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

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.

Inducible transcriptional condensates drive 3D genome reorganization in the heat shock response

Mammalian developmental and disease-associated genes concentrate large quantities of the transcriptional machinery by forming membrane-less compartments known as transcriptional condensates. However, it is unknown whether these structures are evolutionarily conserved or involved in 3D genome reorganization. Here, we identify inducible transcriptional condensates in the yeast heat shock response (HSR). HSR condensates are biophysically dynamic spatiotemporal clusters of the sequence-specific transcription factor heat shock factor 1 (Hsf1) with Mediator and RNA Pol II. Uniquely, HSR condensates drive the coalescence of multiple Hsf1 target genes, even those located on different chromosomes. Binding of the chaperone Hsp70 to a site on Hsf1 represses clustering, whereas an intrinsically disordered region on Hsf1 promotes condensate formation and intergenic interactions. Mutation of both Hsf1 determinants reprograms HSR condensates to become constitutively active without intergenic coalescence, which comes at a fitness cost. These results suggest that transcriptional condensates are ancient and flexible compartments of eukaryotic gene control.

Simulated actin reorganization mediated by motor proteins

Cortical actin networks are highly dynamic and play critical roles in shaping the mechanical properties of cells. The actin cytoskeleton undergoes significant reorganization in many different contexts, including during directed cell migration and over the course of the cell cycle, when cortical actin can transition between different configurations such as open patched meshworks, homogeneous distributions, and aligned bundles. Several types of myosin motor proteins, characterized by different kinetic parameters, have been involved in this reorganization of actin filaments. Given the limitations in studying the interactions of actin with myosin in vivo, we propose stochastic agent-based models and develop a set of data analysis measures to assess how myosin motor proteins mediate various actin organizations. In particular, we identify individual motor parameters, such as motor binding rate and step size, that generate actin networks with different levels of contractility and different patterns of myosin motor localization, which have previously been observed experimentally. In simulations where two motor populations with distinct kinetic parameters interact with the same actin network, we find that motors may act in a complementary way, by tuning the actin network organization, or in an antagonistic way, where one motor emerges as dominant. This modeling and data analysis framework also uncovers parameter regimes where spatial segregation between motor populations is achieved. By allowing for changes in kinetic rates during the actin-myosin dynamic simulations, our work suggests that certain actin-myosin organizations may require additional regulation beyond mediation by motor proteins in order to reconfigure the cytoskeleton network on experimentally-observed timescales.

Cell shape is dictated by a scaffolding network called the cytoskeleton. Actin filaments, a main component of the cytoskeleton, are found predominantly at the periphery of the cell, where they organize into different patterns in response to various stimuli, such as progression through the cell cycle. The actin filament reorganizations are mediated by motor proteins from the myosin superfamily. Using a realistic stochastic model that simulates actin filament and motor protein dynamics and interactions, we systematically vary motor protein kinetics and investigate their effect on actin filament organization. Using novel measures of spatial organization, we quantify conditions under which motor proteins, either alone or in combination, can produce the different actin filament organizations observed in vitro and in vivo. These results yield new insights into the role of motor proteins, as well as into how multiple types of motors can work collectively to produce specific actomyosin network patterns.

Contractile ring constriction and septation in fission yeast are integrated mutually stabilizing processes.. [DOI: 10.1101/2021.06.25.449700] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In common with other cellular machineries, the actomyosin contractile ring that divides cells during cytokinesis does not operate in isolation. Contractile rings in animal cells interact with contiguous actomyosin cortex, while ring constriction in many cell-walled organisms couples tightly to cell wall growth. In fission yeast, a septum grows in the wake of the constricting ring, ensuring cytokinesis leaves two daughter cells fully enclosed by cell wall. Here we mathematical modeled the integrated constriction-septation system in fission yeast, with a kinetic growth model evolving the 3D septum shape coupled to a molecularly explicit simulation of the contractile ring highly constrained by experimental data. Simulations revealed influences in both directions, stabilizing the ring-septum system as a whole. By providing a smooth circular anchoring surface for the ring, the inner septum leading edge stabilized ring organization and tension production; by mechanically regulating septum circularity and in-plane growth, ring tension stabilized septum growth and shape. Genetic or pharmacological perturbation of either subsystem destabilized this delicate balance, precipitating uncontrolled positive feedback with disastrous morphological and functional consequences. Thus, high curvature septum irregularities triggered bridging instabilities, in which contractile ring segments became unanchored. Bridging abolished the local tension-mediated septum shape regulation, exacerbating the irregularity in a mutually destabilizing runaway process. Our model explains a number of previously mysterious experimental observations, including unanchoring of ring segments observed in cells with mutations in the septum-growing β-glucan synthases, and irregular septa in cells with mutations in the contractile ring myosin-II Myo2. Thus, the contractile ring and cell wall growth cellular machineries operate as a single integrated system, whose stability relies on mutual regulation by the two subsystems.

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.

A nuclear-based quality control pathway for non-imported mitochondrial proteins

Mitochondrial import deficiency causes cellular toxicity due to the accumulation of non-imported mitochondrial precursor proteins, termed mitoprotein-induced stress. Despite the burden mis-localized mitochondrial precursors place on cells, our understanding of the systems that dispose of these proteins is incomplete. Here, we cataloged the location and steady-state abundance of mitochondrial precursor proteins during mitochondrial impairment in Saccharomyces cerevisiae. We found that a number of non-imported mitochondrial proteins localize to the nucleus, where they are subjected to proteasome-dependent degradation through a process we term nuclear-associated mitoprotein degradation (mitoNUC). Recognition and destruction of mitochondrial precursors by the mitoNUC pathway requires the presence of an N-terminal mitochondrial targeting sequence and is mediated by combined action of the E3 ubiquitin ligases San1, Ubr1, and Doa10. Impaired breakdown of precursors leads to alternative sequestration in nuclear-associated foci. These results identify the nucleus as an important destination for the disposal of non-imported mitochondrial precursors.

Mechanism of membrane-curvature generation by ER-tubule shaping proteins

The endoplasmic reticulum (ER) network consists of tubules with high membrane curvature in cross-section, generated by the reticulons and REEPs. These proteins have two pairs of trans-membrane (TM) segments, followed by an amphipathic helix (APH), but how they induce curvature is poorly understood. Here, we show that REEPs form homodimers by interaction within the membrane. When overexpressed or reconstituted at high concentrations with phospholipids, REEPs cause extreme curvature through their TMs, generating lipoprotein particles instead of vesicles. The APH facilitates curvature generation, as its mutation prevents ER network formation of reconstituted proteoliposomes, and synthetic L- or D-amino acid peptides abolish ER network formation in Xenopus egg extracts. In Schizosaccharomyces japonicus, the APH is required for reticulon’s exclusive ER-tubule localization and restricted mobility. Thus, the TMs and APH cooperate to generate high membrane curvature. We propose that the formation of splayed REEP/reticulon dimers is responsible for ER tubule formation.

The endoplasmic reticulum network consists of tubules with high membrane curvature in cross-section, generated by the reticulons and REEPs, but how they introduce curvature is poorly understood. Here authors show that REEPs form homodimers and use their amphipathic helix and trans-membrane segments to introduce high membrane curvature that can even lead to the formation of lipoprotein particles.

ER targeting of non-imported mitochondrial carrier proteins is dependent on the GET pathway

The GET pathway is required to target non-imported mitochondrial carrier proteins to the endoplasmic reticulum, which prevents their deposition into Hsp42-dependent protein foci.

Deficiencies in mitochondrial import cause the toxic accumulation of non-imported mitochondrial precursor proteins. Numerous fates for non-imported mitochondrial precursors have been identified in budding yeast, including proteasomal destruction, deposition into protein aggregates, and mistargeting to other organelles. Amongst organelles, the ER has emerged as a key destination for a subset of non-imported mitochondrial proteins. However, how ER targeting of various types of mitochondrial proteins is achieved remains incompletely understood. Here, we show that the ER delivery of endogenous mitochondrial transmembrane proteins, especially those belonging to the SLC25A mitochondrial carrier family, is dependent on the guided entry of tail-anchored proteins (GET) complex. Without a functional GET pathway, non-imported mitochondrial proteins destined for the ER are alternatively sequestered into Hsp42-dependent protein foci. Loss of the GET pathway is detrimental to yeast cells experiencing mitochondrial import failure and prevents re-import of mitochondrial proteins from the ER via the ER-SURF pathway. Overall, this study outlines an important role for the GET complex in ER targeting of non-imported mitochondrial carrier proteins.

Microtubule nucleation promoters Mto1 and Mto2 regulate cytokinesis in fission yeast

Microtubules of the mitotic spindle direct cytokinesis in metazoans but this has not been documented in fungi. We report evidence that microtubule nucleators at the spindle pole body help coordinate cytokinetic furrow formation in fission yeast. The temperature-sensitive cps1-191 strain (Liu et al., 1999) with a D277N substitution in β-glucan synthase 1 (Cps1/Bgs1) was reported to arrest with an unconstricted contractile ring. We discovered that contractile rings in cps1-191 cells constrict slowly and that an mto2S338N mutation is required with the bgs1D277Nmutation to reproduce the cps1-191 phenotype. Complexes of Mto2 and Mto1 with γ-tubulin regulate microtubule assembly. Deletion of Mto1 along with the bgs1D277N mutation also gives the cps1-191 phenotype, which is not observed in mto2S338N or mto1Δ cells expressing bgs1+. Both mto2S338N and mto1Δ cells nucleate fewer astral microtubules than normal and have higher levels of Rho1-GTP at the division site than wild-type cells. We report multiple conditions that sensitize mto1Δ and mto2S338N cells to furrow ingression phenotypes.

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.

Molecular Mechanism of Cytokinesis

Division of amoebas, fungi, and animal cells into two daughter cells at the end of the cell cycle depends on a common set of ancient proteins, principally actin filaments and myosin-II motors. Anillin, formins, IQGAPs, and many other proteins regulate the assembly of the actin filaments into a contractile ring positioned between the daughter nuclei by different mechanisms in fungi and animal cells. Interactions of myosin-II with actin filaments produce force to assemble and then constrict the contractile ring to form a cleavage furrow. Contractile rings disassemble as they constrict. In some cases, knowledge about the numbers of participating proteins and their biochemical mechanisms has made it possible to formulate molecularly explicit mathematical models that reproduce the observed physical events during cytokinesis by computer simulations.

Molecular Mechanism of Cytokinesis

Division of amoebas, fungi, and animal cells into two daughter cells at the end of the cell cycle depends on a common set of ancient proteins, principally actin filaments and myosin-II motors. Anillin, formins, IQGAPs, and many other proteins regulate the assembly of the actin filaments into a contractile ring positioned between the daughter nuclei by different mechanisms in fungi and animal cells. Interactions of myosin-II with actin filaments produce force to assemble and then constrict the contractile ring to form a cleavage furrow. Contractile rings disassemble as they constrict. In some cases, knowledge about the numbers of participating proteins and their biochemical mechanisms has made it possible to formulate molecularly explicit mathematical models that reproduce the observed physical events during cytokinesis by computer simulations.

Actin turnover ensures uniform tension distribution during cytokinetic actomyosin ring contraction

In many eukaryotes, cytokinesis is facilitated by the contraction of an actomyosin ring (AMR). The exact mechanisms that lead to this contractility are unknown, although some models posit that actin turnover in the AMR is essential. The effect of reduced actin dynamics during AMR formation has been well studied in Schizosaccharomyces pombe; however, the corresponding effects on AMR contraction are not well understood. By using mutants of the fission yeast actin severing protein Adf1, we observed that contracting AMRs display a "peeling" phenotype, where bundles of actin and myosin peel off from one side of the AMR, and are pulled across to the opposite side. This occurs multiple times during cytokinesis and is dependent on the activity of myosins Myo2, Myp2, and Myo51. We found that the distribution of Myo2 in the AMR anticorrelates with the location of peeling events, suggesting that peeling is caused by a nonuniform tension distribution around the AMR, and that one of the roles of actin turnover is to maintain a uniform tension distribution around the AMR.

The adaptor protein melanophilin regulates dynamic myosin-Va:cargo interaction and dendrite development in melanocytes

The regulation of organelle transport by the cytoskeleton is fundamental for eukaryotic survival. Cytoskeleton motors are typically modular proteins with conserved motor and diverse cargo-binding domains. Motor:cargo interactions are often indirect and mediated by adaptor proteins, for example, Rab GTPases. Rab27a, via effector melanophilin (Mlph), recruits myosin-Va (MyoVa) to melanosomes and thereby disperses them into melanocyte dendrites. To better understand how adaptors regulate motor:cargo interaction, we used single melanosome fluorescence recovery after photobleaching (smFRAP) to characterize the association kinetics among MyoVa, its adaptors, and melanosomes. We found that MyoVa and Mlph rapidly recovered after smFRAP, whereas Rab27a did not, indicating that MyoVa and Mlph dynamically associate with melanosomes and Rab27a does not. This suggests that dynamic Rab27a:effector interaction rather than Rab27a melanosome:cytosol cycling regulates MyoVa:melanosome association. Accordingly, a Mlph-Rab27a fusion protein reduced MyoVa smFRAP, indicating that it stabilized melanosomal MyoVa. Finally, we tested the functional importance of dynamic MyoVa:melanosome interaction. We found that whereas a MyoVa-Rab27a fusion protein dispersed melanosomes in MyoVa-deficient cells, dendrites were significantly less elongated than in wild-type cells. Given that dendrites are the prime sites of melanosome transfer from melanocytes to keratinocytes, we suggest that dynamic MyoVa:melanosome interaction is important for pigmentation in vivo.

Roles of the fission yeast UNC-13/Munc13 protein Ync13 in late stages of cytokinesis

Cytokinesis is a complicated yet conserved step of the cell-division cycle that requires the coordination of multiple proteins and cellular processes. Here we describe a previously uncharacterized protein, Ync13, and its roles during fission yeast cytokinesis. Ync13 is a member of the UNC-13/Munc13 protein family, whose animal homologues are essential priming factors for soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex assembly during exocytosis in various cell types, but no roles in cytokinesis have been reported. We find that Ync13 binds to lipids in vitro and dynamically localizes to the plasma membrane at cell tips during interphase and at the division site during cytokinesis. Deletion of Ync13 leads to defective septation and exocytosis, uneven distribution of cell-wall enzymes and components of cell-wall integrity pathway along the division site and massive cell lysis during cell separation. Interestingly, loss of Ync13 compromises endocytic site selection at the division plane. Collectively, we find that Ync13 has a novel function as an UNC-13/Munc13 protein in coordinating exocytosis, endocytosis, and cell-wall integrity during fission yeast cytokinesis.

Regulation of class V myosin

Class V myosin (myosin-5) is a molecular motor that functions as an organelle transporter. The activation of myosin-5's motor function has long been known to be associated with a transition from the folded conformation in the off-state to the extended conformation in the on-state, but only recently have we begun to understand the underlying mechanism. The globular tail domain (GTD) of myosin-5 has been identified as the inhibitory domain and has recently been shown to function as a dimer in regulating the motor function. The folded off-state of myosin-5 is stabilized by multiple intramolecular interactions, including head-GTD interactions, GTD-GTD interactions, and interactions between the GTD and the C-terminus of the first coiled-coil segment. Any cellular factor that affects these intramolecular interactions and thus the stability of the folded conformation of myosin-5 would be expected to regulate myosin-5 motor function. Both the adaptor proteins of myosin-5 and Ca2+ are potential regulators of myosin-5 motor function, because they can destabilize its folded conformation. A combination of these regulators provides a versatile scheme in regulating myosin-5 motor function in the cell.

Nine unanswered questions about cytokinesis

Experiments on model systems have revealed that cytokinesis in cells with contractile rings (amoebas, fungi, and animals) depends on shared molecular mechanisms in spite of some differences that emerged during a billion years of divergent evolution. Understanding these fundamental mechanisms depends on identifying the participating proteins and characterizing the mechanisms that position the furrow, assemble the contractile ring, anchor the ring to the plasma membrane, trigger ring constriction, produce force to form a furrow, disassemble the ring, expand the plasma membrane in the furrow, and separate the daughter cell membranes. This review reveals that fascinating questions remain about each step.

Phosphoinositide-mediated ring anchoring resists perpendicular forces to promote medial cytokinesis

Altering phosphoinositide composition through deletion of efr3, a PI4 kinase scaffold, results in type V myosin-dependent cytokinetic ring sliding in Schizosaccharomyces pombe. Membrane-binding proteins contribute to ring anchoring to resist perpendicular forces and thereby promote medial cytokinesis.

Many eukaryotic cells divide by assembling and constricting an actin- and myosin-based contractile ring (CR) that is physically linked to the plasma membrane (PM). In this study, we report that Schizosaccharomyces pombe cells lacking efr3, which encodes a conserved PM scaffold for the phosphatidylinositol-4 kinase Stt4, build CRs that can slide away from the cell middle during anaphase in a myosin V–dependent manner. The Efr3-dependent CR-anchoring mechanism is distinct from previously reported pathways dependent on the Fes/CIP4 homology Bin-Amphiphysin-Rvs167 (F-BAR) protein Cdc15 and paxillin Pxl1. In efr3Δ, the concentrations of several membrane-binding proteins were reduced in the CR and/or on the PM. Our results suggest that proper PM lipid composition is important to stabilize the central position of the CR and resist myosin V–based forces to promote the fidelity of cell division.

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.

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.

Roles of the novel coiled-coil protein Rng10 in septum formation during fission yeast cytokinesis

The regulation of Rho-GAP localization is not well understood. A novel coiled-coil protein Rng10 is characterized that localizes the Rho-GAP Rga7 in fission yeast. Rng10 and Rga7 physically interact and work together to regulate the accumulation and dynamics of glucan synthases for successful septum formation during cytokinesis.

Rho GAPs are important regulators of Rho GTPases, which are involved in various steps of cytokinesis and other processes. However, regulation of Rho-GAP cellular localization and function is not fully understood. Here we report the characterization of a novel coiled-coil protein Rng10 and its relationship with the Rho-GAP Rga7 in fission yeast. Both rng10Δ and rga7Δ result in defective septum and cell lysis during cytokinesis. Rng10 and Rga7 colocalize on the plasma membrane at the cell tips during interphase and at the division site during cell division. Rng10 physically interacts with Rga7 in affinity purification and coimmunoprecipitation. Of interest, Rga7 localization is nearly abolished without Rng10. Moreover, Rng10 and Rga7 work together to regulate the accumulation and dynamics of glucan synthases for successful septum formation in cytokinesis. Our results show that cellular localization and function of the Rho-GAP Rga7 are regulated by a novel protein, Rng10, during cytokinesis in fission yeast.

The Activity-Dependent Regulation of Protein Kinase Stability by the Localization to P-Bodies

The eukaryotic cytoplasm contains a variety of ribonucleoprotein (RNP) granules in addition to the better-understood membrane-bound organelles. These granules form in response to specific stress conditions and contain a number of signaling molecules important for the control of cell growth and survival. However, relatively little is known about the mechanisms responsible for, and the ultimate consequences of, this protein localization. Here, we show that the Hrr25/CK1δ protein kinase is recruited to cytoplasmic processing bodies (P-bodies) in an evolutionarily conserved manner. This recruitment requires Hrr25 kinase activity and the Dcp2 decapping enzyme, a core constituent of these RNP granules. Interestingly, the data indicate that this localization sequesters active Hrr25 away from the remainder of the cytoplasm and thereby shields this enzyme from the degradation machinery during these periods of stress. Altogether, this work illustrates how the presence within an RNP granule can alter the ultimate fate of the localized protein.

Roles of the TRAPP-II Complex and the Exocyst in Membrane Deposition during Fission Yeast Cytokinesis

The cleavage-furrow tip adjacent to the actomyosin contractile ring is believed to be the predominant site for plasma-membrane insertion through exocyst-tethered vesicles during cytokinesis. Here we found that most secretory vesicles are delivered by myosin-V on linear actin cables in fission yeast cytokinesis. Surprisingly, by tracking individual exocytic and endocytic events, we found that vesicles with new membrane are deposited to the cleavage furrow relatively evenly during contractile-ring constriction, but the rim of the cleavage furrow is the main site for endocytosis. Fusion of vesicles with the plasma membrane requires vesicle tethers. Our data suggest that the transport particle protein II (TRAPP-II) complex and Rab11 GTPase Ypt3 help to tether secretory vesicles or tubulovesicular structures along the cleavage furrow while the exocyst tethers vesicles at the rim of the division plane. We conclude that the exocyst and TRAPP-II complex have distinct localizations at the division site, but both are important for membrane expansion and exocytosis during cytokinesis.

Two putative vesicle tethers—the exocyst and TRAPP-II complexes—localize differently at the division plane to ensure efficient plasma-membrane deposition along the whole cleavage furrow during cytokinesis in the fission yeast Schizosaccharomyces pombe.

Cytokinesis partitions a mother cell into two daughter cells at the end of each cell-division cycle. A significant amount of new plasma membrane is needed at the cleavage furrow during cytokinesis in many cell types. Membrane expansion is achieved through the balance of exocytosis and endocytosis. It is poorly understood where and when the membrane is deposited and retrieved during cytokinesis. By tracking individual vesicles with high spatiotemporal resolution and using electron microscopy, we found that new membrane is deposited relatively evenly along the cleavage furrow in fission yeast, while the rim of the division plane is the predominant site for endocytosis. The secretory vesicles/compartments carrying new membrane are mainly delivered along formin-nucleated actin cables by myosin-V motors. Surprisingly, we find that both exocytosis and endocytosis at the division site are ramped up before contractile-ring constriction and last until daughter-cell separation. We discovered that two putative vesicle tethers, the exocyst and TRAPP-II complexes, localize to different sites at the cleavage furrow to promote tethering of different, yet overlapping, classes of secretory vesicles/compartments for exocytosis and new membrane deposition.

Real-Time Visualization and Quantification of Contractile Ring Proteins in Single Living Cells

Single-cell microscopy provides a powerful tool to visualize cellular and subcellular processes in wild-type and mutant cells by observing fluorescently tagged proteins. Here, we describe three simple methods to visualize fission yeast cells: gelatin slides, coverslip-bottom dishes, and tetrad fluorescence microscopy. These imaging methods and data analysis using free software make it possible to quantify protein localization, dynamics, and concentration with high spatial and temporal resolution. In fission yeast, the actomyosin contractile ring is essential for cytokinesis. We use the visualization and quantification of contractile ring proteins as an example to demonstrate how to use these methods.

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.

The F-BAR Cdc15 promotes contractile ring formation through the direct recruitment of the formin Cdc12

Cdc15 contributes to contractile ring formation and cytokinesis by recruiting the formin Cdc12, which defines a novel cytokinetic function for an F-BAR domain.

In Schizosaccharomyces pombe, cytokinesis requires the assembly and constriction of an actomyosin-based contractile ring (CR). Nucleation of F-actin for the CR requires a single formin, Cdc12, that localizes to the cell middle at mitotic onset. Although genetic requirements for formin Cdc12 recruitment have been determined, the molecular mechanisms dictating its targeting to the medial cortex during cytokinesis are unknown. In this paper, we define a short motif within the N terminus of Cdc12 that binds directly to the F-BAR domain of the scaffolding protein Cdc15. Mutations preventing the Cdc12–Cdc15 interaction resulted in reduced Cdc12, F-actin, and actin-binding proteins at the CR, which in turn led to a delay in CR formation and sensitivity to other perturbations of CR assembly. We conclude that Cdc15 contributes to CR formation and cytokinesis via formin Cdc12 recruitment, defining a novel cytokinetic function for an F-BAR domain.

Regulation of Rho-GEF Rgf3 by the arrestin Art1 in fission yeast cytokinesis

The arrestin Art1 and the Rho1 guanine nucleotide exchange factor Rgf3 are interdependent for their localizations to the division site during fission yeast cytokinesis. Art1 physically interacts with Rgf3 to modulate active Rho1 GTPase levels for successful septal formation.

Rho GTPases, activated by guanine nucleotide exchange factors (GEFs), are essential regulators of polarized cell growth, cytokinesis, and many other cellular processes. However, the regulation of Rho-GEFs themselves is not well understood. Rgf3 is an essential GEF for Rho1 GTPase in fission yeast. We show that Rgf3 protein levels and localization are regulated by arrestin-related protein Art1. art1∆ cells lyse during cell separation with a thinner and defective septum. As does Rgf3, Art1 concentrates to the contractile ring starting at early anaphase and spreads to the septum during and after ring constriction. Art1 localization depends on its C-terminus, and Art1 is important for maintaining Rgf3 protein levels. Biochemical experiments reveal that the Rgf3 C-terminus binds to Art1. Using an Rgf3 conditional mutant and mislocalization experiments, we found that Art1 and Rgf3 are interdependent for localization to the division site. As expected, active Rho1 levels at the division site are reduced in art1∆ and rgf3 mutant cells. Taken together, these data reveal that the arrestin family protein Art1 regulates the protein levels and localization of the Rho-GEF Rgf3, which in turn modulates active Rho1 levels during fission yeast cytokinesis.

The Rho-GEF Gef3 interacts with the septin complex and activates the GTPase Rho4 during fission yeast cytokinesis

Rho GTPases, activated by Rho guanine nucleotide exchange factors (GEFs), are conserved molecular switches for signal transductions that regulate diverse cellular processes, including cell polarization and cytokinesis. The fission yeast Schizosaccharomyces pombe has six Rho GTPases (Cdc42 and Rho1-Rho5) and seven Rho GEFs (Scd1, Rgf1-Rgf3, and Gef1-Gef3). The GEFs for Rho2-Rho5 have not been unequivocally assigned. In particular, Gef3, the smallest Rho GEF, was barely studied. Here we show that Gef3 colocalizes with septins at the cell equator. Gef3 physically interacts with septins and anillin Mid2 and depends on them to localize. Gef3 coprecipitates with GDP-bound Rho4 in vitro and accelerates nucleotide exchange of Rho4, suggesting that Gef3 is a GEF for Rho4. Consistently, Gef3 and Rho4 are in the same genetic pathways to regulate septum formation and/or cell separation. In gef3∆ cells, the localizations of two potential Rho4 effectors--glucanases Eng1 and Agn1--are abnormal, and active Rho4 level is reduced, indicating that Gef3 is involved in Rho4 activation in vivo. Moreover, overexpression of active Rho4 or Eng1 rescues the septation defects of mutants containing gef3∆. Together our data support that Gef3 interacts with the septin complex and activates Rho4 GTPase as a Rho GEF for septation in fission yeast.