament nucleation in vitro, yet has no effect on the rate of Bnr1-mediated filament elongation. This activity profile for Bil2 resembles that of another yeast formin regulator, the F-BAR protein Hof1, and we find that bil2Δ with hof1Δ are synthetic lethal. Unlike Hof1, which localizes exclusively to the bud neck, GFP-Bil2 localizes to the cytosol, secretory vesicles, and sites of polarized cell growth. Further, we provide evidence that Hof1 and Bil2 inhibitory effects on Bnr1 are overcome by distinct mechanisms. Together, our results suggest that Bil2 and Hof1 perform distinct yet genetically complementary roles in inhibiting the actin nucleation activity of Bnr1 to control actin cable assembly and polarized secretion.

Discovery of functional interactions among actin regulators by analysis of image fluctuations in an unperturbed motile cell system

Cell migration is driven by propulsive forces derived from polymerizing actin that pushes and extends the plasma membrane. The underlying actin network is constantly undergoing adaptation to new mechano-chemical environments and intracellular conditions. As such, mechanisms that regulate actin dynamics inherently contain multiple feedback loops and redundant pathways. Given the highly adaptable nature of such a system, studies that use only perturbation experiments (e.g. knockdowns, overexpression, pharmacological activation/inhibition, etc.) are challenged by the nonlinearity and redundancy of the pathway. In these pathway configurations, perturbation experiments at best describe the function(s) of a molecular component in an adapting (e.g. acutely drug-treated) or fully adapted (e.g. permanent gene silenced) cell system, where the targeted component now resides in a non-native equilibrium. Here, we propose how quantitative live-cell imaging and analysis of constitutive fluctuations of molecular activities can overcome these limitations. We highlight emerging actin filament barbed-end biology as a prime example of a complex, nonlinear molecular process that requires a fluctuation analytic approach, especially in an unperturbed cellular system, to decipher functional interactions of barbed-end regulators, actin polymerization and membrane protrusion.This article is part of the theme issue 'Self-organization in cell biology'.

A Cdc42 GEF, Gef1, through endocytosis organizes F-BAR Cdc15 along the actomyosin ring and promotes concentric furrowing

During cytokinesis, fission yeast coordinates actomyosin ring constriction with septum ingression, resulting in concentric furrow formation by a poorly defined mechanism. We report that Schizosaccharomyces pombe cells lacking the Cdc42 activator Gef1, combined with an activated allele of the formin, Cdc12, display non-concentric furrowing. Non-concentrically furrowing cells display uneven distribution of the scaffold Cdc15 along the ring. This suggests that, after ring assembly, uniform Cdc15 distribution along the ring enables proper furrow formation. We find that, after assembly, Cdc15 is recruited to the ring in an Arp2/3 complex-dependent manner and is decreased in the activated cdc12 mutant. Cdc15 at cortical endocytic patches shows increased levels and extended lifetimes in gef1 and activated cdc12 mutants. We hypothesize endocytosis helps recruit Cdc15 to assembled rings; uneven Cdc15 distribution at the ring occurs when endocytic patches contain increased Cdc15 levels and the patch-association rate is slow. Based on this, we developed a mathematical model that captures experimentally observed Cdc15 distributions along the ring. We propose that, at the ring, Gef1 and endocytic events promote uniform Cdc15 organization to enable proper septum ingression and concentric furrow formation.

Phospho-regulation of intrinsically disordered proteins for actin assembly and endocytosis

Actin filament assembly contributes to the endocytic pathway pleiotropically, with active roles in clathrin-dependent and clathrin-independent endocytosis as well as subsequent endosomal trafficking. Endocytosis comprises a series of dynamic events, including the initiation of membrane curvature, bud invagination, vesicle abscission and subsequent vesicular transport. The ultimate success of endocytosis requires the coordinated activities of proteins that trigger actin polymerization, recruit actin-binding proteins (ABPs) and organize endocytic proteins (EPs) that promote membrane curvature through molecular crowding or scaffolding mechanisms. A particularly interesting phenomenon is that multiple EPs and ABPs contain a substantial percentage of intrinsically disordered regions (IDRs), which can contribute to protein coacervation and phase separation. In addition, intrinsically disordered proteins (IDPs) frequently contain sites for post-translational modifications (PTMs) such as phosphorylation, and these modifications exhibit a high preference for IDR residues [Groban ES et al. (2006) PLoS Comput Biol 2, e32]. PTMs are implicated in regulating protein function by modulating the protein conformation, protein-protein interactions and the transition between order and disorder states of IDPs. The molecular mechanisms by which IDRs of ABPs and EPs fine-tune actin assembly and endocytosis remain mostly unexplored and elusive. In this review, we analyze protein sequences of budding yeast EPs and ABPs, and discuss the potential underlying mechanisms for regulating endocytosis and actin assembly through the emerging concept of IDR-mediated protein multivalency, coacervation, and phase transition, with an emphasis on the phospho-regulation of IDRs. Finally, we summarize the current understanding of how these mechanisms coordinate actin cytoskeleton assembly and membrane curvature formation during endocytosis in budding yeast.

Yeast Aim21/Tda2 both regulates free actin by reducing barbed end assembly and forms a complex with Cap1/Cap2 to balance actin assembly between patches and cables

Yeast Aim21 is recruited by the SH3-containing proteins Bbc1 and Abp1 to patches and, with Tda2, reduces barbed end assembly to balance the distribution of actin between patches and cables. Aim21/Tda2 also interacts with Cap1/Cap2, revealing a complex interplay between actin assembly regulators.

How cells balance the incorporation of actin into diverse structures is poorly understood. In budding yeast, a single actin monomer pool is used to build both actin cables involved in polarized growth and actin cortical patches involved in endocytosis. Here we report how Aim21/Tda2 is recruited to the cortical region of actin patches, where it negatively regulates actin assembly to elevate the available actin monomer pool. Aim21 has four polyproline regions and is recruited by two SH3-containing patch proteins, Bbc1 and Abp1. The C-terminal region, which is required for its function, binds Tda2. Cell biological and biochemical data reveal that Aim21/Tda2 is a negative regulator of barbed end filamentous actin (F-actin) assembly, and this activity is necessary for efficient endocytosis and plays a pivotal role in balancing the distribution of actin between cables and patches. Aim21/Tda2 also forms a complex with the F-actin barbed end capping protein Cap1/Cap2, revealing an interplay between regulators and showing the complexity of regulation of barbed end assembly.

Global treadmilling coordinates actin turnover and controls the size of actin networks

Various cellular processes (including cell motility) are driven by the regulated, polarized assembly of actin filaments into distinct force-producing arrays of defined size and architecture. Branched, linear, contractile and cytosolic arrays coexist in vivo, and cells intricately control the number, length and assembly rate of filaments in these arrays. Recent in vitro and in vivo studies have revealed novel molecular mechanisms that regulate the number of filament barbed and pointed ends and their respective assembly and disassembly rates, thus defining classes of dynamically different filaments, which coexist in the same cell. We propose that a global treadmilling process, in which a steady-state amount of polymerizable actin monomers is established by the dynamics of each network, is responsible for defining the size and turnover of coexisting actin networks. Furthermore, signal-induced changes in the partitioning of actin to distinct arrays (mediated by RHO GTPases) result in the establishment of various steady-state concentrations of polymerizable monomers, thereby globally influencing the growth rate of actin filaments.

Internetwork competition for monomers governs actin cytoskeleton organization

Cells precisely control the formation of dynamic actin cytoskeleton networks to coordinate fundamental processes, including motility, division, endocytosis and polarization. To support these functions, actin filament networks must be assembled, maintained and disassembled at the correct time and place, and with proper filament organization and dynamics. Regulation of the extent of filament network assembly and of filament network organization has been largely attributed to the coordinated activation of actin assembly factors through signalling cascades. Here, we discuss an intriguing model in which actin monomer availability is limiting and competition between homeostatic actin cytoskeletal networks for actin monomers is an additional crucial regulatory mechanism that influences the density and size of different actin networks, thereby contributing to the organization of the cellular actin cytoskeleton.

Computational model of polarized actin cables and cytokinetic actin ring formation in budding yeast

The budding yeast actin cables and contractile ring are important for polarized growth and division, revealing basic aspects of cytoskeletal function. To study these formin-nucleated structures, we built a three-dimensional (3D) computational model with actin filaments represented as beads connected by springs. Polymerization by formins at the bud tip and bud neck, crosslinking, severing, and myosin pulling, are included. Parameter values were estimated from prior experiments. The model generates actin cable structures and dynamics similar to those of wild type and formin deletion mutant cells. Simulations with increased polymerization rate result in long, wavy cables. Simulated pulling by type V myosin stretches actin cables. Increasing the affinity of actin filaments for the bud neck together with reduced myosin V pulling promotes the formation of a bundle of antiparallel filaments at the bud neck, which we suggest as a model for the assembly of actin filaments to the contractile ring.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.

Profilin-1 serves as a gatekeeper for actin assembly by Arp2/3-dependent and -independent pathways

Cells contain multiple F-actin assembly pathways, including the Arp2/3 complex, formins, and Ena/VASP, which have largely been analyzed separately. They collectively generate the bulk of F-actin from a common pool of G-actin; however, the interplay and/or competition between these pathways remains poorly understood. Using fibroblast lines derived from an Arpc2 conditional knockout mouse, we established matched-pair cells with and without the Arp2/3 complex. Arpc2(-/-) cells lack lamellipodia and migrate more slowly than WT cells but have F-actin levels indistinguishable from controls. Actin assembly in Arpc2(-/-) cells was resistant to cytochalasin-D and was highly dependent on profilin-1 and Ena/VASP but not formins. Profilin-1 depletion in WT cells increased F-actin and Arp2/3 complex in lamellipodia. Conversely, addition of exogenous profilin-1 inhibited Arp2/3 complex actin nucleation in vitro and in vivo. Antagonism of the Arp2/3 complex by profilin-1 in cells appears to maintain actin homeostasis by balancing Arp2/3 complex-dependent and -independent actin assembly pathways.

Eng2 is a component of a dynamic protein complex required for endocytic uptake in fission yeast

Eng2 is a glucanase required for spore release, although it is also expressed during vegetative growth, suggesting that it might play other cellular functions. Its homology to the Saccharomyces cerevisiae Acf2 protein, previously shown to promote actin polymerization at endocytic sites in vitro, prompted us to investigate its role in endocytosis. Interestingly, depletion of Eng2 caused profound defects in endocytic uptake, which were not due to the absence of its glucanase activity. Analysis of the dynamics of endocytic proteins by fluorescence microscopy in the eng2Δ strain unveiled a previously undescribed phenotype, in which assembly of the Arp2/3 complex appeared uncoupled from the internalization of the endocytic coat and resulted in a fission defect. Strikingly also, we found that Eng2-GFP dynamics did not match the pattern of other endocytic proteins. Eng2-GFP localized to bright cytosolic spots that moved around the cellular poles and occasionally contacted assembling endocytic patches just before recruitment of Wsp1, the Schizosaccharomyces pombe WASP. Interestingly, Csh3-YFP, a WASP-interacting protein, interacted with Eng2 by co-immunoprecipitation and was recruited to Eng2 in bright cytosolic spots. Altogether, our work defines a novel endocytic functional module, which probably couples the endocytic coat to the actin module.

Homeostatic actin cytoskeleton networks are regulated by assembly factor competition for monomers

Controlling the quantity and size of organelles through competition for a limited supply of components is quickly emerging as an important cellular regulatory mechanism. Cells assemble diverse actin filament (F-actin) networks for fundamental processes including division, motility, and polarization. F-actin polymerization is tightly regulated by activation of assembly factors such as the Arp2/3 complex and formins at specific times and places. We directly tested an additional hypothesis that diverse F-actin networks are in homeostasis, whereby competition for actin monomers (G-actin) is critical for regulating F-actin network size. Here we show that inhibition of Arp2/3 complex in the fission yeast Schizosaccharomyces pombe not only depletes Arp2/3-complex-mediated endocytic actin patches, but also induces a dramatic excess of formin-assembled F-actin. Conversely, disruption of formin increases the density of Arp2/3-complex-mediated patches. Furthermore, modification of actin levels significantly perturbs the fission yeast actin cytoskeleton. Increasing actin favors Arp2/3-complex-mediated actin assembly, whereas decreasing actin favors formin-mediated contractile rings. Therefore, the specific actin concentration in a cell is critical, and competition for G-actin helps regulate the proper amount of F-actin assembly for diverse processes.

Control of polarized growth by the Rho family GTPase Rho4 in budding yeast: requirement of the N-terminal extension of Rho4 and regulation by the Rho GTPase-activating protein Bem2

In the budding yeast Saccharomyces cerevisiae, Rho4 GTPase partially plays a redundant role with Rho3 in the control of polarized growth, as deletion of RHO4 and RHO3 together, but not RHO4 alone, caused lethality and a loss of cell polarity at 30°C. Here, we show that overexpression of the constitutively active rho4(Q131L) mutant in an rdi1Δ strain caused a severe growth defect and generated large, round, unbudded cells, suggesting that an excess of Rho4 activity could block bud emergence. We also generated four temperature-sensitive rho4-Ts alleles in a rho3Δ rho4Δ strain. These mutants showed growth and morphological defects at 37°C. Interestingly, two rho4-Ts alleles contain mutations that cause amino acid substitutions in the N-terminal region of Rho4. Rho4 possesses a long N-terminal extension that is unique among the six Rho GTPases in the budding yeast but is common in Rho4 homologs in other yeasts and filamentous fungi. We show that the N-terminal extension plays an important role in Rho4 function since rho3Δ rho4(Δ)(61) cells expressing truncated Rho4 lacking amino acids (aa) 1 to 61 exhibited morphological defects at 24°C and a growth defect at 37°C. Furthermore, we show that Rho4 interacts with Bem2, a Rho GTPase-activating protein (RhoGAP) for Cdc42 and Rho1, by yeast two-hybrid, bimolecular fluorescence complementation (BiFC), and glutathione S-transferase (GST) pulldown assays. Bem2 specifically interacts with the GTP-bound form of Rho4, and the interaction is mediated by its RhoGAP domain. Overexpression of BEM2 aggravates the defects of rho3Δ rho4 mutants. These results suggest that Bem2 might be a novel GAP for Rho4.

Fission yeast sec3 bridges the exocyst complex to the actin cytoskeleton

The exocyst complex tethers post-Golgi secretory vesicles to the plasma membrane prior to docking and fusion. In this study, we identify Sec3, the missing component of the Schizosaccharomyces pombe exocyst complex (SpSec3). SpSec3 shares many properties with its orthologs, and its mutants are rescued by human Sec3/EXOC1. Although involved in exocytosis, SpSec3 does not appear to mark the site of exocyst complex assembly at the plasma membrane. It does, however, mark the sites of actin cytoskeleton recruitment and controls the organization of all three yeast actin structures: the actin cables, endocytic actin patches and actomyosin ring. Specifically, SpSec3 physically interacts with For3 and sec3 mutants have no actin cables as a result of a failure to polarize this nucleating formin. SpSec3 also interacts with actin patch components and sec3 mutants have depolarized actin patches of reduced endocytic capacity. Finally, the constriction and disassembly of the cytokinetic actomyosin ring is compromised in these sec3 mutant cells. We propose that a role of SpSec3 is to spatially couple actin machineries and their independently polarized regulators. As a consequence of its dual role in secretion and actin organization, Sec3 appears as a major co-ordinator of cell morphology in fission yeast.

Regulation of the formin Bnr1 by septins anda MARK/Par1-family septin-associated kinase

The septin-associated kinase Gin4 is required for the localization and activation of Bnr1, and the septin Shs1 is essential for Bnr1 activation. The loss of Gin4 or Shs1 phenocopies the loss of Bnr1; these defects are suppressed by constitutive activation of Bnr1. The data reveal novel regulatory links between the actin and septin cytoskeletons.

Formin-family proteins promote the assembly of linear actin filaments and are required to generate cellular actin structures, such as actin stress fibers and the cytokinetic actomyosin contractile ring. Many formin proteins are regulated by an autoinhibition mechanism involving intramolecular binding of a Diaphanous inhibitory domain and a Diaphanous autoregulatory domain. However, the activation mechanism for these Diaphanous-related formins (DRFs) is not completely understood. Although small GTPases play an important role in relieving autoinhibition, other factors likely contribute. Here we describe a requirement for the septin Shs1 and the septin-associated kinase Gin4 for the localization and in vivo activity of the budding yeast DRF Bnr1. In budding yeast strains in which the other formin, Bni1, is conditionally inactivated, the loss of Gin4 or Shs1 results in the loss of actin cables and cell death, similar to the loss of Bnr1. The defects in these strains can be suppressed by constitutive activation of Bnr1. Gin4 is involved in both the localization and activation of Bnr1, whereas the septin Shs1 is required for Bnr1 activation but not its localization. Gin4 promotes the activity of Bnr1 independently of the Gin4 kinase activity, and Gin4 lacking its kinase domain binds to the critical localization region of Bnr1. These data reveal novel regulatory links between the actin and septin cytoskeletons.

Ras and Rho small G proteins: insights from the Schizophyllum commune genome sequence and comparisons to other fungi

Unlike in animal cells and yeasts, the Ras and Rho small G proteins and their regulators have not received extensive research attention in the case of the filamentous fungi. In an effort to begin to rectify this deficiency, the genome sequence of the basidiomycete mushroom Schizophyllum commune was searched for all known components of the Ras and Rho signalling pathways. The results of this study should provide an impetus for further detailed investigations into their role in polarized hyphal growth, sexual reproduction and fruiting body development. These processes have long been the targets for genetic and cell biological research in this fungus.

A systems-biology approach to yeast actin cables

We focus on actin cables in yeast as a model system for understanding cytoskeletal organization and the workings of actin itself. In particular, we highlight quantitative approaches on the kinetics of actin-cable assembly and methods of measuring their morphology by image analysis. Actin cables described by these studies can span greater lengths than a thousand end-to-end actin-monomers. Because of this difference in length scales, control of the actin-cable system constitutes a junction between short-range interactions - among actin-monomers and nucleating, polymerization-facilitating, side-binding, severing, and cross-linking proteins - and the emergence of cell-scale physical form as embodied by the actin cables themselves.

Cotranslational transport of ABP140 mRNA to the distal pole of S. cerevisiae

In budding yeast, several mRNAs are selectively transported into the daughter cell in an actin-dependent manner by a specialized myosin system, the SHE machinery. With ABP140 mRNA, we now describe the first mRNA that is transported in the opposite direction and localizes to the distal pole of the mother cell, independent of the SHE machinery. Distal pole localization is not observed in mutants devoid of actin cables and can be disrupted by latrunculin A. Furthermore, localization of ABP140 mRNA requires the N-terminal actin-binding domain of Abp140p to be expressed. By replacing the N-terminal localization motif, ABP140 mRNA can be retargeted to different subcellular structures. In addition, accumulation of the mRNA at the distal pole can be prevented by disruption of polysomes. Using the MS2 system, the mRNA was found to associate with actin cables and to follow actin cable dynamics. We therefore propose a model of translational coupling, in which ABP140 mRNA is tethered to actin cables via its nascent protein product and is transported to the distal pole by actin retrograde flow.

A domain of the actin binding protein Abp140 is the yeast methyltransferase responsible for 3-methylcytidine modification in the tRNA anti-codon loop

The 3-methylcytidine (m³C) modification is widely found in eukaryotic species of tRNA(Ser), tRNA(Thr), and tRNA(Arg); at residue 32 in the anti-codon loop; and at residue e2 in the variable stem of tRNA(Ser). Little is known about the function of this modification or about the specificity of the corresponding methyltransferase, since the gene has not been identified. We have used a primer extension assay to screen a battery of methyltransferase candidate knockout strains in the yeast Saccharomyces cerevisiae, and find that tRNA(Thr(IGU)) from abp140-Δ strains lacks m³C. Curiously, Abp140p is composed of a poorly conserved N-terminal ORF fused by a programed +1 frameshift in budding yeasts to a C-terminal ORF containing an S-adenosylmethionine (SAM) domain that is highly conserved among eukaryotes. We show that ABP140 is required for m³C modification of substrate tRNAs, since primer extension is similarly affected for all tRNA species expected to have m³C and since quantitative analysis shows explicitly that tRNA(Thr(IGU)) from an abp140-Δ strain lacks m³C. We also show that Abp140p (now named Trm140p) purified after expression in yeast or Escherichia coli has m³C methyltransferase activity, which is specific for tRNA(Thr(IGU)) and not tRNA(Phe) and occurs specifically at C₃₂. We suggest that the C-terminal ORF of Trm140p is necessary and sufficient for activity in vivo and in vitro, based on analysis of constructs deleted for most or all of the N-terminal ORF. We also suggest that m³C has a role in translation, since trm140-Δ trm1-Δ strains (also lacking m²,²G₂₆) are sensitive to low concentrations of cycloheximide.

Cytoskeletal dynamics in fission yeast: a review of models for polarization and division

We review modeling studies concerning cytoskeletal activity of fission yeast. Recent models vary in length and time scales, describing a range of phenomena from cellular morphogenesis to polymer assembly. The components of cytoskeleton act in concert to mediate cell-scale events and interactions such as polarization. The mathematical models reduce these events and interactions to their essential ingredients, describing the cytoskeleton by its bulk properties. On a smaller scale, models describe cytoskeletal subcomponents and how bulk properties emerge.

The yeast formin Bnr1p has two localization regions that show spatially and temporally distinct association with septin structures

Formins are conserved eukaryotic proteins that direct the nucleation and elongation of unbranched actin filaments. We define two nonoverlapping regions, Bnr1p-L1 (1-466) and Bnr1p-L2 (466-733) that can each localize to the bud neck independently of endogenous Bnr1p.

Formins are conserved eukaryotic proteins that direct the nucleation and elongation of unbranched actin filaments. The yeast formins, Bni1p and Bnr1p, assemble actin cables from the bud cortex and bud neck, respectively, to guide overall cell polarity. Here we examine the regions of Bnr1p responsible for bud neck localization. We define two non-overlapping regions, Bnr1p-L1 (1-466) and Bnr1p-L2 (466-733), that can each localize to the bud neck independently of endogenous Bnr1p. Bnr1p-L1 and Bnr1p-L2 localize with septins at the bud neck, but show slightly differently spatial and temporal localization, reflecting the localization (Bnr1p-L1) or cell cycle timing (Bnr1p-L2) of full-length Bnr1p. Bnr1p is known to be very stably localized at the bud neck, and both Bnr1p-L1 and Bnr1p-L2 also show relatively stable localization there. Overexpression of Bnr1p-L1, but not Bnr1p-L2, disrupts septin organization at the bud neck. Thus Bnr1p has two separable regions that each contribute to its bud neck localization.

Formins in development: orchestrating body plan origami

Formins, proteins defined by the presence of an FH2 domain and their ability to nucleate linear F-actin de novo, play a key role in the regulation of the cytoskeleton. Initially thought to primarily regulate actin, recent studies have highlighted a role for formins in the regulation of microtubule dynamics, and most recently have uncovered the ability of some formins to coordinate the organization of both the microtubule and actin cytoskeletons. While biochemical analyses of this family of proteins have yielded many insights into how formins regulate diverse cytoskeletal reorganizations, we are only beginning to appreciate how and when these functional properties are relevant to biological processes in a developmental or organismal context. Developmental genetic studies in fungi, Dictyostelium, vertebrates, plants and other model organisms have revealed conserved roles for formins in cell polarity, actin cable assembly and cytokinesis. However, roles have also been discovered for formins that are specific to particular organisms. Thus, formins perform both global and specific functions, with some of these roles concurring with previous biochemical data and others exposing new properties of formins. While not all family members have been examined across all organisms, the analyses to date highlight the significance of the flexibility within the formin family to regulate a broad spectrum of diverse cytoskeletal processes during development.

How tropomyosin regulates lamellipodial actin-based motility: a combined biochemical and reconstituted motility approach

At the leading edge of migrating cells, protrusive forces are developed by the assembly of actin filaments organised in a lamellipodial dendritic array at the front and a more distal lamellar linear array. Whether these two arrays are distinct or functionally linked and how they contribute to cell migration is an open issue. Tropomyosin severely inhibits lamellipodium formation and facilitates the lamellar array while enhancing migration, by a mechanism that is not understood. Here we show that the complex in vivo effects of tropomyosin are recapitulated in the reconstituted propulsion of neural Wiskott-Aldrich syndrome protein (N-WASP)-functionalised beads, which is based on the sole formation of a dendritic array of actin-related protein (Arp)2/3-branched filaments. Actin-depolymerising factor (ADF) and tropomyosin control the length of the actin tail. By competing with Arp2/3 during filament branching, tropomyosin displays opposite effects on propulsion depending on the surface density of N-WASP. Tropomyosin binding to the dendritic array is facilitated following filament debranching, causing its enrichment at the rear of the actin tail, like in vivo. These results unveil the mechanism by which tropomyosin generates two morphologically and dynamically segregated actin networks from a single one.

Propagating waves separate two states of actin organization in living cells

Propagating actin waves are dynamic supramolecular structures formed by the self-assembly of proteins within living cells. They are built from actin filaments together with single-headed myosin, the Arp23 complex, and coronin in a defined three-dimensional order. The function of these waves in structuring the cell cortex is studied on the substrate-attached surface of Dictyostelium cells by the use of total internal reflection fluorescence (TIRF) microscopy. Actin waves separate two areas of the cell cortex from each other, which are distinguished by the arrangement of actin filaments. The Arp23 complex dominates in the area enclosed by a wave, where it has the capacity of building dendritic structures, while the proteins prevailing in the external area, cortexillin I and myosin-II, bundle actin filaments and arrange them in antiparallel direction. Wave propagation is accompanied by transitions in the state of actin with a preferential period of 5 min. Wave generation is preceded by local fluctuations in actin assembly, some of the nuclei of polymerized actin emanating from clathrin-coated structures, others emerging independently. The dynamics of phase transitions has been analyzed to provide a basis for modeling the nonlinear interactions that produce spatio-temporal patterns in the actin system of living cells.

Pob1 participates in the Cdc42 regulation of fission yeast actin cytoskeleton

Rho GTPases regulate the actin cytoskeleton in all eukaryotes. Fission yeast Cdc42 is involved in actin cable assembly and formin For3 regulation. We isolated cdc42-879 as a thermosensitive strain with actin cable and For3 localization defects. In a multicopy suppressor screening, we identified pob1(+) as suppressor of cdc42-879 thermosensitivity. Pob1 overexpression also partially restores actin cables and localization of For3 in the mutant strain. Pob1 interacts with Cdc42 and this GTPase regulates Pob1 localization and/or stability. The C-terminal pleckstrin homology (PH) domain of Pob1 is required for Cdc42 binding. Pob1 also binds to For3 through its N-terminal sterile alpha motif (SAM) domain and contributes to the formin localization at the cell tips. The previously described pob1-664 mutant strain (Mol. Biol. Cell. 10, 2745-2757, 1999), which carries a mutation in the PH domain, as well as pob1 mutant strains in which Pob1 lacks the N-terminal region (pob1DeltaN) or the SAM domain (pob1DeltaSAM), have cytoskeletal defects similar to that of cdc42-879 cells. Expression of constitutively active For3DAD* partially restores actin organization in cdc42-879, pob1-664, pob1DeltaN, and pob1DeltaSAM. Therefore, we propose that Pob1 is required for For3 localization to the tips and facilitates Cdc42-mediated relief of For3 autoinhibition to stimulate actin cable formation.

Actin nucleation and elongation factors: mechanisms and interplay

Cells require actin nucleators to catalyze the de novo assembly of filaments and actin elongation factors to control the rate and extent of polymerization. Nucleation and elongation factors identified to date include Arp2/3 complex, formins, Ena/VASP, and newcomers Spire, Cobl, and Lmod. Here, we discuss recent advances in understanding their activities and mechanisms and new evidence for their cooperation and interaction in vivo. Earlier models had suggested that different nucleators function independently to assemble distinct actin arrays. However, more recent observations indicate that the construction of most cellular actin networks depends on the activities of multiple actin assembly-promoting factors working in concert.

Model of For3p-mediated actin cable assembly in fission yeast

Formin For3p nucleates actin cables at the tips of fission yeast cells for polarized cell growth. The results of prior experiments have suggested a possible mechanism for actin cable assembly that involves association of For3p near cell tips, For3p-mediated actin polymerization, retrograde flow of actin cables toward the cell center, For3p dissociation from cell tips, and cable disassembly. We used analytical and computational modeling to test the validity and implications of the proposed coupled For3p/actin mechanism. We compared the model to prior experiments quantitatively and generated predictions for the expected behavior of the actin cable system upon changes of parameter values. We found that the model generates stable steady states with realistic values of rate constants and actin and For3p concentrations. Comparison of our results to previous experiments monitoring the FRAP of For3p-3GFP and the response of actin cables to treatments with actin depolymerizing drugs provided further support for the model. We identified the set of parameter values that produces results in agreement with experimental observations. We discuss future experiments that will help test the model's predictions and eliminate other possible mechanisms. The results of the model suggest that flow of actin cables may establish actin and For3p concentration gradients in the cytoplasm that could be important in global cell patterning.