Comparing contractile apparatus-driven cytokinesis mechanisms across kingdoms

Application of Raman spectroscopy to the evaluation of F-actin changes in sea urchin eggs at fertilization

The actin filaments on the surface of echinoderm oocytes and eggs readily undergo massive reorganization during meiotic maturation and fertilization. In sea urchin eggs, the actin cytoskeletal response to the fertilizing sperm is fast enough to accompany Ca2+ signals and to guide sperm's entry into the egg. Although recent work using live cell imaging technology confirmed changes in the actin polymerization status in fertilized eggs, as was previously shown using light and electron microscopy, it failed to provide experimental evidence of F-actin depolymerization a few seconds after insemination, which is concurrent with the sperm-induced Ca2+ release. In the present study, we applied Raman microspectroscopy to tackle this issue by examining the spectral profiles of the egg's subplasmalemmal regions before and after treating the eggs with actin drugs or fertilizing sperm. At both early (15 s) and late (15 min) time points after fertilization, specific peak shifts in the Raman spectra revealed change in the actin structure, and Raman imaging detected the cytoskeletal changes corresponding to the F-actin reorganization visualized with LifeAct-GFP in confocal microscopy. Our observation suggests that the application of Raman spectroscopy, which does not require microinjection of fluorescent probes and exogenous gene expression, may serve as an alternative or even advantageous method in disclosing rapid subtle changes in the subplasmalemmal actin cytoskeleton that are difficult to resolve.

Toxoplasma gondii's Basal Complex: The Other Apicomplexan Business End Is Multifunctional

The Apicomplexa are famously named for their apical complex, a constellation of organelles at their apical end dedicated to invasion of their host cells. In contrast, at the other end of the cell, the basal complex (BC) has been overshadowed since it is much less prominent and specific functions were not immediately obvious. However, in the past decade a staggering array of functions have been associated with the BC and strides have been made in understanding its structure. Here, these collective insights are supplemented with new data to provide an overview of the understanding of the BC in Toxoplasma gondii. The emerging picture is that the BC is a dynamic and multifunctional complex, with a series of (putative) functions. The BC has multiple roles in cell division: it is the site where building blocks are added to the cytoskeleton scaffold; it exerts a two-step stretch and constriction mechanism as contractile ring; and it is key in organelle division. Furthermore, the BC has numerous putative roles in 'import', such as the recycling of mother cell remnants, the acquisition of host-derived vesicles, possibly the uptake of lipids derived from the extracellular medium, and the endocytosis of micronemal proteins. The latter process ties the BC to motility, whereas an additional role in motility is conferred by Myosin C. Furthermore, the BC acts on the assembly and/or function of the intravacuolar network, which may directly or indirectly contribute to the establishment of chronic tissue cysts. Here we provide experimental support for molecules acting in several of these processes and identify several new BC proteins critical to maintaining the cytoplasmic bridge between divided parasites. However, the dispensable nature of many BC components leaves many questions unanswered regarding its function. In conclusion, the BC in T. gondii is a dynamic and multifunctional structure at the posterior end of the parasite.

Cleavage-furrow formation without F-actin in Chlamydomonas

It is widely believed that cleavage-furrow formation during cytokinesis is driven by the contraction of a ring containing F-actin and type-II myosin. However, even in cells that have such rings, they are not always essential for furrow formation. Moreover, many taxonomically diverse eukaryotic cells divide by furrowing but have no type-II myosin, making it unlikely that an actomyosin ring drives furrowing. To explore this issue further, we have used one such organism, the green alga Chlamydomonas reinhardtii We found that although F-actin is associated with the furrow region, none of the three myosins (of types VIII and XI) is localized there. Moreover, when F-actin was eliminated through a combination of a mutation and a drug, furrows still formed and the cells divided, although somewhat less efficiently than normal. Unexpectedly, division of the large Chlamydomonas chloroplast was delayed in the cells lacking F-actin; as this organelle lies directly in the path of the cleavage furrow, this delay may explain, at least in part, the delay in cytokinesis itself. Earlier studies had shown an association of microtubules with the cleavage furrow, and we used a fluorescently tagged EB1 protein to show that microtubules are still associated with the furrows in the absence of F-actin, consistent with the possibility that the microtubules are important for furrow formation. We suggest that the actomyosin ring evolved as one way to improve the efficiency of a core process for furrow formation that was already present in ancestral eukaryotes.

Organization of associating or crosslinked actin filaments in confinement

A key factor of actin cytoskeleton organization in cells is the interplay between the dynamical properties of actin filaments and cell geometry, which restricts, confines and directs their orientation. Crosslinking interactions among actin filaments, together with geometrical cues and regulatory proteins can give rise to contractile rings in dividing cells and actin rings in neurons. Motivated by recent in vitro experiments, in this work we performed computer simulations to study basic aspects of the interplay between confinement and attractive interactions between actin filaments. We used a spring-bead model and Brownian dynamics to simulate semiflexible actin filaments that polymerize in a confining sphere with a rate proportional to the monomer concentration. We model crosslinking, or attraction through the depletion interaction, implicitly as an attractive short-range potential between filament beads. In confining geometries smaller than the persistence length of actin filaments, we show rings can form by curving of filaments of length comparable to, or longer than the confinement diameter. Rings form for optimal ranges of attractive interactions that exist in between open bundles, irregular loops, aggregated, and unbundled morphologies. The probability of ring formation is promoted by attraction to the confining sphere boundary and decreases for large radii and initial monomer concentrations, in agreement with prior experimental data. The model reproduces ring formation along the flat plane of oblate ellipsoids.

Synthetic cell division via membrane-transforming molecular assemblies

Reproduction, i.e. the ability to produce new individuals from a parent organism, is a hallmark of living matter. Even the simplest forms of reproduction require cell division: attempts to create a designer cell therefore should include a synthetic cell division machinery. In this review, we will illustrate how nature solves this task, describing membrane remodelling processes in general and focusing on bacterial cell division in particular. We discuss recent progress made in their in vitro reconstitution, identify open challenges, and suggest how purely synthetic building blocks could provide an additional and attractive route to creating artificial cell division machineries.

F-actin homeostasis through transcriptional regulation and proteasome-mediated proteolysis

Many organisms possess multiple and often divergent actins whose regulation and roles are not understood in detail. For example, Chlamydomonas reinhardtii has both a conventional actin (IDA5) and a highly divergent one (NAP1); only IDA5 is expressed in normal proliferating cells. We showed previously that the drug latrunculin B (LatB) causes loss of filamentous (F-) IDA5 and strong up-regulation of NAP1, which then provides essential actin function(s) by forming LatB-resistant F-NAP1. RNA-sequencing analyses now show that this up-regulation of NAP1 reflects a broad transcriptional response, much of which depends on three proteins (LAT1, LAT2, and LAT3) identified previously as essential for NAP1 transcription. Many of the LAT-regulated genes contain a putative cis-acting regulatory site, the "LRE motif." The LatB transcriptional program appears to be activated by loss of F-IDA5 and deactivated by formation of F-NAP1, thus forming an F-actin-dependent negative-feedback loop. Multiple genes encoding proteins of the ubiquitin-proteasome system are among those induced by LatB, resulting in rapid degradation of IDA5 (but not NAP1). Our results suggest that IDA5 degradation is functionally important because nonpolymerizable LatB-bound IDA5 interferes with the formation of F-NAP1. The genes for the actin-interacting proteins cofilin and profilin are also induced. Cofilin induction may further the clearance of IDA5 by promoting the scission of F-IDA5, whereas profilin appears to function in protecting monomeric IDA5 from degradation. This multifaceted regulatory system allows rapid and quantitative turnover of F-actin in response to cytoskeletal perturbations and probably also maintains F-actin homeostasis under normal growth conditions.

Evolution of polymer formation within the actin superfamily

While many are familiar with actin as a well-conserved component of the eukaryotic cytoskeleton, it is less often appreciated that actin is a member of a large superfamily of structurally related protein families found throughout the tree of life. Actin-related proteins include chaperones, carbohydrate kinases, and other enzymes, as well as a staggeringly diverse set of proteins that use the energy from ATP hydrolysis to form dynamic, linear polymers. Despite differing widely from one another in filament structure and dynamics, these polymers play important roles in ordering cell space in bacteria, archaea, and eukaryotes. It is not known whether these polymers descended from a single ancestral polymer or arose multiple times by convergent evolution from monomeric actin-like proteins. In this work, we provide an overview of the structures, dynamics, and functions of this diverse set. Then, using a phylogenetic analysis to examine actin evolution, we show that the actin-related protein families that form polymers are more closely related to one another than they are to other nonpolymerizing members of the actin superfamily. Thus all the known actin-like polymers are likely to be the descendants of a single, ancestral, polymer-forming actin-like protein.

Mechanisms of Cytokinesis in Basidiomycetous Yeasts

While mechanisms of cytokinesis exhibit considerable plasticity, it is difficult to precisely define the level of conservation of this essential part of cell division in fungi, as majority of our knowledge is based on ascomycetous yeasts. However, in the last decade more details have been uncovered regarding cytokinesis in the second largest fungal phylum, basidiomycetes, specifically in two yeasts, Cryptococcus neoformans and Ustilago maydis. Based on these findings, and current sequenced genomes, we summarize cytokinesis in basidiomycetous yeasts, indicating features that may be unique to this phylum, species-specific characteristics, as well as mechanisms that may be common to all eukaryotes.

Beyond force generation: Why is a dynamic ring of FtsZ polymers essential for bacterial cytokinesis?

We propose that the essential function of the most highly conserved protein in bacterial cytokinesis, FtsZ, is not to generate a mechanical force to drive cell division. Rather, we suggest that FtsZ acts as a signal-processing hub to coordinate cell wall synthesis at the division septum with a diverse array of cellular processes, ensuring that the cell divides smoothly at the correct time and place, and with the correct septum morphology. Here, we explore how the polymerization properties of FtsZ, which have been widely attributed to force generation, can also be advantageous in this signal processing role. We suggest mechanisms by which FtsZ senses and integrates both mechanical and biochemical signals, and conclude by proposing experiments to investigate how FtsZ contributes to the remarkable spatial and temporal precision of bacterial cytokinesis.

A New Membrane Protein Sbg1 Links the Contractile Ring Apparatus and Septum Synthesis Machinery in Fission Yeast

Cytokinesis in many organisms requires a plasma membrane anchored actomyosin ring, whose contraction facilitates cell division. In yeast and fungi, actomyosin ring constriction is also coordinated with division septum assembly. How the actomyosin ring interacts with the plasma membrane and the plasma membrane-localized septum synthesizing machinery remains poorly understood. In Schizosaccharomyces pombe, an attractive model organism to study cytokinesis, the β-1,3-glucan synthase Cps1p / Bgs1p, an integral membrane protein, localizes to the plasma membrane overlying the actomyosin ring and is required for primary septum synthesis. Through a high-dosage suppressor screen we identified an essential gene, sbg1⁺ (suppressor of beta glucan synthase 1), which suppressed the colony formation defect of Bgs1-defective cps1-191 mutant at higher temperatures. Sbg1p, an integral membrane protein, localizes to the cell ends and to the division site. Sbg1p and Bgs1p physically interact and are dependent on each other to localize to the division site. Loss of Sbg1p results in an unstable actomyosin ring that unravels and slides, leading to an inability to deposit a single contiguous division septum and an important reduction of the β-1,3-glucan proportion in the cell wall, coincident with that observed in the cps1-191 mutant. Sbg1p shows genetic and / or physical interaction with Rga7p, Imp2p, Cdc15p, and Pxl1p, proteins known to be required for actomyosin ring integrity and efficient septum synthesis. This study establishes Sbg1p as a key member of a group of proteins that link the plasma membrane, the actomyosin ring, and the division septum assembly machinery in fission yeast.

Cell division in many organisms requires the function of an actomyosin ring, an apparatus that resembles the force generating machinery in the muscle. This ring apparatus is attached to the cell periphery (cell membranes) such that when it contracts, it brings the cell periphery together with it, leading to cell division. How the actomyosin ring is attached to the cell membrane at the division site is unknown. In this manuscript, we identify and describe Sbg1, a protein that links the actomyosin ring and the cell membranes since Sbg1 has a sequence that allows it to be inserted into the cell membrane. Sbg1 specifically localizes to the cell division site and also cooperates with a cell wall biosynthetic enzyme Bgs1 to achieve cell division. Consistently, in the absence of Sbg1, cells fail to divide leading to lethality. Sbg1 interacts with a number of cell division proteins, such as Cdc15, Rga7, Imp2, and Pxl1, to achieve its function as a bridge between the cell membrane and the actomyosin ring. Our work identifies a direct molecular link between the actomyosin ring and the cell membranes, explaining how ring contraction leads to inward movement of the cell periphery.

Investigation of the role of four mitotic septins and chitin synthase 2 for cytokinesis in Kluyveromyces lactis

Septins are key components of the cell division machinery from yeast to humans. The model yeast Saccharomyces cerevisiae has five mitotic septins, Cdc3, Cdc10, Cdc11, Cdc12, and Shs1. Here we characterized the five orthologs from the genetically less-redundant milk yeast Kluyveromyces lactis. We found that except for KlSHS1 all septin genes are essential. Klshs1 deletions displayed temperature-sensitive growth and morphological defects. Heterologous complementation analyses revealed that all five K. lactis genes encode functional orthologs of their S. cerevisiae counterparts. Fluorophore-tagged versions of the K. lactis septins localized to a ring at the incipient bud site and split into two separate rings at the bud neck later in cytokinesis. One of the key proteins recruited to the bud neck by septins in S. cerevisiae is the chitin synthase Chs2, which synthesizes the primary septum. KlCHS2 was found to be essential and deletions showed cytokinetic defects upon spore germination. KlChs2-GFP also localized to the bud neck and to punctate structures in K. lactis. We conclude that cytokinesis in K. lactis is similar to S. cerevisiae and chimeric septin complexes are fully functional in both yeasts. In contrast to some S. cerevisiae strains, KlChs2 and KlCdc10 were found to be essential.

Coupling changes in cell shape to chromosome segregation

Animal cells undergo dramatic changes in shape, mechanics and polarity as they progress through the different stages of cell division. These changes begin at mitotic entry, with cell-substrate adhesion remodelling, assembly of a cortical actomyosin network and osmotic swelling, which together enable cells to adopt a near spherical form even when growing in a crowded tissue environment. These shape changes, which probably aid spindle assembly and positioning, are then reversed at mitotic exit to restore the interphase cell morphology. Here, we discuss the dynamics, regulation and function of these processes, and how cell shape changes and sister chromatid segregation are coupled to ensure that the daughter cells generated through division receive their fair inheritance.

A MORN1-associated HAD phosphatase in the basal complex is essential for Toxoplasma gondii daughter budding

Apicomplexan parasites replicate by several budding mechanisms with two well-characterized examples being Toxoplasma endodyogeny and Plasmodium schizogony. Completion of budding requires the tapering of the nascent daughter buds toward the basal end, driven by contraction of the basal complex. This contraction is not executed by any of the known cell division associated contractile mechanisms and in order to reveal new components of the unusual basal complex we performed a yeast two-hybrid screen with its major scaffolding protein, TgMORN1. Here we report on a conserved protein with a haloacid dehalogenase (HAD) phosphatase domain, hereafter named HAD2a, identified by yeast two-hybrid. HAD2a has demonstrated enzyme-activity in vitro, localizes to the nascent daughter buds, and co-localizes with MORN1 to the basal complex during its contraction. Conditional knockout of HAD2a in Toxoplasma interferes with basal complex assembly, which leads to incomplete cytokinesis and conjoined daughters that ultimately results in disrupted proliferation. In Plasmodium, we further confirmed localization of the HAD2a ortholog to the basal complex toward the end of schizogony. In conclusion, our work highlights an essential role for this HAD phosphatase across apicomplexan budding and suggests a regulatory mechanism of differential phosphorylation on the structure and/or contractile function of the basal complex.

Evidence That an Unconventional Actin Can Provide Essential F-Actin Function and That a Surveillance System Monitors F-Actin Integrity in Chlamydomonas

Actin is one of the most conserved eukaryotic proteins. It is thought to have multiple essential cellular roles and to function primarily or exclusively as filaments ("F-actin"). Chlamydomonas has been an enigma, because a null mutation (ida5-1) in its single gene for conventional actin does not affect growth. A highly divergent actin gene, NAP1, is upregulated in ida5-1 cells, but it has been unclear whether NAP1 can form filaments or provide actin function. Here, we used the actin-depolymerizing drug latrunculin B (LatB), the F-actin-specific probe Lifeact-Venus, and genetic and molecular methods to resolve these issues. LatB-treated wild-type cells continue to proliferate; they initially lose Lifeact-stained structures but recover them concomitant with upregulation of NAP1. Thirty-nine LatB-sensitive mutants fell into four genes (NAP1 and LAT1-LAT3) in which we identified the causative mutations using a novel combinatorial pool-sequencing strategy. LAT1-LAT3 are required for NAP1 upregulation upon LatB treatment, and ectopic expression of NAP1 largely rescues the LatB sensitivity of the lat1-lat3 mutants, suggesting that the LAT gene products comprise a regulatory hierarchy with NAP1 expression as the major functional output. Selection of LatB-resistant revertants of a nap1 mutant yielded dominant IDA5 mutations that presumably render F-IDA5 resistant to LatB, and nap1 and lat mutations are synthetically lethal with ida5-1 in the absence of LatB. We conclude that both IDA5 and the divergent NAP1 can form filaments and redundantly provide essential F-actin functions and that a novel surveillance system, probably responding to a loss of F-actin, triggers NAP1 expression and perhaps other compensatory responses.

Tissue-specific control of midbody microtubule stability by Citron kinase through modulation of TUBB3 phosphorylation

Cytokinesis, the physical separation of daughter cells at the end of cell cycle, is commonly considered a highly stereotyped phenomenon. However, in some specialized cells this process may involve specific molecular events that are still largely unknown. In mammals, loss of Citron-kinase (CIT-K) leads to massive cytokinesis failure and apoptosis only in neuronal progenitors and in male germ cells, resulting in severe microcephaly and testicular hypoplasia, but the reasons for this specificity are unknown. In this report we show that CIT-K modulates the stability of midbody microtubules and that the expression of tubulin β-III (TUBB3) is crucial for this phenotype. We observed that TUBB3 is expressed in proliferating CNS progenitors, with a pattern correlating with the susceptibility to CIT-K loss. More importantly, depletion of TUBB3 in CIT-K-dependent cells makes them resistant to CIT-K loss, whereas TUBB3 overexpression increases their sensitivity to CIT-K knockdown. The loss of CIT-K leads to a strong decrease in the phosphorylation of S444 on TUBB3, a post-translational modification associated with microtubule stabilization. CIT-K may promote this event by interacting with TUBB3 and by recruiting at the midbody casein kinase-2α (CK2α) that has previously been reported to phosphorylate the S444 residue. Indeed, CK2α is lost from the midbody in CIT-K-depleted cells. Moreover, expression of the nonphosphorylatable TUBB3 mutant S444A induces cytokinesis failure, whereas expression of the phospho-mimetic mutant S444D rescues the cytokinesis failure induced by both CIT-K and CK2α loss. Altogether, our findings reveal that expression of relatively low levels of TUBB3 in mitotic cells can be detrimental for their cytokinesis and underscore the importance of CIT-K in counteracting this event.

The fission yeast cytokinetic contractile ring regulates septum shape and closure

During cytokinesis, fission yeast and other fungi and bacteria grow a septum that divides the cell in two. In fission yeast closure of the circular septum hole by the β-glucan synthases (Bgs) and other glucan synthases in the plasma membrane is tightly coupled to constriction of an actomyosin contractile ring attached to the membrane. It is unknown how septum growth is coordinated over scales of several microns to maintain septum circularity. Here, we documented the shapes of ingrowing septum edges by measuring the roughness of the edges, a measure of the deviation from circularity. The roughness was small, with spatial correlations indicative of spatially coordinated growth. We hypothesized that Bgs-mediated septum growth is mechanosensitive and coupled to contractile ring tension. A mathematical model showed that ring tension then generates almost circular septum edges by adjusting growth rates in a curvature-dependent fashion. The model reproduced experimental roughness statistics and showed that septum synthesis sets the mean closure rate. Our results suggest that the fission yeast cytokinetic ring tension does not set the constriction rate but regulates septum closure by suppressing roughness produced by inherently stochastic molecular growth processes.

Cooperation between Paxillin-like Protein Pxl1 and Glucan Synthase Bgs1 Is Essential for Actomyosin Ring Stability and Septum Formation in Fission Yeast

In fungal cells cytokinesis requires coordinated closure of a contractile actomyosin ring (CAR) and synthesis of a special cell wall structure known as the division septum. Many CAR proteins have been identified and characterized, but how these molecules interact with the septum synthesis enzymes to form the septum remains unclear. Our genetic study using fission yeast shows that cooperation between the paxillin homolog Pxl1, required for ring integrity, and Bgs1, the enzyme responsible for linear β(1,3)glucan synthesis and primary septum formation, is required for stable anchorage of the CAR to the plasma membrane before septation onset, and for cleavage furrow formation. Thus, lack of Pxl1 in combination with Bgs1 depletion, causes failure of ring contraction and lateral cell wall overgrowth towards the cell lumen without septum formation. We also describe here that Pxl1 concentration at the CAR increases during cytokinesis and that this increase depends on the SH3 domain of the F-BAR protein Cdc15. In consequence, Bgs1 depletion in cells carrying a cdc15_ΔSH3 allele causes ring disassembly and septation blockage, as it does in cells lacking Pxl1. On the other hand, the absence of Pxl1 is lethal when Cdc15 function is affected, generating a large sliding of the CAR with deposition of septum wall material along the cell cortex, and suggesting additional functions for both Pxl1 and Cdc15 proteins. In conclusion, our findings indicate that CAR anchorage to the plasma membrane through Cdc15 and Pxl1, and concomitant Bgs1 activity, are necessary for CAR maintenance and septum formation in fission yeast.

Cytokinesis requires assembly of an actomyosin ring adjacent to the plasma membrane, which upon contraction pulls the membrane to form a cleavage furrow. In fungi ring closure is coordinated with the synthesis of a cell wall septum. Knowledge about the molecules anchoring the ring to the membrane is very limited. We have found that fission yeast paxillin, located at the ring, and Bgs1, the enzyme responsible for primary septum formation, located at the membrane, cooperate during cytokinesis. Both are required to anchor the ring to the membrane and to maintain it during cytokinesis. Moreover, both proteins cooperate to form the septum. Accordingly, paxillin is essential when Bgs1 is depleted. When both proteins are missing, the contractile ring forms but the lateral cell wall overgrows inwards without a defined cleavage furrow and septum formation. During cytokinesis there is an increase of paxillin which depends on the SH3 domain of the F-BAR protein Cdc15. Consequently the absence of this domain mimics the phenotype of paxillin absence in Bgs1-depleted cells. Interestingly, a decreased function of both Cdc15 and paxillin uncouples the septum synthesis from the ring contraction, indicating an essential cooperation between these proteins and Bgs1 for proper cytokinesis.

Formin-based control of the actin cytoskeleton during cytokinesis

Cytokinesis, the terminal event in the canonical cell cycle, physically separates daughter cells following mitosis. For cleavage to occur in many eukaryotes, a cytokinetic ring must assemble and constrict between divided genomes. Although dozens of different molecules localize to and participate within the cytokinetic ring, the core machinery comprises linear actin filaments. Accordingly, formins, which nucleate and elongate F-actin (filamentous actin) for the cytokinetic ring, are required for cytokinesis in diverse species. In the present article, we discuss specific modes of formin-based actin regulation during cell division and highlight emerging mechanisms and questions on this topic.

Regulation of cytokinesis in the milk yeast Kluyveromyces lactis

Cytokinesis in yeast and mammalian cells is a highly coordinated process mediated by the constriction of an actomyosin ring. In yeasts, it is accompanied by the formation of a chitinous primary septum. Although much is known about the regulation of cytokinesis in budding yeast, overlapping functions of redundant genes complicates genetic analyses. Here, we investigated the effects of various deletion mutants on cytokinesis in the milk yeast Kluyveromyces lactis. To determine the spatiotemporal parameters of cytokinesis components, live-cell imaging of fluorophor-tagged KlMyo1 and a new Lifeact probe for KlAct1 was employed. In contrast to Saccharomyces cerevisiae, where deletion of ScMYO1 is lethal, Klmyo1 deletion was temperature-sensitive. Transmission and scanning electron microscopy demonstrated that the Klmyo1 deletion cells had a defect in the formation of the primary septum and in cell separation; this result was confirmed by FACS analyses. Deletion of KlCYK3 was lethal, whereas in S. cerevisiae a cyk3 deletion is synthetically lethal with hof1 deletion. Growth of Klhof1 mutants was osmoremedial at 25°C, as it is in S. cerevisiae. CYK3 and HOF1 genes cross-complemented in both species, suggesting that they are functional homologs. Inn1, a common interactor for these two regulators, was essential in both yeasts and the encoding genes did not cross-complement. The C2 domain of the Inn1 homologs conferred species specificity. Thus, our work establishes K. lactis as a model yeast to study cytokinesis with less genetic redundancy than S. cerevisiae. The viability of Klmyo1 deletions provides an advantage over budding yeast to study actomyosin-independent cytokinesis. Moreover, the lethality of Klcyk3 null mutants suggests that there are fewer functional redundancies with KlHof1 in K. lactis.

Pom1 regulates the assembly of Cdr2-Mid1 cortical nodes for robust spatial control of cytokinesis

Pom1 regulation of Cdr2 membrane association and interaction with Mid1 prevents Cdr2 assembly into stable nodes in the cell tip region, which ensures proper positioning of cytokinetic ring precursors and accurate division plane positioning in fission yeast.

Proper division plane positioning is essential to achieve faithful DNA segregation and to control daughter cell size, positioning, or fate within tissues. In Schizosaccharomyces pombe, division plane positioning is controlled positively by export of the division plane positioning factor Mid1/anillin from the nucleus and negatively by the Pom1/DYRK (dual-specificity tyrosine-regulated kinase) gradients emanating from cell tips. Pom1 restricts to the cell middle cortical cytokinetic ring precursor nodes organized by the SAD-like kinase Cdr2 and Mid1/anillin through an unknown mechanism. In this study, we show that Pom1 modulates Cdr2 association with membranes by phosphorylation of a basic region cooperating with the lipid-binding KA-1 domain. Pom1 also inhibits Cdr2 interaction with Mid1, reducing its clustering ability, possibly by down-regulation of Cdr2 kinase activity. We propose that the dual regulation exerted by Pom1 on Cdr2 prevents Cdr2 assembly into stable nodes in the cell tip region where Pom1 concentration is high, which ensures proper positioning of cytokinetic ring precursors at the cell geometrical center and robust and accurate division plane positioning.

The putative exchange factor Gef3p interacts with Rho3p GTPase and the septin ring during cytokinesis in fission yeast

The small GTP-binding proteins of the Rho family and its regulatory proteins play a central role in cytokinetic actomyosin ring assembly and cytokinesis. Here we show that the fission yeast guanine nucleotide exchange factor Gef3p interacts with Rho3p at the division site. Gef3p contains a putative DH homology domain and a BAR/IMD-like domain. The protein localized to the division site late in mitosis, where it formed a ring that did not constrict with actomyosin ring (cytokinetic actomyosin ring) invagination; instead, it split into a double ring that resembled the septin ring. Gef3p co-localized with septins and Mid2p and required septins and Mid2p for its localization. Gef3p interacts physically with the GTP-bound form of Rho3p. Although Gef3p is not essential for cell separation, the simultaneous disruption of gef3(+) and Rho3p-interacting proteins, such as Sec8p, an exocyst component, Apm1p, a subunit of the clathrin adaptor complex or For3p, an actin-polymerizing protein, yielded cells with strong defects in septation and polarity respectively. Our results suggest that interactions between septins and Rho-GEFs provide a new targeting mechanism for GTPases in cytokinesis, in this case probably contributing to Rho3p function in vesicle tethering and vesicle trafficking in the later steps of cell separation.

Divided we stand: splitting synthetic cells for their proliferation

With the recent dawn of synthetic biology, the old idea of man-made artificial life has gained renewed interest. In the context of a bottom-up approach, this entails the de novo construction of synthetic cells that can autonomously sustain themselves and proliferate. Reproduction of a synthetic cell involves the synthesis of its inner content, replication of its information module, and growth and division of its shell. Theoretical and experimental analysis of natural cells shows that, whereas the core synthesis machinery of the information module is highly conserved, a wide range of solutions have been realized in order to accomplish division. It is therefore to be expected that there are multiple ways to engineer division of synthetic cells. Here we survey the field and review potential routes that can be explored to accomplish the division of bottom-up designed synthetic cells. We cover a range of complexities from simple abiotic mechanisms involving splitting of lipid-membrane-encapsulated vesicles due to physical or chemical principles, to potential division mechanisms of synthetic cells that are based on prokaryotic division machineries.

Proper actin ring formation and septum constriction requires coordinated regulation of SIN and MOR pathways through the germinal centre kinase MST-1

Nuclear DBF2p-related (NDR) kinases constitute a functionally conserved protein family of eukaryotic regulators that control cell division and polarity. In fungi, they function as effector kinases of the morphogenesis (MOR) and septation initiation (SIN) networks and are activated by pathway-specific germinal centre (GC) kinases. We characterized a third GC kinase, MST-1, that connects both kinase cascades. Genetic and biochemical interactions with SIN components and life cell imaging identify MST-1 as SIN-associated kinase that functions in parallel with the GC kinase SID-1 to activate the SIN-effector kinase DBF-2. SID-1 and MST-1 are both regulated by the upstream SIN kinase CDC-7, yet in an opposite manner. Aberrant cortical actomyosin rings are formed in Δmst-1, which resulted in mis-positioned septa and irregular spirals, indicating that MST-1-dependent regulation of the SIN is required for proper formation and constriction of the septal actomyosin ring. However, MST-1 also interacts with several components of the MOR network and modulates MOR activity at multiple levels. MST-1 functions as promiscuous enzyme and also activates the MOR effector kinase COT-1 through hydrophobic motif phosphorylation. In addition, MST-1 physically interacts with the MOR kinase POD-6, and dimerization of both proteins inactivates the GC kinase hetero-complex. These data specify an antagonistic relationship between the SIN and MOR during septum formation in the filamentous ascomycete model Neurospora crassa that is, at least in part, coordinated through the GC kinase MST-1. The similarity of the SIN and MOR pathways to the animal Hippo and Ndr pathways, respectively, suggests that intensive cross-communication between distinct NDR kinase modules may also be relevant for the homologous NDR kinases of higher eukaryotes.

Cytokinesis is a fundamental cellular process essential for cell proliferation of uni- and multicellular organisms. The molecular pathways that regulate cytokinesis are highly complex and involve a large number of components that form elaborate interactive networks. The fungal septation initiation network (SIN) functions as tripartite kinase cascade that connects cell cycle progression with the control of cell division. Mis-regulation of the homologous Hippo pathway in animals results in excessive proliferation and formation of tumors, underscoring the conservation and importance of these kinase networks. A second morphogenesis (MOR) pathway involves homologous components and is controlling cell polarity in fungi and higher eukaryotes. Here we show that the promiscuous functioning Ste20-related kinase MST-1 has a dual role in regulating SIN and MOR network function. Moreover, SIN and MOR coordination through MST-1 can be achieved in an enzyme-independent manner through hetero-dimerization of germinal centre kinases, providing an additional level for activity regulation of signaling networks that is not dependent on phosphate transfer. Given the functional conservation of NDR kinase signaling modules and their regulation, our work may define general mechanisms by which NDR kinase pathway are coordinated in fungi and higher eukaryotes.

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.

A role for myosin II in mammalian mitochondrial fission

Mitochondria are dynamic organelles, undergoing both fission and fusion regularly in interphase cells. Mitochondrial fission is thought to be part of a quality-control mechanism whereby damaged mitochondrial components are segregated from healthy components in an individual mitochondrion, followed by mitochondrial fission and degradation of the damaged daughter mitochondrion. Fission also plays a role in apoptosis. Defects in mitochondrial dynamics can lead to neurodegenerative diseases such as Alzheimer's disease. Mitochondrial fission requires the dynamin GTPase Drp1, which assembles in a ring around the mitochondrion and appears to constrict both outer and inner mitochondrial membranes. However, mechanisms controlling Drp1 assembly on mammalian mitochondria are unclear. Recent results show that actin polymerization, driven by the endoplasmic reticulum-bound formin protein INF2, stimulates Drp1 assembly at fission sites. Here, we show that myosin II also plays a role in fission. Chemical inhibition by blebbistatin or small interfering RNA (siRNA)-mediated suppression of myosin IIA or myosin IIB causes an increase in mitochondrial length in both control cells and cells expressing constitutively active INF2. Active myosin II accumulates in puncta on mitochondria in an actin- and INF2-dependent manner. In addition, myosin II inhibition decreases Drp1 association with mitochondria. Based on these results, we propose a mechanistic model in which INF2-mediated actin polymerization leads to myosin II recruitment and constriction at the fission site, enhancing subsequent Drp1 accumulation and fission.

Extracellular cell wall β(1,3)glucan is required to couple septation to actomyosin ring contraction

β(1,3)glucan is critical for contractile ring positioning and for coupling septum synthesis to constriction of the contractile ring and plasma membrane extension during cytokinesis.

Cytokinesis has been extensively studied in different models, but the role of the extracellular cell wall is less understood. Here we studied this process in fission yeast. The essential protein Bgs4 synthesizes the main cell wall β(1,3)glucan. We show that Bgs4-derived β(1,3)glucan is required for correct and stable actomyosin ring positioning in the cell middle, before the start of septum formation and anchorage to the cell wall. Consequently, β(1,3)glucan loss generated ring sliding, oblique positioned rings and septa, misdirected septum synthesis indicative of relaxed rings, and uncoupling between a fast ring and membrane ingression and slow septum synthesis, suggesting that cytokinesis can progress with defective septum pushing and/or ring pulling forces. Moreover, Bgs4-derived β(1,3)glucan is essential for secondary septum formation and correct primary septum completion. Therefore, our results show that extracellular β(1,3)glucan is required for cytokinesis to connect the cell wall with the plasma membrane and for contractile ring function, as proposed for the equivalent extracellular matrix in animal cells.

The design of MACs (minimal actin cortices)

The actin cell cortex in eukaryotic cells is a key player in controlling and maintaining the shape of cells, and in driving major shape changes such as in cytokinesis. It is thereby constantly being remodeled. Cell shape changes require forces acting on membranes that are generated by the interplay of membrane coupled actin filaments and assemblies of myosin motors. Little is known about how their interaction regulates actin cell cortex remodeling and cell shape changes. Because of the vital importance of actin, myosin motors and the cell membrane, selective in vivo experiments and manipulations are often difficult to perform or not feasible. Thus, the intelligent design of minimal in vitro systems for actin-myosin-membrane interactions could pave a way for investigating actin cell cortex mechanics in a detailed and quantitative manner. Here, we present and discuss the design of several bottom-up in vitro systems accomplishing the coupling of actin filaments to artificial membranes, where key parameters such as actin densities and membrane properties can be varied in a controlled manner. Insights gained from these in vitro systems may help to uncover fundamental principles of how exactly actin-myosin-membrane interactions govern actin cortex remodeling and membrane properties for cell shape changes. © 2013 Wiley Periodicals, Inc.

The Nitrosopumilus maritimus CdvB, but not FtsZ, assembles into polymers

Euryarchaeota and Crenarchaeota are two major phyla of archaea which use distinct molecular apparatuses for cell division. Euryarchaea make use of the tubulin-related protein FtsZ, while Crenarchaea, which appear to lack functional FtsZ, employ the Cdv (cell division) components to divide. Ammonia oxidizing archaeon (AOA) Nitrosopumilus maritimus belongs to another archaeal phylum, the Thaumarchaeota, which has both FtsZ and Cdv genes in the genome. Here, we used a heterologous expression system to characterize FtsZ and Cdv proteins from N. maritimus by investigating the ability of these proteins to form polymers. We show that one of the Cdv proteins in N. maritimus, the CdvB (Nmar_0816), is capable of forming stable polymers when expressed in fission yeast. The N. maritimus CdvB is also capable of assembling into filaments in mammalian cells. However, N. maritimus FtsZ does not assemble into polymers in our system. The ability of CdvB, but not FtsZ, to polymerize is consistent with a recent finding showing that several Cdv proteins, but not FtsZ, localize to the mid-cell site in the dividing N. maritimus. Thus, we propose that it is Cdv proteins, rather than FtsZ, that function as the cell division apparatus in N. maritimus.