Myosin Vs organize actin cables in fission yeast

Acetylation of fission yeast tropomyosin does not promote differential association with cognate formins

It was proposed from cellular studies that S. pombe tropomyosin Cdc8 (Tpm) segregates into two populations due to the presence or absence of an amino-terminal acetylation that specifies which formin-mediated F-actin networks it binds, but with no supporting biochemistry. To address this mechanism in vitro, we developed methods for S. pombe actin expression in Sf9 cells. We then employed 3-color TIRF microscopy using all recombinant S. pombe proteins to probe in vitro multicomponent mechanisms involving actin, acetylated and unacetylated Tpm, formins, and myosins. Acetyl-Tpm exhibits tight binding to actin in contrast to weaker binding by unacetylated Tpm. In disagreement with the differential recruitment model, Tpm showed no preferential binding to filaments assembled by the FH1-FH2-domains of two S. pombe formins, nor did Tpm binding have any bias towards the growing formin-bound actin filament barbed end. Although our in vitro findings do not support a direct formin-tropomyosin interaction, it is possible that formins bias differential tropomyosin isoform recruitment through undiscovered mechanisms. Importantly, despite a 12% sequence divergence between skeletal and S. pombe actin, S. pombe myosins Myo2 and Myo51 exhibited similar motile behavior with these two actins, validating key prior findings with these myosins that used skeletal actin.

Rab11 negatively regulates wingless preventing JNK-mediated apoptosis in Drosophila epithelium during embryonic dorsal closure

Rab11, a small Ras like GTPase marking the recycling endosomes, plays instrumental roles in Drosophila embryonic epithelial morphogenesis where an array of reports testify its importance in the maintenance of cyto-architectural as well as functional attributes of the concerned cells. Proper Rab11 functions ensure a precise regulation of developmentally active cell signaling pathways which in turn promote the expression of morphogens and other physico-chemical cues which finally forge an embryo out of a single layer of cells. Earlier reports have established that Rab11 functions are vital for fly embryonic development where amorphic mutants such as EP3017 homozygotes show a fair degree of epithelial defects along with incomplete dorsal closure. Here, we present a detailed account of the effects of Rab11 loss of function in the dorso-lateral epithelium which resulted in severe dorsal closure defects along with an elevated JNK-Dpp expression. We further observed that the dorso-lateral epithelial cells undergo epithelial to mesenchymal transition as well as apoptosis in Rab11 mutants with elevated expression levels of MMP1 and Caspase-3, where Caspase-3 contributes to the Rab11 knockout phenotype contrary to the knockdown mutants or hypomorphs. Interestingly, the elevated expressions of the core JNK-Dpp signaling could be rescued with a simultaneous knockdown of wingless in the Rab11 knockout mutants suggesting a genetic interaction of Rab11 with the Wingless pathway during dorsal closure, an ideal model of epithelial wound healing.

The Multiple Functions of Rho GTPases in Fission Yeasts

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Rapid adaptation of endocytosis, exocytosis and eisosomes after an acute increase in membrane tension in yeast cells

During clathrin-mediated endocytosis (CME) in eukaryotes, actin assembly is required to overcome large membrane tension and turgor pressure. However, the molecular mechanisms by which the actin machinery adapts to varying membrane tension remain unknown. In addition, how cells reduce their membrane tension when they are challenged by hypotonic shocks remains unclear. We used quantitative microscopy to demonstrate that cells rapidly reduce their membrane tension using three parallel mechanisms. In addition to using their cell wall for mechanical protection, yeast cells disassemble eisosomes to buffer moderate changes in membrane tension on a minute time scale. Meanwhile, a temporary reduction in the rate of endocytosis for 2–6 min and an increase in the rate of exocytosis for at least 5 min allow cells to add large pools of membrane to the plasma membrane. We built on these results to submit the cells to abrupt increases in membrane tension and determine that the endocytic actin machinery of fission yeast cells rapidly adapts to perform CME. Our study sheds light on the tight connection between membrane tension regulation, endocytosis, and exocytosis.

Microtubule-independent movement of the fission yeast nucleus

Movement of the cell nucleus typically involves the cytoskeleton and either polymerization-based pushing forces or motor-based pulling forces. In the fission yeast Schizosaccharomyces pombe, nuclear movement and positioning are thought to depend on microtubule polymerization-based pushing forces. Here, we describe a novel, microtubule-independent, form of nuclear movement in fission yeast. Microtubule-independent nuclear movement is directed towards growing cell tips, and it is strongest when the nucleus is close to a growing cell tip, and weakest when the nucleus is far from that tip. Microtubule-independent nuclear movement requires actin cables but does not depend on actin polymerization-based pushing or myosin V-based pulling forces. The vesicle-associated membrane protein (VAMP)-associated proteins (VAPs) Scs2 and Scs22, which are critical for endoplasmic reticulum-plasma membrane contact sites in fission yeast, are also required for microtubule-independent nuclear movement. We also find that in cells in which microtubule-based pushing forces are present, disruption of actin cables leads to increased fluctuations in interphase nuclear positioning and subsequent altered septation. Our results suggest two non-exclusive mechanisms for microtubule-independent nuclear movement, which may help illuminate aspects of nuclear positioning in other cells.

Escape from mitotic catastrophe by actin-dependent nuclear displacement in fission yeast

Eukaryotic cells position the nucleus within the proper intracellular space, thereby safeguarding a variety of cellular processes. In fission yeast, the interphase nucleus is placed in the cell middle in a microtubule-dependent manner. By contrast, how the mitotic nucleus is positioned remains elusive. Here we show that several cell-cycle mutants that arrest in mitosis all displace the nucleus toward one end of the cell. Intriguingly, the actin cytoskeleton is responsible for nuclear movement. Time-lapse live imaging indicates that mitosis-specific F-actin cables possibly push the nucleus through direct interaction with the nuclear envelope, and subsequently actomyosin ring constriction further shifts the nucleus away from the center. This nuclear movement is beneficial, because if the nuclei were retained in the center, unseparated chromosomes would be intersected by the contractile actin ring and the septum, imposing the lethal cut phenotype. Thus, fission yeast escapes from mitotic catastrophe by means of actin-dependent nuclear movement.

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Actin-dependent mitotic nuclear positioning in fission yeast

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Actin cables and ring closure drive nuclear displacement upon mitotic arrest

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Nuclear displacement evades cut-mediated cell death

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Survivors resume cell division as diploids

Stress-activated MAPK signaling controls fission yeast actomyosin ring integrity by modulating formin For3 levels

Cytokinesis, which enables the physical separation of daughter cells once mitosis has been completed, is executed in fungal and animal cells by a contractile actin- and myosin-based ring (CAR). In the fission yeast Schizosaccharomyces pombe, the formin For3 nucleates actin cables and also co-operates for CAR assembly during cytokinesis. Mitogen-activated protein kinases (MAPKs) regulate essential adaptive responses in eukaryotic organisms to environmental changes. We show that the stress-activated protein kinase pathway (SAPK) and its effector, MAPK Sty1, downregulates CAR assembly in S. pombe when its integrity becomes compromised during cytoskeletal damage and stress by reducing For3 levels. Accurate control of For3 levels by the SAPK pathway may thus represent a novel regulatory mechanism of cytokinesis outcome in response to environmental cues. Conversely, SAPK signaling favors CAR assembly and integrity in its close relative Schizosaccharomyces japonicus, revealing a remarkable evolutionary divergence of this response within the fission yeast clade.

DYRK kinase Pom1 drives F-BAR protein Cdc15 from the membrane to promote medial division

In many organisms, positive and negative signals cooperate to position the division site for cytokinesis. In the rod-shaped fission yeast Schizosaccharomyces pombe, symmetric division is achieved through anillin/Mid1-dependent positive cues released from the central nucleus and negative signals from the DYRK-family polarity kinase Pom1 at cell tips. Here we establish that Pom1's kinase activity prevents septation at cell tips even if Mid1 is absent or mislocalized. We also find that Pom1 phosphorylation of F-BAR protein Cdc15, a major scaffold of the division apparatus, disrupts Cdc15's ability to bind membranes and paxillin, Pxl1, thereby inhibiting Cdc15's function in cytokinesis. A Cdc15 mutant carrying phosphomimetic versions of Pom1 sites or deletion of Cdc15 binding partners suppresses division at cell tips in cells lacking both Mid1 and Pom1 signals. Thus, inhibition of Cdc15-scaffolded septum formation at cell poles is a key Pom1 mechanism that ensures medial division.

Capping Protein Insulates Arp2/3-Assembled Actin Patches from Formins

How actin structures of distinct identities and functions coexist within the same environment is a critical self-organization question. Fission yeast cells have a simple actin cytoskeleton made of four structures: Arp2/3 assembles actin patches around endocytic pits, and the formins For3, Cdc12, and Fus1 assemble actin cables, the cytokinetic ring during division, and the fusion focus during sexual reproduction, respectively. The focus concentrates the delivery of hydrolases by myosin V to digest the cell wall for cell fusion. We discovered that cells lacking capping protein (CP), a heterodimer that blocks barbed-end dynamics and associates with actin patches, exhibit a delay in fusion. Consistent with CP-formin competition for barbed-end binding, Fus1, F-actin, and the linear filament marker tropomyosin hyper-accumulate at the fusion focus in cells lacking CP. CP deletion also rescues the fusion defect of a mutation in the Fus1 knob region. However, myosin V and exocytic cargoes are reduced at the fusion focus and diverted to ectopic foci, which underlies the fusion defect. Remarkably, the ectopic foci coincide with Arp2/3-assembled actin patches, which now contain low levels of Fus1. We further show that CP localization to actin patches is required to prevent the formation of ectopic foci and promote efficient cell fusion. During mitotic growth, actin patches lacking CP similarly display a dual identity, as they accumulate the formins For3 and Cdc12, normally absent from patches, and are co-decorated by the linear filament-binding protein tropomyosin and the patch marker fimbrin. Thus, CP serves to protect Arp2/3-nucleated structures from formin activity.

Sliding filament and fixed filament mechanisms contribute to ring tension in the cytokinetic contractile ring

A fundamental challenge in cell biology is to understand how cells generate actomyosin-based contractile force. Here we study the actomyosin contractile ring that divides cells during cytokinesis and generates tension by a mechanism that remains poorly understood. Long ago a muscle-like sliding filament mechanism was proposed, but evidence for sarcomeric organization in contractile rings is lacking. We develop a coarse-grained model of the fission yeast cytokinetic ring, incorporating the two myosin-II isoforms Myo2 and Myp2 and severely constrained by experimental data. The model predicts that ring tension is indeed generated by a sliding filament mechanism, but a spatially and temporally homogeneous version of that in muscle. In this mechanism all pairs of oppositely oriented actin filaments are rendered tense as they are pulled toward one another and slide through clusters of myosin-II. The mechanism relies on anchoring of actin filament barbed ends to the plasma membrane, which resists lateral motion and enables filaments to become tense when pulled by myosin-II. A second fixed filament component is independent of lateral anchoring, generated by chains of like-oriented actin filaments. Myo2 contributes to both components, while Myp2 contributes to the sliding filament component only. In the face of instabilities inherent to actomyosin contractility, organizational homeostasis is maintained by rapid turnover of Myo2 and Myp2, and by drag forces that resist lateral motion of actin, Myo2 and Myp2. Thus, sliding and fixed filament mechanisms contribute to tension in the disordered contractile ring without the need for the sarcomeric architecture of muscle.

The actin motor MYO-5 effect in the intracellular organization of Neurospora crassa

In filamentous fungi, polarized growth is the result of vesicle secretion at the hyphal apex. Motor proteins mediate vesicle transport to target destinations on the plasma membrane via actin and microtubule cytoskeletons. Myosins are motor proteins associated with actin filaments. Specifically, class V myosins are responsible for cargo transport in eukaryotes. We studied the dynamics and localization of myosin V in wild type hyphae of Neurospora crassa and in hyphae that lacked MYO-5. In wild type hyphae, MYO-5-GFP was localized concentrated in the hyphal apex and colocalized with Spitzenkörper. Photobleaching studies showed that MYO-5-GFP was transported to the apex from subapical hyphal regions. The deletion of the class V myosin resulted in a reduced rate of hyphal growth, apical hyperbranching, and intermittent loss of hyphal polarity. MYO-5 did not participate in breaking the symmetrical growth during germination but contributed in the apical organization upon establishment of polarized growth. In the Δmyo-5 mutant, actin was organized into thick cables in the apical and subapical hyphal regions, and the number of endocytic patches was reduced. The microvesicles-chitosomes observed with CHS-1-GFP were distributed as a cloud occupying the apical dome and not in the Spitzenkörper as the WT strain. The mitochondrial movement was not associated with MYO-5, but tubular vacuole position is MYO-5-dependent. These results suggest that MYO-5 plays a role in maintaining apical organization and the integrity of the Spitzenkörper and is required for normal hyphal growth, polarity, septation, conidiation, and proper conidial germination.

Exportin Crm1 is repurposed as a docking protein to generate microtubule organizing centers at the nuclear pore

Non-centrosomal microtubule organizing centers (MTOCs) are important for microtubule organization in many cell types. In fission yeast Schizosaccharomyces pombe, the protein Mto1, together with partner protein Mto2 (Mto1/2 complex), recruits the γ-tubulin complex to multiple non-centrosomal MTOCs, including the nuclear envelope (NE). Here, we develop a comparative-interactome mass spectrometry approach to determine how Mto1 localizes to the NE. Surprisingly, we find that Mto1, a constitutively cytoplasmic protein, docks at nuclear pore complexes (NPCs), via interaction with exportin Crm1 and cytoplasmic FG-nucleoporin Nup146. Although Mto1 is not a nuclear export cargo, it binds Crm1 via a nuclear export signal-like sequence, and docking requires both Ran in the GTP-bound state and Nup146 FG repeats. In addition to determining the mechanism of MTOC formation at the NE, our results reveal a novel role for Crm1 and the nuclear export machinery in the stable docking of a cytoplasmic protein complex at NPCs.

Active transport of cytoophidia in Schizosaccharomyces pombe

The metabolic enzyme cytidine triphosphate synthase has recently been found to form micrometer-sized filamentous structures termed cytoophidia, which are evolutionarily conserved across prokaryotes and eukaryotes. The cytoophidium represents a novel type of membraneless organelle and behaves dynamically inside the cell. The question of how cytoophidia transport is mediated, however, remains unanswered. For the first time, we detected in this study the active transport of cytoophidia, taking advantage of the fission yeast Schizosaccharomyces pombe as an excellent model for studying membraneless organelles. We demonstrated that actin filaments, not microtubules, are responsible for this transport. Furthermore, we determined that Myo52, a type of myosin V, is required for the active transport of cytoophidia. These results reveal the major players critical to the dynamics of cytoophidia and extend our understanding of intracellular transport of membraneless organelles.—Li, H., Ye, F., Ren, J.-Y., Wang, P.-Y., Du, L.-L., Liu, J.-L. Active transport of cytoophidia in Schizosaccharomyces pombe.

Extracellular matrix stiffness and cell contractility control RNA localization to promote cell migration

Numerous RNAs are enriched within cellular protrusions, but the underlying mechanisms are largely unknown. We had shown that the APC (adenomatous polyposis coli) protein controls localization of some RNAs at protrusions. Here, using protrusion-isolation schemes and RNA-Seq, we find that RNAs localized in protrusions of migrating fibroblasts can be distinguished in two groups, which are differentially enriched in distinct types of protrusions, and are additionally differentially dependent on APC. APC-dependent RNAs become enriched in high-contractility protrusions and, accordingly, their localization is promoted by increasing stiffness of the extracellular matrix. Dissecting the underlying mechanism, we show that actomyosin contractility activates a RhoA-mDia1 signaling pathway that leads to formation of a detyrosinated-microtubule network, which in turn is required for localization of APC-dependent RNAs. Importantly, a competition-based approach to specifically mislocalize APC-dependent RNAs suggests that localization of the APC-dependent RNA subgroup is functionally important for cell migration.Adenomatous polyposis coli (APC) regulates the localization of some mRNAs at cellular protrusions but the underlying mechanisms and functional roles are not known. Here the authors show that APC-dependent RNAs are enriched in contractile protrusions, via detyrosinated microtubules, and enhance cell migration.

A systematic screen for morphological abnormalities during fission yeast sexual reproduction identifies a mechanism of actin aster formation for cell fusion

In non-motile fungi, sexual reproduction relies on strong morphogenetic changes in response to pheromone signaling. We report here on a systematic screen for morphological abnormalities of the mating process in fission yeast Schizosaccharomyces pombe. We derived a homothallic (self-fertile) collection of viable deletions, which, upon visual screening, revealed a plethora of phenotypes affecting all stages of the mating process, including cell polarization, cell fusion and sporulation. Cell fusion relies on the formation of the fusion focus, an aster-like F-actin structure that is marked by strong local accumulation of the myosin V Myo52, which concentrates secretion at the fusion site. A secondary screen for fusion-defective mutants identified the myosin V Myo51-associated coiled-coil proteins Rng8 and Rng9 as critical for the coalescence of the fusion focus. Indeed, rng8Δ and rng9Δ mutant cells exhibit multiple stable dots at the cell-cell contact site, instead of the single focus observed in wildtype. Rng8 and Rng9 accumulate on the fusion focus, dependent on Myo51 and tropomyosin Cdc8. A tropomyosin mutant allele, which compromises Rng8/9 localization but not actin binding, similarly leads to multiple stable dots instead of a single focus. By contrast, myo51 deletion does not strongly affect fusion focus coalescence. We propose that focusing of the actin filaments in the fusion aster primarily relies on Rng8/9-dependent cross-linking of tropomyosin-actin filaments.

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

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

Nuclear displacement and fluorescence recovery after photobleaching (FRAP) assays to study division site placement and cytokinesis in fission yeast

Cytokinesis is an essential cellular event that completes the cell division cycle. It begins with the assembly of an actomyosin contractile ring that undergoes constriction concomitant with the septum formation to divide the cell in two. Placement of the septum at the right position is important to ensure fidelity of the division process. In fission yeast, the medially placed nucleus is a major spatial cue to position the site of division. In this chapter, we describe a simple synthetic biology-based approach to displace the nucleus and study the consequence on division site positioning. We also describe how to perform fluorescence recovery after photobleaching to follow the dynamics of cytokinetic proteins at defined time points by live-cell microscopy.

A single-headed fission yeast myosin V transports actin in a tropomyosin-dependent manner

Myo51, a class V myosin in fission yeast, localizes to and assists in the assembly of the contractile ring, a conserved eukaryotic actomyosin structure that facilitates cytokinesis. Rng8 and Rng9 are binding partners that dictate the cellular localization and function of Myo51. Myo51 was expressed in insect cells in the presence or absence of Rng8/9. Surprisingly, electron microscopy of negatively stained images and hydrodynamic measurements showed that Myo51 is single headed, unlike most class V myosins. When Myo51-Rng8/9 was bound to actin-tropomyosin, two attachment sites were observed: the typical ATP-dependent motor domain attachment and a novel ATP-independent binding of the tail mediated by Rng8/9. A modified motility assay showed that this additional binding site anchors Myo51-Rng8/9 so that it can cross-link and slide actin-tropomyosin filaments relative to one another, functions that may explain the role of this motor in contractile ring assembly.

Lipid Droplets Form from Distinct Regions of the Cell in the Fission Yeast Schizosaccharomyces pombe

Eukaryotic cells store cholesterol/sterol esters (SEs) and triacylglycerols (TAGs) in lipid droplets, which form from the contiguous endoplasmic reticulum (ER) network. However, it is not known if droplets preferentially form from certain regions of the ER over others. Here, we used fission yeast Schizosaccharomyces pombe cells where the nuclear and cortical/peripheral ER domains are distinguishable by light microscopy to show that SE-enriched lipid droplets form away from the nucleus at the cell tips, whereas TAG-enriched lipid droplets form around the nucleus. Sterols localize to the regions of the cells where droplets enriched in SEs are observed. TAG droplet formation around the nucleus appears to be a strong function of diacylglycerol (DAG) homeostasis with Cpt1p, which coverts DAG into phosphatidylcholine and phosphatidylethanolamine localized exclusively to the nuclear ER. Also, Dgk1p, which converts DAG into phosphatidic acid localized strongly to the nuclear ER over the cortical/peripheral ER. We also show that TAG more readily translocates from the ER to lipid droplets than do SEs. The results augment the standard lipid droplet formation model, which has SEs and TAGs flowing into the same nascent lipid droplet regardless of its biogenesis point in the cell.

The DYRK-family kinase Pom1 phosphorylates the F-BAR protein Cdc15 to prevent division at cell poles

Division site positioning is critical for both symmetric and asymmetric cell divisions. In many organisms, positive and negative signals cooperate to position the contractile actin ring for cytokinesis. In rod-shaped fission yeast Schizosaccharomyces pombe cells, division at midcell is achieved through positive Mid1/anillin-dependent signaling emanating from the central nucleus and negative signals from the dual-specificity tyrosine phosphorylation-regulated kinase family kinase Pom1 at the cell poles. In this study, we show that Pom1 directly phosphorylates the F-BAR protein Cdc15, a central component of the cytokinetic ring. Pom1-dependent phosphorylation blocks Cdc15 binding to paxillin Pxl1 and C2 domain protein Fic1 and enhances Cdc15 dynamics. This promotes ring sliding from cell poles, which prevents septum assembly at the ends of cells with a displaced nucleus or lacking Mid1. Pom1 also slows down ring constriction. These results indicate that a strong negative signal from the Pom1 kinase at cell poles converts Cdc15 to its closed state, destabilizes the actomyosin ring, and thus promotes medial septation.

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.

Three myosins contribute uniquely to the assembly and constriction of the fission yeast cytokinetic contractile ring

Cytokinesis in fission yeast cells depends on conventional myosin-II (Myo2) to assemble and constrict a contractile ring of actin filaments. Less is known about the functions of an unconventional myosin-II (Myp2) and a myosin-V (Myo51) that are also present in the contractile ring. Myo2 appears in cytokinetic nodes around the equator 10 min before spindle pole body separation (cell-cycle time, -10 min) independent of actin filaments, followed by Myo51 at time zero and Myp2 at time +20 min, both located between nodes and dependent on actin filaments. We investigated the contributions of these three myosins to cytokinesis using a severely disabled mutation of the essential myosin-II heavy-chain gene (myo2-E1) and deletion mutations of the other myosin heavy-chain genes. Cells with only Myo2 assemble contractile rings normally. Cells with either Myp2 or Myo51 alone can assemble nodes and actin filaments into contractile rings but complete assembly later than normal. Both Myp2 and Myo2 contribute to constriction of fully assembled rings at rates 55% that of normal in cells relying on Myp2 alone and 25% that of normal in cells with Myo2 alone. Myo51 alone cannot constrict rings but increases the constriction rate by Myo2 in Δmyp2 cells or Myp2 in myo2-E1 cells. Three myosins function in a hierarchal, complementary manner to accomplish cytokinesis, with Myo2 and Myo51 taking the lead during contractile ring assembly and Myp2 making the greatest contribution to constriction.

A formin-nucleated actin aster concentrates cell wall hydrolases for cell fusion in fission yeast

The formin Fus1 nucleates a novel actin structure in fission yeast, named the actin fusion focus, which consists of an aster of actin filaments whose barbed ends are focalized at a membrane proximal site and serves to focalize cell wall hydrolase delivery for cell fusion.

Cell–cell fusion is essential for fertilization. For fusion of walled cells, the cell wall must be degraded at a precise location but maintained in surrounding regions to protect against lysis. In fission yeast cells, the formin Fus1, which nucleates linear actin filaments, is essential for this process. In this paper, we show that this formin organizes a specific actin structure—the actin fusion focus. Structured illumination microscopy and live-cell imaging of Fus1, actin, and type V myosins revealed an aster of actin filaments whose barbed ends are focalized near the plasma membrane. Focalization requires Fus1 and type V myosins and happens asynchronously always in the M cell first. Type V myosins are essential for fusion and concentrate cell wall hydrolases, but not cell wall synthases, at the fusion focus. Thus, the fusion focus focalizes cell wall dissolution within a broader cell wall synthesis zone to shift from cell growth to cell fusion.

Myosin motor isoforms direct specification of actomyosin function by tropomyosins

Myosins and tropomyosins represent two cytoskeletal proteins that often work together with actin filaments in contractile and motile cellular processes. While the specialized role of tropomyosin in striated muscle myosin-II regulation is well characterized, its role in nonmuscle myosin regulation is poorly understood. We previously showed that fission yeast tropomyosin (Cdc8p) positively regulates myosin-II (Myo2p) and myosin-V (Myo52p) motors. To understand the broader implications of this regulation we examined the role of two mammalian tropomyosins (Tpm3.1cy/Tm5NM1 and Tpm4.2cy/Tm4) recently implicated in cancer cell proliferation and metastasis. Like Cdc8p, the Tpm3.1cy and Tpm4.2cy isoforms significantly enhance Myo2p and Myo52p motor activity, converting nonprocessive Myo52p molecules into processive motors that can walk along actin tracks as single molecules. In contrast to the positive regulation of Myo2p and Myo52p, Cdc8p and the mammalian tropomyosins potently inhibited skeletal muscle myosin-II, while having negligible effects on the highly processive mammalian myosin-Va. In support of a conserved role for certain tropomyosins in regulating nonmuscle actomyosin structures, Tpm3.1cy supported normal contractile ring function in fission yeast. Our work reveals that actomyosin regulation by tropomyosin is dependent on the myosin isoform, highlighting a general role for specific isoforms of tropomyosin in sorting myosin motor outputs.

Rapid glucose depletion immobilizes active myosin V on stabilized actin cables

Polarization of eukaryotic cells requires organelles and protein complexes to be transported to their proper destinations along the cytoskeleton. When nutrients are abundant, budding yeast grows rapidly transporting secretory vesicles for localized growth and actively segregating organelles. This is mediated by myosin Vs transporting cargos along F-actin bundles known as actin cables. Actin cables are dynamic structures regulated by assembly, stabilization, and disassembly. Polarized growth and actin filament dynamics consume energy. For most organisms, glucose is the preferred energy source and generally represses alternative carbon source usage. Thus, upon abrupt glucose depletion, yeast shuts down pathways consuming large amounts of energy, including the vacuolar-ATPase, translation, and phosphoinositide metabolism. Here we show that glucose withdrawal rapidly (<1 min) depletes ATP levels and that the yeast myosin V, Myo2, responds by relocalizing to actin cables, making it the fastest response documented. Myo2 immobilized on cables releases its secretory cargo, defining a new rigor-like state of a myosin V in vivo. Only actively transporting Myo2 can be converted to the rigor-like state. Glucose depletion has differential effects on the actin cytoskeleton, resulting in disassembly of actin patches with concomitant inhibition of endocytosis and strong stabilization of actin cables, thereby revealing a selective and previously unappreciated ATP requirement for actin cable disassembly. A similar response is seen in HeLa cells to ATP depletion. These findings reveal a new fast-acting energy conservation strategy halting growth by immobilizing myosin V in a newly described state on selectively stabilized actin cables.

Actin cable distribution and dynamics arising from cross-linking, motor pulling, and filament turnover

A computational model of actin cables in fission yeast is presented that includes polymerization, severing, cross-linking, and motor pulling. Results reproduce observations in wild-type cells and cells lacking myosin V and are compared to images of cells overexpressing α-actinin. Formin clustering at cell tips is predicted to promote cable formation.

The growth of fission yeast relies on the polymerization of actin filaments nucleated by formin For3p, which localizes at tip cortical sites. These actin filaments bundle to form actin cables that span the cell and guide the movement of vesicles toward the cell tips. A big challenge is to develop a quantitative understanding of these cellular actin structures. We used computer simulations to study the spatial and dynamical properties of actin cables. We simulated individual actin filaments as semiflexible polymers in three dimensions composed of beads connected with springs. Polymerization out of For3p cortical sites, bundling by cross-linkers, pulling by type V myosin, and severing by cofilin are simulated as growth, cross-linking, pulling, and turnover of the semiflexible polymers. With the foregoing mechanisms, the model generates actin cable structures and dynamics similar to those observed in live-cell experiments. Our simulations reproduce the particular actin cable structures in myoVΔ cells and predict the effect of increased myosin V pulling. Increasing cross-linking parameters generates thicker actin cables. It also leads to antiparallel and parallel phases with straight or curved cables, consistent with observations of cells overexpressing α-actinin. Finally, the model predicts that clustering of formins at cell tips promotes actin cable formation.

The novel proteins Rng8 and Rng9 regulate the myosin-V Myo51 during fission yeast cytokinesis

The myosin-V family of molecular motors is known to be under sophisticated regulation, but our knowledge of the roles and regulation of myosin-Vs in cytokinesis is limited. Here, we report that the myosin-V Myo51 affects contractile ring assembly and stability during fission yeast cytokinesis, and is regulated by two novel coiled-coil proteins, Rng8 and Rng9. Both rng8Δ and rng9Δ cells display similar defects as myo51Δ in cytokinesis. Rng8 and Rng9 are required for Myo51's localizations to cytoplasmic puncta, actin cables, and the contractile ring. Myo51 puncta contain multiple Myo51 molecules and walk continuously on actin filaments in rng8(+) cells, whereas Myo51 forms speckles containing only one dimer and does not move efficiently on actin tracks in rng8Δ. Consistently, Myo51 transports artificial cargos efficiently in vivo, and this activity is regulated by Rng8. Purified Rng8 and Rng9 form stable higher-order complexes. Collectively, we propose that Rng8 and Rng9 form oligomers and cluster multiple Myo51 dimers to regulate Myo51 localization and functions.

The yeast actin cytoskeleton

The actin cytoskeleton is a complex network of dynamic polymers, which plays an important role in various fundamental cellular processes, including maintenance of cell shape, polarity, cell division, cell migration, endocytosis, vesicular trafficking, and mechanosensation. Precise spatiotemporal assembly and disassembly of actin structures is regulated by the coordinated activity of about 100 highly conserved accessory proteins, which nucleate, elongate, cross-link, and sever actin filaments. Both in vivo studies in a wide range of organisms from yeast to metazoans and in vitro studies of purified proteins have helped shape the current understanding of actin dynamics and function. Molecular genetics, genome-wide functional analysis, sophisticated real-time imaging, and ultrastructural studies in concert with biochemical analysis have made yeast an attractive model to understand the actin cytoskeleton, its molecular dynamics, and physiological function. Studies of the yeast actin cytoskeleton have contributed substantially in defining the universal mechanism regulating actin assembly and disassembly in eukaryotes. Here, we review some of the important insights generated by the study of actin cytoskeleton in two important yeast models the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe.

Getting myosin-V on the right track: tropomyosin sorts transport in yeast

Recent studies have revealed a novel mechanism of myosin regulation in which the actin-binding protein tropomyosin converts atypical type-V myosins into processive cargo transporters. To achieve this, tropomyosin's primary role appears to lie in its ability to influence myosin's enzyme kinetics, prolonging the strong actin-bound ADP/apo state to enable hand-over-hand walking of myosin-V dimers along actin tracks. Activation of myosin-V mediated transport by tropomyosin underscores its function in helping to direct cargos to specific actin tracks and subcellular destinations. This type of regulation supports the broader notion that tropomyosin plays a key role in actomyosin sorting.

Understanding myosin functions in plants: are we there yet?

Myosins are motor proteins that drive movements along actin filaments and have long been assumed to be responsible for cytoplasmic streaming in plant cells. This conjecture is now firmly established by genetic analysis in the reference species, Arabidopsis thaliana. This work and similar approaches in the moss, Physcomitrella patens, also established that myosin-driven movements are necessary for cell growth and polarity, organelle distribution and shape, and actin organization and dynamics. Identification of a mechanistic link between intracellular movements and cell expansion has proven more challenging, not the least because of the high level of apparent genetic redundancy among myosin family members. Recent progress in the creation of functional complementation constructs and identification of interaction partners promises a way out of this dilemma.

Fission yeast tropomyosin specifies directed transport of myosin-V along actin cables

Fission yeast tropomyosin targets myosin-V to actin cables by favoring processivity of the motor. Live-cell imaging is used to estimate the number of myosin-V molecules per motile particle in vivo. In vitro reconstitution demonstrates the physiological relevance of tropomyosin-based targeting of this motor.

A hallmark of class-V myosins is their processivity—the ability to take multiple steps along actin filaments without dissociating. Our previous work suggested, however, that the fission yeast myosin-V (Myo52p) is a nonprocessive motor whose activity is enhanced by tropomyosin (Cdc8p). Here we investigate the molecular mechanism and physiological relevance of tropomyosin-mediated regulation of Myo52p transport, using a combination of in vitro and in vivo approaches. Single molecules of Myo52p, visualized by total internal reflection fluorescence microscopy, moved processively only when Cdc8p was present on actin filaments. Small ensembles of Myo52p bound to a quantum dot, mimicking the number of motors bound to physiological cargo, also required Cdc8p for continuous motion. Although a truncated form of Myo52p that lacked a cargo-binding domain failed to support function in vivo, it still underwent actin-dependent movement to polarized growth sites. This result suggests that truncated Myo52p lacking cargo, or single molecules of wild-type Myo52p with small cargoes, can undergo processive movement along actin-Cdc8p cables in vivo. Our findings outline a mechanism by which tropomyosin facilitates sorting of transport to specific actin tracks within the cell by switching on myosin processivity.

Myosin XIK of Arabidopsis thaliana accumulates at the root hair tip and is required for fast root hair growth

Myosin motor proteins are thought to carry out important functions in the establishment and maintenance of cell polarity by moving cellular components such as organelles, vesicles, or protein complexes along the actin cytoskeleton. In Arabidopsis thaliana, disruption of the myosin XIK gene leads to reduced elongation of the highly polar root hairs, suggesting that the encoded motor protein is involved in this cell growth. Detailed live-cell observations in this study revealed that xik root hairs elongated more slowly and stopped growth sooner than those in wild type. Overall cellular organization including the actin cytoskeleton appeared normal, but actin filament dynamics were reduced in the mutant. Accumulation of RabA4b-containing vesicles, on the other hand, was not significantly different from wild type. A functional YFP-XIK fusion protein that could complement the mutant phenotype accumulated at the tip of growing root hairs in an actin-dependent manner. The distribution of YFP-XIK at the tip, however, did not match that of the ER or several tip-enriched markers including CFP-RabA4b. We conclude that the myosin XIK is required for normal actin dynamics and plays a role in the subapical region of growing root hairs to facilitate optimal growth.