Cell polarity: Adapting the PAR cascade to diverse cellular contexts

Two new studies shed light on the intricacies of Caenorhabditis elegans embryo patterning, revealing how the conserved interaction and crosstalk of PAR proteins are adapted to perceive distinct cues, ultimately shaping unique asymmetries in form and function.

Fluid flow dynamics in cellular patterning

The development of complex forms of multicellular organisms depends on the spatial arrangement of cellular architecture and functions. The interior design of the cell is patterned by spatially biased distributions of molecules and biochemical reactions in the cytoplasm and/or on the plasma membrane. In recent years, a dynamic change in the cytoplasmic fluid flow has emerged as a key physical process of driving long-range transport of molecules to particular destinations within the cell. Here, recent experimental advances in the understanding of the generation of the various types of cytoplasmic flows and contributions to intracellular patterning are reviewed with a particular focus on feedback mechanisms between the mechanical properties of fluid flow and biochemical signaling during animal cell polarization.

A balance between antagonizing PAR proteins specifies the pattern of asymmetric and symmetric divisions in C. elegans embryogenesis

Coordination between cell differentiation and proliferation during development requires the balance between asymmetric and symmetric modes of cell division. However, the cellular intrinsic cue underlying the choice between these two division modes remains elusive. Here, we show evidence in Caenorhabditis elegans that the invariable lineage of the division modes is specified by the balance between antagonizing complexes of partitioning-defective (PAR) proteins. By uncoupling unequal inheritance of PAR proteins from that of fate determinants during cell division, we demonstrate that changes in the balance between PAR-2 and PAR-6 can be sufficient to re-program the division modes from symmetric to asymmetric and vice versa in two daughter cells. The division mode adopted occurs independently of asymmetry in cytoplasmic fate determinants, cell-size asymmetry, and cell-cycle asynchrony between sister cells. We propose that the balance between PAR proteins represents an intrinsic self-organizing cue for the specification of the two division modes during development.

Mechanochemical Control of Symmetry Breaking in the Caenorhabditis elegans Zygote

Cell polarity is the asymmetric organization of cellular components along defined axes. A key requirement for polarization is the ability of the cell to break symmetry and achieve a spatially biased organization. Despite different triggering cues in various systems, symmetry breaking (SB) usually relies on mechanochemical modulation of the actin cytoskeleton, which allows for advected movement and reorganization of cellular components. Here, the mechanisms underlying SB in Caenorhabditis elegans zygote, one of the most popular models to study cell polarity, are reviewed. A zygote initiates SB through the centrosome, which modulates mechanics of the cell cortex to establish advective flow of cortical proteins including the actin cytoskeleton and partitioning defective (PAR) proteins. The chemical signaling underlying centrosomal control of the Aurora A kinase–mediated cascade to convert the organization of the contractile actomyosin network from an apolar to polar state is also discussed.

Novel approaches to link apicobasal polarity to cell fate specification

Understanding the development of apicobasal polarity (ABP) is a long-standing problem in biology. The molecular components involved in the development and maintenance of APB have been largely identified and are known to have ubiquitous roles across organisms. Our knowledge of the functional consequences of ABP establishment and maintenance is far less comprehensive. Recent studies using novel experimental approaches and cellular models have revealed a growing link between ABP and the genetic program of cell lineage. This mini-review describes some of the most recent advances in this new field, highlighting examples from Caenorhabditis elegans and mouse embryos, human pluripotent stem cells, and epithelial cells. We also speculate on the most interesting and challenging avenues that can be explored.

TPXL-1 activates Aurora A to clear contractile ring components from the polar cortex during cytokinesis

During cytokinesis, a signal from the central spindle that forms between the separating anaphase chromosomes promotes the accumulation of contractile ring components at the cell equator, while a signal from the centrosomal microtubule asters inhibits accumulation of contractile ring components at the cell poles. However, the molecular identity of the inhibitory signal has remained unknown. To identify molecular components of the aster-based inhibitory signal, we developed a means to monitor the removal of contractile ring proteins from the polar cortex after anaphase onset. Using this assay, we show that polar clearing is an active process that requires activation of Aurora A kinase by TPXL-1. TPXL-1 concentrates on astral microtubules coincident with polar clearing in anaphase, and its ability to recruit Aurora A and activate its kinase activity are essential for clearing. In summary, our data identify Aurora A kinase as an aster-based inhibitory signal that restricts contractile ring components to the cell equator during cytokinesis.

ImaEdge: a platform for the quantitative analysis of cortical proteins spatiotemporal dynamics during cell polarization

Cell polarity involves the compartmentalization of the cell cortex. The establishment of cortical compartments arises from the spatial bias in the activity and concentration of cortical proteins. The mechanistic dissection of cell polarity requires the accurate detection of dynamic changes in cortical proteins, but the fluctuations of cell shape and the inhomogeneous distributions of cortical proteins greatly complicate the quantitative extraction of their global and local changes during cell polarization. To address these problems, we introduce an open-source software package, ImaEdge, which automates the segmentation of the cortex from time-lapse movies, and enables quantitative extraction of cortical protein intensities. We demonstrate that ImaEdge enables efficient and rigorous analysis of the dynamic evolution of cortical PAR proteins during C. elegans embryogenesis. It is also capable of accurate tracking of varying levels of transgene expression and discontinuous signals of the actomyosin cytoskeleton during multiple rounds of cell division. ImaEdge provides a unique resource for the quantitative studies of cortical polarization, with the potential for application to many types of polarized cells.

The PAR network: redundancy and robustness in a symmetry-breaking system

To become polarized, cells must first 'break symmetry'. Symmetry breaking is the process by which an unpolarized, symmetric cell develops a singularity, often at the cell periphery, that is used to develop a polarity axis. The Caenorhabditis elegans zygote breaks symmetry under the influence of the sperm-donated centrosome, which causes the PAR polarity regulators to sort into distinct anterior and posterior cortical domains. Modelling analyses have shown that cortical flows induced by the centrosome combined with antagonism between anterior and posterior PARs (mutual exclusion) are sufficient, in principle, to break symmetry, provided that anterior and posterior PAR activities are precisely balanced. Experimental evidence indicates, however, that the system is surprisingly robust to changes in cortical flows, mutual exclusion and PAR balance. We suggest that this robustness derives from redundant symmetry-breaking inputs that engage two positive feedback loops mediated by the anterior and posterior PAR proteins. In particular, the PAR-2 feedback loop stabilizes the polarized state by creating a domain where posterior PARs are immune to exclusion by anterior PARs. The two feedback loops in the PAR network share characteristics with the two feedback loops in the Cdc42 polarization network of Saccharomyces cerevisiae.

Microtubules induce self-organization of polarized PAR domains in Caenorhabditis elegans zygotes

A hallmark of polarized cells is the segregation of the PAR polarity regulators into asymmetric domains at the cell cortex^{1, 2}. Antagonistic interactions involving two conserved kinases, atypical protein kinase C (aPKC) and PAR-1, have been implicated in polarity maintenance^{1, 2}, but the mechanisms that initiate the formation of asymmetric PAR domains are not understood. Here, we describe one pathway used by the sperm-donated centrosome to polarize the PAR proteins in Caenorhabditis elegans zygotes. Before polarization, cortical aPKC excludes PAR-1 kinase and its binding partner PAR-2 by phosphorylation. During symmetry breaking, microtubules nucleated by the centrosome locally protect PAR-2 from phosphorylation by aPKC, allowing PAR-2 and PAR-1 to access the cortex nearest the centrosome. Cortical PAR-1 phosphorylates PAR-3, causing the PAR-3/aPKC complex to leave the cortex. Our findings illustrate how microtubules, independent of actin dynamics, stimulate the self-organization of PAR proteins by providing local protection against a global barrier imposed by aPKC.

Basement membrane sliding and targeted adhesion remodels tissue boundaries during uterine-vulval attachment in Caenorhabditis elegans

Large gaps in basement membrane (BM) occur at sites of cell invasion and tissue remodelling in development and cancer. Though never followed directly in vivo, BM dissolution or reduced synthesis have been postulated to create these gaps. Using landmark photobleaching and optical highlighting of laminin and type IV collagen, we find that a new mechanism, BM sliding, underlies BM gap enlargement during uterine-vulval attachment in C. elegans. Laser ablation and mutant analysis reveal that the invaginating vulval cells promote BM movement. Further, an RNA interference and expression screen identify the integrin INA-1/PAT-3 and VAB-19, homolog of the tumour suppressor Kank, as regulators of BM opening. Both concentrate within vulval cells at the BM gap boundary and halt expansion of the shifting BM. BM sliding followed by targeted adhesion represents a new mechanism for creating precise BM breaches that can be used by cells to break down compartment boundaries.

Cytoplasmic partitioning of P granule components is not required to specify the germline in C. elegans

Asymmetric segregation of P granules during the first four divisions of the Caenorhabditis elegans embryo is a classic example of cytoplasmic partitioning of germline determinants. It is thought that asymmetric partitioning of P granule components during mitosis is essential to distinguish germline from soma. We have identified a mutant (pptr-1) in which P granules become unstable during mitosis and P granule proteins and RNAs are distributed equally to somatic and germline blastomeres. Despite symmetric partitioning of P granule components, pptr-1 mutants segregate a germline that uniquely expresses P granules during postembryonic development. pptr-1 mutants are fertile, except at high temperatures. Hence, asymmetric partitioning of maternal P granules is not essential to specify germ cell fate. Instead, it may serve to protect the nascent germline from stress.

Caenorhabditis elegans ortholog of the p24/p22 subunit, DNC-3, is essential for the formation of the dynactin complex by bridging DNC-1/p150(Glued) and DNC-2/dynamitin

Dynactin is a multisubunit protein complex required for the activity of cytoplasmic dynein. In Caenorhabditis elegans, although 10 of the 11 dynactin subunits were identified based on the sequence similarities to their orthologs, the p24/p22 subunit has not been detected in the genome. Here, we demonstrate that DNC-3 (W10G11.20) is the functional counterpart of the p24/p22 subunit in C. elegans. RNAi phenotypes and subcellular localization of DNC-3 in early C. elegans embryos were nearly identical to those of the known dynactin components. All other dynactin subunits were co-immunoprecipitated with DNC-3, indicating that DNC-3 is a core component of dynactin. Furthermore, the overall secondary structure of DNC-3 resembles to those of the mammalian and yeast p24/p22. We found that DNC-3 is required for the localization of the DNC-1/p150(Glued) and DNC-2/dynamitin, the two components of the projection arm of dynactin, to the nuclear envelope of meiotic nuclei in the adult gonad. Moreover, DNC-3 physically interacted with DNC-1 and DNC-2 and significantly enhanced the binding ability between DNC-1 and DNC-2 in vitro. These results suggest that DNC-3 is essential for the formation of the projection arm subcomplex of dynactin.

Symmetry breaking and polarization of the C. elegans zygote by the polarity protein PAR-2

Polarization of the C. elegans zygote is initiated by ECT-2-dependent cortical flows, which mobilize the anterior PAR proteins (PAR-3, PAR-6 and PKC-3) away from the future posterior end of the embryo marked by the sperm centrosome. Here, we demonstrate the existence of a second, parallel and redundant pathway that can polarize the zygote in the absence of ECT-2-dependent cortical flows. This second pathway depends on the polarity protein PAR-2. We show that PAR-2 localizes to the cortex nearest the sperm centrosome even in the absence of cortical flows. Once on the cortex, PAR-2 antagonizes PAR-3-dependent recruitment of myosin, creating myosin flows that transport the anterior PAR complex away from PAR-2 in a positive-feedback loop. We propose that polarity in the C. elegans zygote is initiated by redundant ECT-2- and PAR-2-dependent mechanisms that lower PAR-3 levels locally, triggering a positive-feedback loop that polarizes the entire cortex.

A new mechanism controlling kinetochore-microtubule interactions revealed by comparison of two dynein-targeting components: SPDL-1 and the Rod/Zwilch/Zw10 complex

Chromosome segregation requires stable bipolar attachments of spindle microtubules to kinetochores. The dynein/dynactin motor complex localizes transiently to kinetochores and is implicated in chromosome segregation, but its role remains poorly understood. Here, we use the Caenorhabditis elegans embryo to investigate the function of kinetochore dynein by analyzing the Rod/Zwilch/Zw10 (RZZ) complex and the associated coiled-coil protein SPDL-1. Both components are essential for Mad2 targeting to kinetochores and spindle checkpoint activation. RZZ complex inhibition, which abolishes both SPDL-1 and dynein/dynactin targeting to kinetochores, slows but does not prevent the formation of load-bearing kinetochore-microtubule attachments and reduces the fidelity of chromosome segregation. Surprisingly, inhibition of SPDL-1, which abolishes dynein/dynactin targeting to kinetochores without perturbing RZZ complex localization, prevents the formation of load-bearing attachments during most of prometaphase and results in extensive chromosome missegregation. Coinhibition of SPDL-1 along with the RZZ complex reduces the phenotypic severity to that observed following RZZ complex inhibition alone. We propose that the RZZ complex can inhibit the formation of load-bearing attachments and that this activity of the RZZ complex is normally controlled by dynein/dynactin localized via SPDL-1. This mechanism could coordinate the hand-off from initial weak dynein-mediated lateral attachments, which help orient kinetochores and enhance their ability to capture microtubules, to strong end-coupled attachments that drive chromosome segregation.

Revisiting the role of microtubules in C. elegans polarity

Cells must break symmetry to acquire polarity. Microtubules have been implicated in the induction of asymmetry in several cell types, but their role in the Caenorhabditis elegans zygote, a classic polarity model, has remained uncertain. One study (see Tsai and Ahringer on p. 397 of this issue) brings new light to this problem by demonstrating that severe loss of microtubules impairs polarity onset in C. elegans.

Sequential functioning of the ECT-2 RhoGEF, RHO-1 and CDC-42 establishes cell polarity in Caenorhabditis elegans embryos

During development, the establishment of cell polarity is important for cells to undergo asymmetric cell divisions that give rise to diverse cell types. In C. elegans embryos, cues from the centrosome trigger the cortical flow of an actomyosin network, leading to the formation of anterior-posterior polarity. However, its precise mechanism is poorly understood. Here, we show that small GTPases have sequential and crucial functions in this process. ECT-2, a potential guanine nucleotide-exchange factor (GEF) for RHO-1, was uniformly distributed at the cortex before polarization, but was excluded from the posterior cortex by the polarity cue from the centrosomes. This local exclusion of ECT-2 led to an asymmetric RHO-1 distribution, which generated a cortical flow of the actomyosin that translocated PAR proteins and CDC-42 (Refs 4, 5) to the anterior cortex. Polarized CDC-42 was, in turn, involved in maintaining the established anterior-cortical domains. Our results suggest that a local change in the function of ECT-2 and RHO-1 links the centrosomal polarity cue with the polarization of the cell cortex.

Two phases of astral microtubule activity during cytokinesis in C. elegans embryos

Microtubules of the mitotic spindle are believed to provide positional cues for the assembly of the actin-based contractile ring and the formation of the subsequent cleavage furrow during cytokinesis. In Caenorhabditis elegans, astral microtubules have been thought to inhibit cortical contraction outside the cleavage furrow. Here, we demonstrate by live imaging and RNA interference (RNAi) that astral microtubules play two distinct roles in initiating cleavage furrow formation. In early anaphase, microtubules are required for contractile ring assembly; in late anaphase, microtubules show different cortical behavior and seem to suppress cortical contraction at the poles, as suggested in previous studies. These two distinct phases of microtubule behavior depend on distinct regulatory pathways, one involving the gamma-tubulin complex and the other requiring aurora-A kinase. We propose that temporal and spatial regulation of two distinct phases of astral microtubule behavior is crucial in specifying the position and timing of furrowing.

Myosin-II reorganization during mitosis is controlled temporally by its dephosphorylation and spatially by Mid1 in fission yeast

Cytokinesis in many eukaryotes requires an actomyosin contractile ring. Here, we show that in fission yeast the myosin-II heavy chain Myo2 initially accumulates at the division site via its COOH-terminal 134 amino acids independently of F-actin. The COOH-terminal region can access to the division site at early G2, whereas intact Myo2 does so at early mitosis. Ser1444 in the Myo2 COOH-terminal region is a phosphorylation site that is dephosphorylated during early mitosis. Myo2 S1444A prematurely accumulates at the future division site and promotes formation of an F-actin ring even during interphase. The accumulation of Myo2 requires the anillin homologue Mid1 that functions in proper ring placement. Myo2 interacts with Mid1 in cell lysates, and this interaction is inhibited by an S1444D mutation in Myo2. Our results suggest that dephosphorylation of Myo2 liberates the COOH-terminal region from an intramolecular inhibition. Subsequently, dephosphorylated Myo2 is anchored by Mid1 at the medial cortex and promotes the ring assembly in cooperation with F-actin.

Importance of a myosin II-containing progenitor for actomyosin ring assembly in fission yeast

An actomyosin-based contractile ring provides the forces necessary for cell cleavage in several organisms [1-3]. Myosin II is an essential component of the actomyosin ring and has also been detected as a "spot" in interphase Schizosaccharomyces pombe cells [4-5]. It is currently unknown if this myosin II-containing spot is important for cytokinesis. In this study, we characterize this myosin II-containing spot using a combination of genetic and cell biological analyses. Whereas myosin II at the actomyosin ring undergoes rapid turnover, myosin II at the spot does not. Maintenance of the myosin II-containing spot is independent of F-actin function. Interestingly, maintenance of this myosin II spot in interphase requires the function of Rng3p, a UCS domain-containing protein, the Caenorhabditis elegans homolog of which has recently been shown to be a cochaperone for myosin II assembly [6]. Disassembly of the spot in interphase prevents actomyosin ring formation in the subsequent mitosis, implying that the spot might represent a progenitor that is important for assembly of the actomyosin ring. Given that mitosis represents a short period of the fission yeast cell cycle, organization of this progenitor structure in interphase might ensure proper assembly of the actomyosin ring and successful cell division.

Contractile ring formation in Xenopus egg and fission yeast

How actin filaments (F-actin) and myosin II (myosin) assemble to form the contractile ring was investigated with fission yeast and Xenopus egg. In fission yeast cells, an aster-like structure composed of F-actin cables is formed at the medial cortex of the cell during prophase to metaphase, and a single F-actin cable(s) extends from this structure, which seems to be a structural basis of the contractile ring. In early mitosis, myosin localizes as dots in the medial cortex independently of F-actin. Then they fuse with each other and are packed into a thin contractile ring. At the growing ends of the cleavage furrow of Xenopus eggs, F-actin at first assembles to form patches. Next they fuse with each other to form short F-actin bundles. The short bundles then form long bundles. Myosin seems to be transported by the cortical movement to the growing end and assembles there as spots earlier than F-actin. Actin polymerization into the patches is likely to occur after accumulation of myosin. The myosin spots and the F-actin patches are simultaneously reorganized to form the contractile ring bundles. The idea that a Ca signal triggers cleavage furrow formation was tested with Xenopus eggs during the first cleavage. We could not detect any Ca signals such as a Ca wave, Ca puffs or even Ca blips at the growing end of the cleavage furrow. Furthermore, cleavages are not affected by Ca-chelators injected into the eggs at concentrations sufficient to suppress the Ca waves. Thus we conclude that formation of the contractile ring is not induced by a Ca signal at the growing end of the cleavage furrow.

Identification of two type V myosins in fission yeast, one of which functions in polarized cell growth and moves rapidly in the cell

We characterized the novel Schizosaccharomyces pombe genes myo4(+) and myo5(+), both of which encode myosin-V heavy chains. Disruption of myo4 caused a defect in cell growth and led to an abnormal accumulation of secretory vesicles throughout the cytoplasm. The mutant cells were rounder than normal, although the sites for cell polarization were still established. Elongation of the cell ends and completion of septation required more time than in wild-type cells, indicating that Myo4 functions in polarized growth both at the cell ends and during septation. Consistent with this conclusion, Myo4 was localized around the growing cell ends, the medial F-actin ring, and the septum as a cluster of dot structures. In living cells, the dots of green fluorescent protein-tagged Myo4 moved rapidly around these regions. The localization and movement of Myo4 were dependent on both F-actin cables and its motor activity but seemed to be independent of microtubules. Moreover, the motor activity of Myo4 was essential for its function. These results suggest that Myo4 is involved in polarized cell growth by moving with a secretory vesicle along the F-actin cables around the sites for polarization. In contrast, the phenotype of myo5 null cells was indistinguishable from that of wild-type cells. This and other data suggest that Myo5 has a role distinct from that of Myo4.

Identification and functional analysis of the gene for type I myosin in fission yeast

BACKGROUND

Type I myosin is highly conserved among eukaryotes, and apparently plays important roles in a number of cellular processes. In the budding yeast, two myosin I species have been identified and their role in F-actin assembly has been inferred.

RESULTS

We cloned the fission yeast myo1 gene, which apparently encoded a myosin I protein. Disruption of myo1 was not lethal, but it caused growth retardation at high and low temperatures, sensitivity to a high concentration of KCl, and aberrance in cell morphology associated with an abnormal distribution of F-actin patches. An abnormal deposition of cell wall materials was also seen. Homothallic myo1Delta cells could mate, but heterothallic myo1Delta cells were poor in conjugation. Myo1p was necessary for the encapsulation of spores. The tail domain of Myo1p was pivotal for its function. Calmodulin could bind to Myo1p through the IQ domain at the neck.

CONCLUSIONS

Myo1p appears to control the redistribution of F-actin patches during the cell cycle. Loss of Myo1p function is likely to slow down the actin assembly/disassembly process, which results in a failure of the actin cycle to catch up with other events in both the mitotic and meiotic cell cycles, including extension of the conjugation tubes.

The S. pombe rlc1 gene encodes a putative myosin regulatory light chain that binds the type II myosins myo3p and myo2p

In order to identify additional components important for cell division in the fission yeast Schizosaccharomyces pombe we have screened a bank of conditional cold-sensitive mutants for cytokinesis defects. One of these mutants showed a delay in cell cleavage, and strong genetic interactions with other genes implicated in medial ring formation. Cloning of the corresponding gene indicates that it encodes a protein with significant homology to the regulatory light chain of non-muscle myosins. We have named the gene rlc1 (regulatory light chain 1). The gene is not essential for division, but null mutants display a cell cleavage defect and form an aberrant F-actin ring. Two myosin-II heavy chains have been identified in fission yeast: Co-immunoprecipitation experiments indicate that rlc1p associates more strongly with myo3p than myo2p.

Molecular mechanism of myosin-II assembly at the division site in Schizosaccharomyces pombe

Schizosaccharomyces pombe cells divide by virtue of the F-actin-based contractile ring (F-actin ring). Two myosin-II heavy chains, Myo2 and Myp2/Myo3, have been localized to the F-actin ring. Here, we investigated the mechanism of myosin-II assembly at the division site in S. pombe cells. First, we showed that Cdc4, an EF-hand protein, appears to be a common myosin light chain associated with both Myo2 and Myo3. Loss of function of both Myo2 and Myo3 caused a defect in F-actin assembly at the division site, like the phenotype of cdc4 null cells. It is suggested that Myo2, Myo3 and Cdc4 function in a cooperative manner in the formation of the F-actin ring during mitosis. Next, we investigated the dynamics of myosin-II during mitosis in S. pombe cells. In early mitosis when accumulation of F-actin cables in the medial region was not yet observed, Myo2 was detected primarily as dots widely located in the medial cortex. Myo2 fibers also became visible following the appearance of the dots. The Myo2 dots and fibers then fused with each other to form a medial cortical network. Some Myo2 dots appeared to be localized with F-actin cables which are also accumulated in the medial region. Finally these structures were packed into a thin contractile ring. In mutant cells that cannot form the F-actin ring such as cdc3(ts), cdc8(ts) and cdc12(ts), Myo2 was able to accumulate as dots in the medial cortex, whereas no accumulation of Myo2 dots was detected in cdc4(ts) cells. Moreover, disruption of F-actin in the cell by applying latrunculin-A did not affect the accumulation of Myo2 dots, suggesting that F-actin is not required for their accumulation. A truncated Myo2 which lacks putative Cdc4-binding sites (Myo2dIQs) was able to rescue myo2 null cells, myo3 null cells, cdc4(ts) mutant cells and cdc4 null cells. The Myo2dIQs could assemble into a normal-shaped ring in these cells. Therefore, its assembly at the division site does not require the function of either Cdc4 or Myo3.

Overproduction of elongation factor 1alpha, an essential translational component, causes aberrant cell morphology by affecting the control of growth polarity in fission yeast

BACKGROUND

Elongation factor 1alpha (EF1alpha), an essential component of the eukaryotic translational machinery, has been shown to possess various biochemical and biological activities, including F-actin-binding and -bundling, microtubule- severing, and the activity of making fibroblasts highly susceptible to transformation. However, our understanding of the biological significance of EF1alpha with respect to these various biochemical or biological activities remains limited. Here we report the identification of EF1alpha-encoding genes as genes whose over-expression causes aberrant cell morphology in fission yeast.

RESULTS

Overproduction of EF1alpha caused aberrant cell morphology-elliptic, curved or branched-and growth defects in yeast cells at high temperatures. EF1alpha-overproducing cells showed a supersensitivity to the actin inhibitor cytochalasin D and to the tubulin inhibitor thiabendazole. Genetic analyses using cdc mutants suggested that excess EF1alpha disturbed the establishment and the maintenance of growth polarity in the G1 phase by pre- venting the localization of F-actin to the polarized growing site and the organization of microtubules. Results from DNase I column chromatography indicated that EF1alpha was bound to G-actin. Indeed, the fission yeast actin was immunoprecipitated along with EF1alpha. Moreover, the temperature sensitivity caused by the overproduction of EF1alpha was restored by co-overproduction of actin.

CONCLUSIONS

Fission yeast EF1alpha has the ability to alter the cell morphology of yeast by affecting the control of actin and microtubule cytoskeletons.

Identification of Myo3, a second type-II myosin heavy chain in the fission yeast Schizosaccharomyces pombe

We cloned the myo3+ gene of Schizosaccharomyces pombe which encodes a type-II myosin heavy chain. myo3 null cells showed a defect in cytokinesis under certain conditions. Overproduction of Myo3 also showed a defect in cytokinesis. Double mutant analysis indicated that Myo3 genetically interacts with Cdc8 tropomyosin and actin. Myo3 may be implicated in cytokinesis and stabilization of F-actin cables. Moreover, the function of Myo2 can be replaced by overexpressed Myo3. We observed a modest synthetic interaction between Myo2 and Myo3. Thus, Myo2 and Myo3 seem to cooperate in the formation of the F-actin ring in S. pombe.