Regulation and function of the fission yeast myosins

Effects of rpl1001 Gene Deletion on Cell Division of Fission Yeast and Its Molecular Mechanism

The rpl1001 gene encodes 60S ribosomal protein L10, which is involved in intracellular protein synthesis and cell growth. However, it is not yet known whether it is involved in the regulation of cell mitosis dynamics. This study focuses on the growth, spore production, cell morphology, the dynamics of microtubules, chromosomes, actin, myosin, and mitochondria of fission yeast (Schizosaccharomyces pombe) to investigate the impact of rpl1001 deletion on cell mitosis. RNA-Seq and bioinformatics analyses were also used to reveal key genes, such as hsp16, mfm1 and isp3, and proteasome pathways. The results showed that rpl1001 deletion resulted in slow cell growth, abnormal spore production, altered cell morphology, and abnormal microtubule number and length during interphase. The cell dynamics of the rpl1001Δ strain showed that the formation of a monopolar spindle leads to abnormal chromosome segregation with increased rate of spindle elongation in anaphase of mitosis, decreased total time of division, prolonged formation time of actin and myosin loops, and increased expression of mitochondrial proteins. Analysis of the RNA-Seq sequencing results showed that the proteasome pathway, up-regulation of isp3, and down-regulation of mfm1 and mfm2 in the rpl1001Δ strain were the main factors underpinning the increased number of spore production. Also, in the rpl1001Δ strain, down-regulation of dis1 caused the abnormal microtubule and chromosome dynamics, and down-regulation of hsp16 and pgk1 were the key genes affecting the delay of actin ring and myosin ring formation. This study reveals the effect and molecular mechanism of rpl1001 gene deletion on cell division, which provides the scientific basis for further clarifying the function of the Rpl1001 protein in cell division.

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.

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.

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.

Interacting-heads motif has been conserved as a mechanism of myosin II inhibition since before the origin of animals

Electron microscope studies have shown that the switched-off state of myosin II in muscle involves intramolecular interaction between the two heads of myosin and between one head and the tail. The interaction, seen in both myosin filaments and isolated molecules, inhibits activity by blocking actin-binding and ATPase sites on myosin. This interacting-heads motif is highly conserved, occurring in invertebrates and vertebrates, in striated, smooth, and nonmuscle myosin IIs, and in myosins regulated by both Ca²⁺ binding and regulatory light-chain phosphorylation. Our goal was to determine how early this motif arose by studying the structure of inhibited myosin II molecules from primitive animals and from earlier, unicellular species that predate animals. Myosin II from Cnidaria (sea anemones, jellyfish), the most primitive animals with muscles, and Porifera (sponges), the most primitive of all animals (lacking muscle tissue) showed the same interacting-heads structure as myosins from higher animals, confirming the early origin of the motif. The social amoeba Dictyostelium discoideum showed a similar, but modified, version of the motif, while the amoeba Acanthamoeba castellanii and fission yeast (Schizosaccharomyces pombe) showed no head-head interaction, consistent with the different sequences and regulatory mechanisms of these myosins compared with animal myosin IIs. Our results suggest that head-head/head-tail interactions have been conserved, with slight modifications, as a mechanism for regulating myosin II activity from the emergence of the first animals and before. The early origins of these interactions highlight their importance in generating the inhibited (relaxed) state of myosin in muscle and nonmuscle cells.

Analysis of biophysical and functional consequences of tropomyosin-fluorescent protein fusions

The dynamic nature of actin polymers is modulated to facilitate a diverse range of cellular processes. These dynamic properties are determined by different isoforms of tropomyosin which are recruited to distinct subpopulations of actin polymers to differentially regulate their functional properties. This makes tropomyosin an attractive target for labelling discrete actin populations. We have assessed the effect of different fluorescent labelling strategies for this protein. Although tropomyosin–fluorescent fusions decorate actin in vivo, they are either nonfunctional or perturb regulation of actin nucleation and cell cycle timings. Thus, conclusions and physiological relevance should be carefully evaluated when using tropomyosin fusions.

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.

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.

Evolution and classification of myosins, a paneukaryotic whole-genome approach

Myosins are key components of the eukaryotic cytoskeleton, providing motility for a broad diversity of cargoes. Therefore, understanding the origin and evolutionary history of myosin classes is crucial to address the evolution of eukaryote cell biology. Here, we revise the classification of myosins using an updated taxon sampling that includes newly or recently sequenced genomes and transcriptomes from key taxa. We performed a survey of eukaryotic genomes and phylogenetic analyses of the myosin gene family, reconstructing the myosin toolkit at different key nodes in the eukaryotic tree of life. We also identified the phylogenetic distribution of myosin diversity in terms of number of genes, associated protein domains and number of classes in each taxa. Our analyses show that new classes (i.e., paralogs) and domain architectures were continuously generated throughout eukaryote evolution, with a significant expansion of myosin abundance and domain architectural diversity at the stem of Holozoa, predating the origin of animal multicellularity. Indeed, single-celled holozoans have the most complex myosin complement among eukaryotes, with paralogs of most myosins previously considered animal specific. We recover a dynamic evolutionary history, with several lineage-specific expansions (e.g., the myosin III-like gene family diversification in choanoflagellates), convergence in protein domain architectures (e.g., fungal and animal chitin synthase myosins), and important secondary losses. Overall, our evolutionary scheme demonstrates that the ancestral eukaryote likely had a complex myosin repertoire that included six genes with different protein domain architectures. Finally, we provide an integrative and robust classification, useful for future genomic and functional studies on this crucial eukaryotic gene family.

Changes in cell ultrastructure and inhibition of JAK1/STAT3 signaling pathway in CBRH-7919 cells with astaxanthin

Astaxanthin (AST), a xanthophylls carotenoid, possesses significant anticancer effects. However, to date, the molecular mechanism of anticancer remains unclear. In the present research, we studied the anticancer mechanism of AST, including the changes in cell ultrastructure, such as the mitochondrion, rough endoplasmic reticulum (RER), Golgi complex, and cytoskeleton, the inhibition of Janus kinase 1(JAK1)/transduction and the activators of the transcription-3 (STAT3) signaling pathway using rat hepatocellular carcinoma CBRH-7919 cells. Cell apoptosis was evaluated and the expressions of JAK1, STAT3, non-metastasis23-1 (nm23-1), and apoptotic gene like B-cell lymphoma/leukemia-2 (bcl-2), B-cell lymphoma-extra large (bcl-xl), proto-oncogene proteins c myc (c-myc) and bcl-2- associated X (bax) were also examined. The results showed that AST could induce cancer cell apoptosis. Under transmission electron microscope, the ultrastructure of treated cells were not clearly distinguishable, the membranes of the mitochondrion, RER, Golgi complex were broken or loosened, and the endoplasmic reticulum (ER) was degranulated. Cytoskeleton depolymerization of the microtubule system led to the collapse of extended vimentin intermediate filament bundles into short agglomerations with disordered distributions. AST inhibited the expression of STAT3, its upstream activator JAK1, and the STAT3 target antiapoptotic genes bcl-2, bcl-xl, and c-myc. Conversely, AST enhanced the expressions of nm23-1 and bax. Overall, our findings demonstrate that AST could induce the apoptosis of CBRH-7919 cells, which are involved in cell ultrastructure and the JAK1/STAT3 signaling pathway.