Central Roles of Small GTPases in the Development of Cell Polarity in Yeast and Beyond

Summary: The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.     

The Glc7p-interacting protein Bud14p attenuates polarized growth,pheromone response,and filamentous growth in Saccharomyces cerevisiae

A genetic selection in Saccharomyces cerevisiae for mutants that stimulate the mating pathway uncovered a mutant that had a hyperactive pheromone response pathway and also had hyperpolarized growth. Cloning and segregation analysis demonstrated that BUD14 was the affected gene. Disruption of BUD14 in wild-type cells caused mild stimulation of pheromone response pathway reporters, an increase in sensitivity to mating factor, and a hyperelongated shmoo morphology. The bud14 mutant also had hyperfilamentous growth. Consistent with a role in the control of cell polarity, a Bud14p-green fluorescent protein fusion was localized to sites of polarized growth in the cell. Bud14p shared morphogenetic functions with the Ste20p and Bni1p proteins as well as with the type 1 phosphatase Glc7p. The genetic interactions between BUD14 and GLC7 suggested a role for Glc7p in filamentous growth, and Glc7p was found to have a positive function in filamentous growth in yeast.     

A link between very long chain fatty acid elongation and mating-specific yeast cell cycle arrest

Ceramides and sphingolipid intermediates are well-established regulators of the cell cycle. In the budding yeast Saccharomyces cerevisae, the complex sphingolipid backbone, ceramide, comprises a long chain sphingoid base, a polar head group, and a very long chain fatty acid (VLCFA). While ceramides and long chain bases have been extensively studied as to their roles in regulating cell cycle arrest under multiple conditions, the roles of VLCFAs are not well understood. Here, we used the yeast elo2 and elo3 mutants, which are unable to elongate fatty acids, as tools to explore if maintaining VLCFA elongation is necessary for cell cycle arrest in response to yeast mating. We found that both elo2 and elo3 cells had severely reduced mating efficiencies and were unable to form polarized shmoo projections that are necessary for cell-cell contact during mating. They also lacked functional MAP kinase signaling activity and were defective in initiating a cell cycle arrest in response to pheromone. Additional data suggests that mislocalization of the Ste5 scaffold in elo2 and elo3 mutants upon mating initiation may be responsible for the inability to initiate a cell cycle arrest. Moreover, the lack of proper Ste5 localization may be caused by the inability of mutant cells to mobilize PIP₂. We suggest that VLCFAs are required for Ste5 localization, which is a necessary event for initiating MAP kinase signaling and cell cycle arrest during yeast mating initiation.     

Cryptococcus neoformans Hyperfilamentous Strain Is Hypervirulent in a Murine Model of Cryptococcal Meningoencephalitis

Cryptococcus neoformans is a human fungal pathogen that causes lethal infections of the lung and central nervous system in immunocompromised individuals. C. neoformans has a defined bipolar sexual life cycle with a and α mating types. During the sexual cycle, which can occur between cells of opposite mating types (bisexual reproduction) or cells of one mating type (unisexual reproduction), a dimorphic transition from yeast to hyphal growth occurs. Hyphal development and meiosis generate abundant spores that, following inhalation, penetrate deep into the lung to enter the alveoli, germinate, and establish a pulmonary infection growing as budding yeast cells. Unisexual reproduction has been directly observed only in the Cryptococcus var. neoformans (serotype D) lineage under laboratory conditions. However, hyphal development has been previously associated with reduced virulence and the serotype D lineage exhibits limited pathogenicity in the murine model. In this study we show that the serotype D hyperfilamentous strain XL280α is hypervirulent in an animal model. It can grow inside the lung of the host, establish a pulmonary infection, and then disseminate to the brain to cause cryptococcal meningoencephalitis. Surprisingly, this hyperfilamentous strain triggers an immune response polarized towards Th2-type immunity, which is usually observed in the highly virulent sibling species C. gattii, responsible for the Pacific Northwest outbreak. These studies provide a technological advance that will facilitate analysis of virulence genes and attributes in C. neoformans var. neoformans, and reveal the virulence potential of serotype D as broader and more dynamic than previously appreciated.     

Microscopy of Fission Yeast Sexual Lifecycle

The fission yeast Schizosaccharomyces pombe has been an invaluable model system in studying the regulation of the mitotic cell cycle progression, the mechanics of cell division and cell polarity. Furthermore, classical experiments on its sexual reproduction have yielded results pivotal to current understanding of DNA recombination and meiosis. More recent analysis of fission yeast mating has raised interesting questions on extrinsic stimuli response mechanisms, polarized cell growth and cell-cell fusion. To study these topics in detail we have developed a simple protocol for microscopy of the entire sexual lifecycle. The method described here is easily adjusted to study specific mating stages. Briefly, after being grown to exponential phase in a nitrogen-rich medium, cell cultures are shifted to a nitrogen-deprived medium for periods of time suited to the stage of the sexual lifecycle that will be explored. Cells are then mounted on custom, easily built agarose pad chambers for imaging. This approach allows cells to be monitored from the onset of mating to the final formation of spores.     

Specific protein targeting during cell differentiation: polarized localization of Fus1p during mating depends on Chs5p in Saccharomyces cerevisiae

In budding yeast, chs5 mutants are defective in chitin synthesis and cell fusion during mating. Chs5p is a late-Golgi protein required for the polarized transport of the chitin synthase Chs3p to the membrane. Here we show that Chs5p is also essential for the polarized targeting of Fus1p, but not of other cell fusion proteins, to the membrane during mating.     

Symmetry Breaking in the Life Cycle of the Budding Yeast

The budding yeast Saccharomyces cerevisiae has been an invaluable model system for the study of the establishment of cellular asymmetry and growth polarity in response to specific physiological cues. A large body of experimental observations has shown that yeast cells are able to break symmetry and establish polarity through two coupled and partially redundant intrinsic mechanisms, even in the absence of any pre-existing external asymmetry. One of these mechanisms is dependent upon interplay between the actin cytoskeleton and the Rho family GTPase Cdc42, whereas the other relies on a Cdc42 GTPase signaling network. Integral to these mechanisms appear to be positive feedback loops capable of amplifying small and stochastic asymmetries. Spatial cues, such as bud scars and pheromone gradients, orient cell polarity by modulating the regulation of the Cdc42 GTPase cycle, thereby biasing the site of asymmetry amplification.The budding yeast Saccharomyces cerevisiae is a gift of nature, not just for its superb ability in fermentation to provide us food for hunger and pastime, but also for its relatively simple physiology, which has illuminated our understanding of many fundamental cellular processes. In particular, asymmetry is a way of life for the budding yeast, both when it grows vegetatively and initiates sexual reproductive cycles; as such, yeast has been an invaluable model for studying the establishment of cellular asymmetry. A haploid yeast cell in the G1 phase, which is round and grows isotropically, faces two options: to enter the mitotic cell cycle and grow a bud, or to refrain from cell cycle entry and form a mating projection (shmoo) toward a cell of the opposite mating type. In either case, the cell has to break symmetry to switch from isotropic growth to growth along a polarized axis (Fig. 1). These processes of cell polarity establishment are triggered either by internal signals from the cell cycle engine (budding) or by an external signal in the form of a pheromone gradient (mating).Open in a separate windowFigure 1.Symmetry breaking processes in the life cycle of budding yeast. Shown are the locations of actin patches, actin cables, and Cdc42 during polarized growth for both cycling cells and cells undergoing pheromone response. In G1 cells, Cdc42 is distributed symmetrically, and the actin cytoskeleton is not polarized. In response to cell cycle signals or mating pheromone stimulation, Cdc42 and the actin cytoskeleton become polarized: Cdc42 forms a “polar cap” and actin cables become oriented to allow for targeted secretion. Polarized growth further leads to formation of a bud (cell cycle signal) or formation of a mating projection (pheromone signal). Images represent GFP-Cdc42 (green), and rhodamine-phalloidin staining of filamentous actin (red).Pioneering work involving isolation and characterization of mutants deficient in various aspects of budding and shmoo formation identified key components of the molecular pathways underlying yeast polarized morphogenesis. Despite the relative simplicity of yeast, it has become increasingly clear that many of the genes that control the establishment of cell polarity are conserved between yeast and more complex eukaryotic organisms (see McCaffrey and Macara 2009; Munro and Bowerman 2009; Wang 2009; Nelson 2009). In particular, the small GTPase Cdc42, first discovered in yeast (Adams et al. 1990) and subsequently shown to be required for cell polarization in many eukaryotic organisms (Etienne-Manneville 2004), is the central regulator of yeast polarity.Common principles have begun to emerge to explain symmetry breaking under varying physiological conditions. One of these principles is the self-organizing nature of cell polarity. Whereas under physiological conditions yeast cells polarize toward an environmental asymmetry (pheromone gradient) or a “landmark,” i.e., the bud scar, deposited on the cell surface from a previous division (in a process called bud site selection), it is clear that the ability to undergo symmetry breaking to establish polarity in a random orientation is independent of these cues. It is tempting to speculate that the basic molecular machinery for symmetry breaking, which is required for asexual proliferation through budding, might have evolved independently of the machinery underlying mating and bud site selection.As in all polarized cell systems, yeast polarity is manifested as both an asymmetry in the distribution of signaling molecules and in the organization of the cytoskeleton. In yeast, the switch from an isotropic distribution of Cdc42 on the plasma membrane to a polarized distribution (Fig. 1) is required for the polarized organization of the actin cytoskeleton and membrane trafficking systems, and eventually orientated cell growth. Recent work also showed that the cytoskeleton and the membrane trafficking system can in turn impact the localization of Cdc42 and possibly other membrane‐associated regulatory molecules (Karpova et al. 2000; Wedlich-Soldner et al. 2004; Irazoqui et al. 2005; Zajac et al. 2005). A combination of experimental and theoretical analyses strongly suggests that the interplay between signaling and structural pathways is at the heart of the cell’s intrinsic ability to break symmetry.As there have been recent review articles on the polarized organization of budding yeast growth systems (Bretscher 2003; Pruyne et al. 2004b) and on the molecular parts list involved in cell polarization (Park and Bi 2007), this article is specifically focused on the mechanisms of symmetry breaking at two levels: first as a self-organization process accomplished through dynamic interplay between intrinsic signaling and cytoskeletal systems, which enables vegetative proliferation through bud formation; and second, as an adaptive process where polarity is spatially harnessed by physical cues that arise during bud-site selection and mating. Finally, we briefly extend our discussion to include the role of polarity in yeast aging and cell fate determination. This exciting, relatively new area of research has made important advances in our understanding of how asymmetry can be an important mechanism to ensure long-lasting fitness of a fast proliferating population.     

Specification of sites for polarized growth in Saccharomyces cerevisiae and the influence of external factors on site selection.

Many eucaryotic cell types exhibit polarized cell growth and polarized cell division at nonrandom sites. The sites of polarized growth were investigated in G1 arrested haploid Saccharomyces cerevisiae cells. When yeast cells are arrested during G1 either by treatment with alpha-factor or by shifting temperature-sensitive cdc28-1 cells to the restrictive temperature, the cells form a projection. Staining with Calcofluor reveals that in both cases the projection usually forms at axial sites (i.e., next to the previous bud scar); these are the same sites where bud formation is expected to occur. These results indicate that sites of polarized growth are specified before the end of G1. Sites of polarized growth can be influenced by external conditions. Cells grown to stationary phase and diluted into fresh medium preferentially select sites for polarized growth opposite the previous bud scar (i.e., distal sites). Incubation of cells in a mating mixture results in projection formation at nonaxial sites: presumably cells form projections toward their mating partner. These observations have important implications in understanding three aspects of cell polarity in yeast: 1) how yeast cell shape is influenced by growth conditions 2) how sites of polarized growth are chosen, and 3) the pathway by which polarity is affected and redirected during the mating process.     

The Coordination of Cell Growth during Fission Yeast Mating Requires Ras1-GTP Hydrolysis

The spatial and temporal control of polarity is fundamental to the survival of all organisms. Cells define their polarity using highly conserved mechanisms that frequently rely upon the action of small GTPases, such as Ras and Cdc42. Schizosaccharomyces pombe is an ideal system with which to study the control of cell polarity since it grows from defined tips using Cdc42-mediated actin remodeling. Here we have investigated the importance of Ras1-GTPase activity for the coordination of polarized cell growth during fission yeast mating. Following pheromone stimulation, Ras1 regulates both a MAPK cascade and the activity of Cdc42 to enable uni-directional cell growth towards a potential mating partner. Like all GTPases, when bound to GTP, Ras1 adopts an active conformation returning to an inactive state upon GTP-hydrolysis, a process accelerated through interaction with negative regulators such as GAPs. Here we show that, at low levels of pheromone stimulation, loss of negative regulation of Ras1 increases signal transduction via the MAPK cascade. However, at the higher concentrations observed during mating, hyperactive Ras1 mutations promote cell death. We demonstrate that these cells die due to their failure to coordinate active Cdc42 into a single growth zone resulting in disorganized actin deposition and unsustainable elongation from multiple tips. These results provide a striking demonstration that the deactivation stage of Ras signaling is fundamentally important in modulating cell polarity.     

Polarized growth in fungi: symmetry breaking and hyphal formation

Cell shape is a critical determinant for function. The baker's yeast Saccharomyces cerevisiae changes shape in response to its environment, growing by budding in rich nutrients, forming invasive pseudohyphal filaments in nutrient poor conditions and pear shaped shmoos for growth towards a partner during mating. The human opportunistic pathogen Candida albicans can switch from budding to hyphal growth, in response to numerous environmental stimuli to colonize and invade its host. Hyphal growth, typical of filamentous fungi, is not observed in S. cerevisiae. A number of internal cues regulate when and where yeast cells break symmetry leading to polarized growth and ultimately distinct cell shapes. This review discusses how cells break symmetry using the yeast S. cerevisiae paradigm and how polarized growth is initiated and maintained to result in dramatic morphological changes during C. albicans hyphal growth.     

A Model for Cell Wall Dissolution in Mating Yeast Cells: Polarized Secretion and Restricted Diffusion of Cell Wall Remodeling Enzymes Induces Local Dissolution

Mating of the budding yeast, Saccharomyces cerevisiae, occurs when two haploid cells of opposite mating types signal using reciprocal pheromones and receptors, grow towards each other, and fuse to form a single diploid cell. To fuse, both cells dissolve their cell walls at the point of contact. This event must be carefully controlled because the osmotic pressure differential between the cytoplasm and extracellular environment causes cells with unprotected plasma membranes to lyse. If the cell wall-degrading enzymes diffuse through the cell wall, their concentration would rise when two cells touched each other, such as when two pheromone-stimulated cells adhere to each other via mating agglutinins. At the surfaces that touch, the enzymes must diffuse laterally through the wall before they can escape into the medium, increasing the time the enzymes spend in the cell wall, and thus raising their concentration at the point of attachment and restricting cell wall dissolution to points where cells touch each other. We tested this hypothesis by studying pheromone treated cells confined between two solid, impermeable surfaces. This confinement increases the frequency of pheromone-induced cell death, and this effect is diminished by reducing the osmotic pressure difference across the cell wall or by deleting putative cell wall glucanases and other genes necessary for efficient cell wall fusion. Our results support the model that pheromone-induced cell death is the result of a contact-driven increase in the local concentration of cell wall remodeling enzymes and suggest that this process plays an important role in regulating cell wall dissolution and fusion in mating cells.     

Prm1 Targeting to Contact Sites Enhances Fusion during Mating in Saccharomyces cerevisiae

Prm1 is a pheromone-regulated membrane glycoprotein involved in the plasma membrane fusion event of Saccharomyces cerevisiae mating. Although this function suggests that Prm1 should act at contact sites in pairs of mating yeast cells where plasma membrane fusion occurs, only a small percentage of the total Prm1 was actually detected on the plasma membrane. We therefore investigated the intracellular transport of Prm1 and how this transport contributes to cell fusion. Two Prm1 chimeras that were sorted away from the contact site had reduced fusion activity, indicating that Prm1 indeed functions at contact sites. However, most Prm1 is located in endosomes and other cytoplasmic organelles and is targeted to vacuoles for degradation. Mutations in a putative endocytosis signal in a cytoplasmic loop partially stabilized the Prm1 protein and caused it to accumulate on the plasma membrane, but this endocytosis mutant actually had reduced mating activity. When Prm1 was expressed from a galactose-regulated promoter and its synthesis was repressed at the start of mating, vanishingly small amounts of Prm1 protein remained at the time when the plasma membranes came into contact. Nevertheless, this stable pool of Prm1 was retained at polarized sites on the plasma membrane and was sufficient to promote plasma membrane fusion. Thus, the amount of Prm1 expressed in mating yeast is far in excess of the amount required to facilitate fusion.Membrane fusion has been studied extensively in the context of viral infection and intracellular membrane fusion. These fusion events are mediated by fusases—proteins that mediate membrane fusion. Some of the best-studied fusases are the SNAREs (soluble N-ethylmaleimide-sensitive factors) that mediate fusion of intracellular organelles and the hemagglutinin (HA) protein of influenza virus that mediates fusion of the viral envelope membrane with host endosomes (13). However, little is known about how the plasma membranes of two cells fuse during cell fusion.Cell fusion is essential for the development of multicellular organisms. Some cell fusion processes involve a single pair of cells, as in sperm-egg fusion. Many other developmental processes require multiple fusion events, as in fusion of myoblasts for muscle formation. However, all fusion events must overcome a common obstacle—maintaining the integrity and selective permeability of the two plasma membranes while fusing the hydrophobic cores of their phospholipid bilayers.We study cell fusion in mating pairs of the yeast Saccharomyces cerevisiae. This organism offers a genetically tractable model amenable to many biochemical and cell biological assays. The mating pathway in yeast is comprised of 5 steps: pheromone signaling, adhesion, degradation of the intervening cell walls, plasma membrane fusion, and karyogamy. S. cerevisiae has two haploid mating types: MATa and MATα. Haploid cells secrete pheromones that bind to G-protein-coupled receptors on the surface of cells of the opposite mating type. Pheromone binding activates a signaling cascade that causes cell cycle arrest, expression of pheromone-inducible genes, and polarized growth to form a mating projection (or shmoo tip). The binding of two cells of opposite mating type to form a mating pair is mediated by complementary agglutinins located on the shmoo tips. Then, the cell walls of the two cells are joined to form a unified wall protecting the mating pair, and the walls between the two cells are degraded. This allows the plasma membranes to come into contact and fuse. The initial fusion pore between cells expands to allow cytoplasmic mixing and, ultimately, karyogamy. After mating is complete, the mitotic cell cycle resumes, and a diploid daughter cell buds from the neck connecting the two parent cells (5, 30).This work focuses on Prm1, a glycoprotein that promotes the plasma membrane fusion step of mating. PRM1 was discovered in a bioinformatic screen designed to identify Prm (pheromone-regulated membrane) proteins (11). Prm1 has four transmembrane domains and functions as a disulfide-linked dimer (20). Prm1-deficient mating pairs experience one of three fates: arrest as late prezygotes (unfused mating pairs with no intervening cell walls), lysis once their plasma membranes come into contact, or fusion. Electron microscopy revealed that the two plasma membranes in a late prezygote were only ∼8 nm apart but did not fuse. Additional studies showed that ∼30% of prm1Δ mating pairs lyse after membrane contact (1, 14). However, 50% of prm1Δ mating pairs fuse on standard yeast extract-peptone-dextrose (YPD) medium, implying that Prm1 is important, but not required, for fusion. Mating becomes more dependent upon Prm1 activity if Ca²⁺ or ergosterol is limiting (1, 15).On the basis of its apparent role in membrane fusion, Prm1 should be targeted to the contact sites where membranes fuse. Surprisingly, only a small amount of Prm1 was found at contact sites, and even less was at shmoo tips or at bud tips in mitotic cells expressing Prm1 from a constitutive promoter. These observations prompted further investigation of Prm1''s intracellular transport. The results revealed that Prm1 does indeed function at contact sites. However, except for the small pool that promotes fusion, Prm1 proteins are transported to vacuoles and rapidly degraded.     

The unfolded protein response has a protective role in yeast models of classic galactosemia

Classic galactosemia is a human autosomal recessive disorder caused by mutations in the GALT gene (GAL7 in yeast), which encodes the enzyme galactose-1-phosphate uridyltransferase. Here we show that the unfolded protein response pathway is triggered by galactose in two yeast models of galactosemia: lithium-treated cells and the gal7Δ mutant. The synthesis of galactose-1-phosphate is essential to trigger the unfolded protein response under these conditions because the deletion of the galactokinase-encoding gene GAL1 completely abolishes unfolded protein response activation and galactose toxicity. Impairment of the unfolded protein response in both yeast models makes cells even more sensitive to galactose, unmasking its cytotoxic effect. These results indicate that endoplasmic reticulum stress is induced under galactosemic conditions and underscores the importance of the unfolded protein response pathway to cellular adaptation in these models of classic galactosemia.KEY WORDS: Yeast, Galactosemia, UPR, Lithium, Galactose     

Pheromone-encoding mRNA is transported to the yeast mating projection by specific RNP granules

Association of messenger RNAs with large complexes such as processing bodies (PBs) plays a pivotal role in regulating their translation and decay. Little is known about other possible functions of these assemblies. Exposure of haploid yeast cells, carrying mating type “a,” to “α pheromone” stimulates polarized growth resulting in a “shmoo” projection; it also induces synthesis of “a pheromone,” encoded by MFA2. In this paper, we show that, in response to α pheromone, MFA2 mRNA is assembled with two types of granules; both contain some canonical PB proteins, yet they differ in size, localization, motility, and sensitivity to cycloheximide. Remarkably, one type is involved in mRNA transport to the tip of the shmoo, whereas the other—in local translation in the shmoo. Normal assembly of these granules is critical for their movement, localization, and for mating. Thus, MFA2 mRNAs are transported to the shmoo tip, in complex with PB-like particles, where they are locally translated.     

Interconversion of Yeast Mating Types II. Restoration of Mating Ability to Sterile Mutants in Homothallic and Heterothallic Strains

The two mating types of the yeast Saccharomyces cerevisiae can be interconverted in both homothallic and heterothallic strains. Previous work indicates that all yeast cells contain the information to be both a and α and that the HO gene (in homothallic strains) promotes a change in mating type by causing a change at the mating type locus itself. In both heterothallic and homothallic strains, a defective α mating type locus can be converted to a functional a locus and subsequently to a functional α locus. In contrast, action of the HO gene does not restore mating ability to a strain defective in another gene for mating which is not at the mating type locus. These observations indicate that a yeast cell contains an additional copy (or copies) of α information, and lead to the "cassette" model for mating type interconversion. In this model, HMa and hmα loci are blocs of unexpressed α regulatory information, and HMα and hma loci are blocs of unexpressed a regulatory information. These blocs are silent because they lack an essential site for expression, and become active upon insertion of this information (or a copy of the information) into the mating type locus by action of the HO gene.     

Central roles of small GTPases in the development of cell polarity in yeast and beyond.

Summary: The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.     

Pheromone communication in the fission yeast Schizosaccharomyces pombe

Conjugation between two haploid yeast cells is generally controlled by the reciprocal action of diffusible mating pheromones, cells of each mating type releasing pheromones that induce mating-specific changes in cells of the opposite type. Recent studies into pheromone signalling in the fission yeast Schizosaccharomyces pombe have revealed significant parallels with processes in higher eukaryotes and could provide the opportunity for investigating communication in an organism that is amenable to both biochemical and genetic manipulation.