Prm1 prevents contact-dependent lysis of yeast mating pairs

Live imaging-based assay for visualising species-specific interactions in gamete adhesion molecules

Successful gamete fusion requires species-specific membrane adhesion. However, the interaction of adhesion molecules in gametes is difficult to study in real time through low-throughput microscopic observation. Therefore, we developed a live imaging-based adhesion molecule (LIAM) assay to study gamete adhesion molecule interactions in cultured cells. First, we modified a fusion assay previously established for fusogens introduced into cultured cells, and confirmed that our live imaging technique could visualise cell-cell fusion in the modified fusion assay. Next, instead of fusogen, we introduced adhesion molecules including a mammalian gamete adhesion molecule pair, IZUMO1 and JUNO, and detected their temporal accumulation at the contact interfaces of adjacent cells. Accumulated IZUMO1 or JUNO was partly translocated to the opposite cells as discrete spots; the mutation in amino acids required for their interaction impaired accumulation and translocation. By using the LIAM assay, we investigated the species specificity of IZUMO1 and JUNO of mouse, human, hamster, and pig in all combinations. IZUMO1 and JUNO accumulation and translocation were observed in conspecific, and some interspecific, combinations, suggesting potentially interchangeable combinations of IZUMO1 and JUNO from different species.

Chemotropism and Cell-Cell Fusion in Fungi

Fungi exhibit an enormous variety of morphologies, including yeast colonies, hyphal mycelia, and elaborate fruiting bodies. This diversity arises through a combination of polar growth, cell division, and cell fusion. Because fungal cells are nonmotile and surrounded by a protective cell wall that is essential for cell integrity, potential fusion partners must grow toward each other until they touch and then degrade the intervening cell walls without impacting cell integrity. Here, we review recent progress on understanding how fungi overcome these challenges. Extracellular chemoattractants, including small peptide pheromones, mediate communication between potential fusion partners, promoting the local activation of core cell polarity regulators to orient polar growth and cell wall degradation. However, in crowded environments, pheromone gradients can be complex and potentially confusing, raising the question of how cells can effectively find their partners. Recent findings suggest that the cell polarity circuit exhibits searching behavior that can respond to pheromone cues through a remarkably flexible and effective strategy called exploratory polarization.

Plasma Membrane Fusion Is Specifically Impacted by the Molecular Structure of Membrane Sterols During Vegetative Development of Neurospora crassa

Cell-to-cell fusion is crucial for the development and propagation of most eukaryotic organisms. Despite this importance, the molecular mechanisms mediating this process are only poorly understood in biological systems. In particular, the step of plasma membrane merger and the contributing proteins and physicochemical factors remain mostly unknown. Earlier studies provided the first evidence of a role of membrane sterols in cell-to-cell fusion. By characterizing different ergosterol biosynthesis mutants of the fungus Neurospora crassa, which accumulate different ergosterol precursors, we show that the structure of the sterol ring system specifically affects plasma membrane merger during the fusion of vegetative spore germlings. Genetic analyses pinpoint this defect to an event prior to engagement of the fusion machinery. Strikingly, this effect is not observed during sexual fusion, suggesting that the specific sterol precursors do not generally block membrane merger, but rather impair subcellular processes exclusively mediating fusion of vegetative cells. At a colony-wide level, the altered structure of the sterol ring system affects a subset of differentiation processes, including vegetative sporulation and steps before and after fertilization during sexual propagation. Together, these observations corroborate the notion that the accumulation of particular sterol precursors has very specific effects on defined cellular processes rather than nonspecifically disturbing membrane functioning. Given the phenotypic similarities of the ergosterol biosynthesis mutants of N. crassa during vegetative fusion and of Saccharomyces cerevisiae cells undergoing mating, our data support the idea that yeast mating is evolutionarily and mechanistically more closely related to vegetative than sexual fusion of filamentous fungi.

The molecular foundations of zygosis

Zygosis is the generation of new biological individuals by the sexual fusion of gamete cells. Our current understanding of eukaryotic phylogeny indicates that sex is ancestral to all extant eukaryotes. Although sexual development is extremely diverse, common molecular elements have been retained. HAP2-GCS1, a protein that promotes the fusion of gamete cell membranes that is related in structure to certain viral fusogens, is conserved in many eukaryotic lineages, even though gametes vary considerably in form and behaviour between species. Similarly, although zygotes have dramatically different forms and fates in different organisms, diverse eukaryotes share a common developmental programme in which homeodomain-containing transcription factors play a central role. These common mechanistic elements suggest possible common evolutionary histories that, if correct, would have profound implications for our understanding of eukaryogenesis.

Plasma Membrane Integrity During Cell-Cell Fusion and in Response to Pore-Forming Drugs Is Promoted by the Penta-EF-Hand Protein PEF1 in Neurospora crassa

Plasma membrane damage commonly occurs during cellular growth and development. To counteract these potentially lethal injuries, membrane repair mechanisms have evolved, which promote the integrity of the lipid bilayer. Although the membrane of fungi is the target of important clinical drugs and agricultural fungicides, the molecular mechanisms which mediate membrane repair in these organisms remain elusive. Here we identify the penta-EF-hand protein PEF1 of the genetic model fungus Neurospora crassa as part of a cellular response mechanism against different types of membrane injury. Deletion of the pef1 gene in the wild type and different lysis-prone gene knockout mutants revealed a function of the protein in maintaining cell integrity during cell-cell fusion and in the presence of pore-forming drugs, such as the plant defense compound tomatine. By fluorescence and live-cell imaging we show that green fluorescent protein (GFP)-tagged PEF1 accumulates at the sites of membrane injury in a Ca2+-dependent manner. Site-directed mutagenesis identified Ca2+-binding domains essential for the spatial dynamics and function of the protein. In addition, the subcellular localization of PEF1 revealed that the syncytial fungal colony undergoes compartmentation in response to antifungal treatment. We propose that plasma membrane repair in fungi constitutes an additional line of defense against membrane-disturbing drugs, thereby expanding the current model of fungal drug resistance mechanisms.

Molecular mechanisms of chemotropism and cell fusion in unicellular fungi

In all eukaryotic phyla, cell fusion is important for many aspects of life, from sexual reproduction to tissue formation. Fungal cells fuse during mating to form the zygote, and during vegetative growth to connect mycelia. Prior to fusion, cells first detect gradients of pheromonal chemoattractants that are released by their partner and polarize growth in their direction. Upon pairing, cells digest their cell wall at the site of contact and merge their plasma membrane. In this Review, I discuss recent work on the chemotropic response of the yeast models Saccharomyces cerevisiae and Schizosaccharomyces pombe, which has led to a novel model of gradient sensing: the cell builds a motile cortical polarized patch, which acts as site of communication where pheromones are released and sensed. Initial patch dynamics serve to correct its position and align it with the gradient from the partner cell. Furthermore, I highlight the transition from cell wall expansion during growth to cell wall digestion, which is imposed by physical and signaling changes owing to hyperpolarization that is induced by cell proximity. To conclude, I discuss mechanisms of membrane fusion, whose characterization remains a major challenge for the future.

Cell fusion in yeast is negatively regulated by components of the cell wall integrity pathway

During mating, Saccharomyces cerevisiae cells must degrade the intervening cell wall to allow fusion of the partners. Because improper timing or location of cell wall degradation would cause lysis, the initiation of cell fusion must be highly regulated. Here, we find that yeast cell fusion is negatively regulated by components of the cell wall integrity (CWI) pathway. Loss of the cell wall sensor, MID2, specifically causes “mating-induced death” after pheromone exposure. Mating-induced death is suppressed by mutations in cell fusion genes (FUS1, FUS2, RVS161, CDC42), implying that mid2Δ cells die from premature fusion without a partner. Consistent with premature fusion, mid2Δ shmoos had thinner cell walls and lysed at the shmoo tip. Normally, Cdc42p colocalizes with Fus2p to form a focus only when mating cells are in contact (prezygotes) and colocalization is required for cell fusion. However, Cdc42p was aberrantly colocalized with Fus2p to form a focus in mid2Δ shmoos. A hyperactive allele of the CWI kinase Pkc1p (PKC1*) caused decreased cell fusion and Cdc42p localization in prezygotes. In shmoos, PKC1* increased Cdc42p localization; however, it was not colocalized with Fus2p or associated with cell death. We conclude that Mid2p and Pkc1p negatively regulate cell fusion via Cdc42p and Fus2p.

Cholesterol suppresses membrane leakage by decreasing water penetrability

Membrane fusion is a fundamental biological process that lies at the heart of enveloped virus infection, synaptic signaling, intracellular vesicle trafficking, gamete fertilization, and cell-cell fusion. Membrane fusion is initiated as two apposed membranes merge to a single bilayer called a hemifusion diaphragm. It is believed that the contents of the two fusing membranes are released through a fusion pore formed at the hemifusion diaphragm, and yet another possible pathway has been proposed in which an undefined pore may form outside the hemifusion diaphragm at the apposed membranes, leading to the so-called leaky fusion. Here, we performed all-atom molecular dynamics simulations to study the evolution of the hemifusion diaphragm structure with various lipid compositions. We found that the lipid cholesterol decreased water penetrability to inhibit leakage pore formation. Biochemical leakage experiments support these simulation results. This study may shed light on the underlying mechanism of the evolution pathways of the hemifusion structure, especially the understanding of content leakage during membrane fusion.

Identification and characterization of LFD-2, a predicted fringe protein required for membrane integrity during cell fusion in neurospora crassa

The molecular mechanisms of membrane merger during somatic cell fusion in eukaryotic species are poorly understood. In the filamentous fungus Neurospora crassa, somatic cell fusion occurs between genetically identical germinated asexual spores (germlings) and between hyphae to form the interconnected network characteristic of a filamentous fungal colony. In N. crassa, two proteins have been identified to function at the step of membrane fusion during somatic cell fusion: PRM1 and LFD-1. The absence of either one of these two proteins results in an increase of germling pairs arrested during cell fusion with tightly appressed plasma membranes and an increase in the frequency of cell lysis of adhered germlings. The level of cell lysis in ΔPrm1 or Δlfd-1 germlings is dependent on the extracellular calcium concentration. An available transcriptional profile data set was used to identify genes encoding predicted transmembrane proteins that showed reduced expression levels in germlings cultured in the absence of extracellular calcium. From these analyses, we identified a mutant (lfd-2, for late fusion defect-2) that showed a calcium-dependent cell lysis phenotype. lfd-2 encodes a protein with a Fringe domain and showed endoplasmic reticulum and Golgi membrane localization. The deletion of an additional gene predicted to encode a low-affinity calcium transporter, fig1, also resulted in a strain that showed a calcium-dependent cell lysis phenotype. Genetic analyses showed that LFD-2 and FIG1 likely function in separate pathways to regulate aspects of membrane merger and repair during cell fusion.

Identification and characterization of LFD1, a novel protein involved in membrane merger during cell fusion in Neurospora crassa

Despite its essential role in development, molecular mechanisms of membrane merger during cell-cell fusion in most eukaryotic organisms remain elusive. In the filamentous fungus Neurospora crassa, cell fusion occurs during asexual spore germination, where genetically identical germlings show chemotropic interactions and cell-cell fusion. Fusion of germlings and hyphae is required for the formation of the interconnected mycelial network characteristic of filamentous fungi. Previously, a multipass membrane protein, PRM1, was characterized and acts at the step of bilayer fusion in N. crassa. Here we describe the identification and characterization of lfd-1, encoding a single pass transmembrane protein, which is also involved in membrane merger. lfd-1 was identified by a targeted analysis of a transcriptional profile of a transcription factor mutant (Δpp-1) defective in germling fusion. The Δlfd-1 mutant shows a similar, but less severe, membrane merger defect as a ΔPrm1 mutant. By genetic analyses, we show that LFD1 and PRM1 act independently, but share a redundant function. The cell fusion frequency of both Δlfd-1 and ΔPrm1 mutants was sensitive to extracellular calcium concentration and was associated with an increase in cell lysis, which was suppressed by a calcium-dependent mechanism involving a homologue to synaptotagmin.

Membrane organization and cell fusion during mating in fission yeast requires multipass membrane protein Prm1

The involvement of Schizosaccharomyces pombe prm1(+) in cell fusion during mating and its relationship with other genes required for this process have been addressed. S. pombe prm1Δ mutant exhibits an almost complete blockade in cell fusion and an abnormal distribution of the plasma membrane and cell wall in the area of cell-cell interaction. The distribution of cellular envelopes is similar to that described for mutants devoid of the Fig1-related claudin-like Dni proteins; however, prm1(+) and the dni(+) genes act in different subpathways. Time-lapse analyses show that in the wild-type S. pombe strain, the distribution of phosphatidylserine in the cytoplasmic leaflet of the plasma membrane undergoes some modification before an opening is observed in the cross wall at the cell-cell contact region. In the prm1Δ mutant, this membrane modification does not take place, and the cross wall between the mating partners is not extensively degraded; plasma membrane forms invaginations and fingers that sometimes collapse/retract and that are sometimes strengthened by the synthesis of cell-wall material. Neither prm1Δ nor prm1Δ dniΔ zygotes lyse after cell-cell contact in medium containing and lacking calcium. Response to drugs that inhibit lipid synthesis or interfere with lipids is different in wild-type, prm1Δ, and dni1Δ strains, suggesting that membrane structure/organization/dynamics is different in all these strains and that Prm1p and the Dni proteins exert some functions required to guarantee correct membrane organization that are critical for cell fusion.

Mate and fuse: how yeast cells do it

Many cells are able to orient themselves in a non-uniform environment by responding to localized cues. This leads to a polarized cellular response, where the cell can either grow or move towards the cue source. Fungal haploid cells secrete pheromones to signal mating, and respond by growing a mating projection towards a potential mate. Upon contact of the two partner cells, these fuse to form a diploid zygote. In this review, we present our current knowledge on the processes of mating signalling, pheromone-dependent polarized growth and cell fusion in Saccharomyces cerevisiae and Schizosaccharomyces pombe, two highly divergent ascomycete yeast models. While the global architecture of the mating response is very similar between these two species, they differ significantly both in their mating physiologies and in the molecular connections between pheromone perception and downstream responses. The use of both yeast models helps enlighten both conserved solutions and species-specific adaptations to a general biological problem.

Genetic basis of cell-cell fusion mechanisms

Cell-cell fusion in sexually reproducing organisms is a mechanism to merge gamete genomes and, in multicellular organisms, it is a strategy to sculpt organs, such as muscle, bone, and placenta. Moreover, this mechanism has been implicated in pathological conditions, such as infection and cancer. Studies of genetic model organisms have uncovered a unifying principle: cell fusion is a genetically programmed process. This process can be divided in three stages: competence (cell induction and differentiation); commitment (cell determination, migration, and adhesion); and cell fusion (membrane merging and cytoplasmic mixing). Recent work has led to the discovery of fusogens, which are cell fusion proteins that are necessary and sufficient to fuse cell membranes. Two unrelated families of fusogens have been discovered, one in mouse placenta and one in Caenorhabditis elegans (syncytins and F proteins, respectively). Current research aims to identify new fusogens and determine the mechanisms by which they merge membranes.

Activation of an essential calcium signaling pathway in Saccharomyces cerevisiae by Kch1 and Kch2, putative low-affinity potassium transporters

In the budding yeast Saccharomyces cerevisiae, mating pheromones activate a high-affinity Ca(2+) influx system (HACS) that activates calcineurin and is essential for cell survival. Here we identify extracellular K(+) and a homologous pair of transmembrane proteins, Kch1 and Kch2 (Prm6), as necessary components of the HACS activation mechanism. Expression of Kch1 and especially Kch2 was strongly induced during the response to mating pheromones. When forcibly overexpressed, Kch1 and Kch2 localized to the plasma membrane and activated HACS in a fashion that depended on extracellular K(+) but not pheromones. They also promoted growth of trk1 trk2 mutant cells in low K(+) environments, suggesting they promote K(+) uptake. Voltage-clamp recordings of protoplasts revealed diminished inward K(+) currents in kch1 kch2 double-mutant cells relative to the wild type. Conversely, heterologous expression of Kch1 in HEK293T cells caused the appearance of inwardly rectifying K(+) currents. Collectively, these findings suggest that Kch1 and Kch2 directly promote K(+) influx and that HACS may electrochemically respond to K(+) influx in much the same way as the homologous voltage-gated Ca(2+) channels in most animal cell types.

Cdc42p and Fus2p act together late in yeast cell fusion

Cdc42p is the master regulator of morphogenesis in eukaryotic cells. It has an additional role in cell fusion, acting later in the pathway, after cells have undergone the changes in polarization and growth required for fusion. Cdc42p acts in concert with Fus2p to allow cell fusion.

Cell fusion is the key event of fertilization that gives rise to the diploid zygote and is a nearly universal aspect of eukaryotic biology. In the yeast Saccharomyces cerevisiae, several mutants have been identified that are defective for cell fusion, and yet the molecular mechanism of this process remains obscure. One obstacle has been that genetic screens have mainly focused on mating-specific factors, whereas the process likely involves housekeeping proteins as well. Here we implicate Cdc42p, an essential protein with roles in multiple aspects of morphogenesis, as a core component of the yeast cell fusion pathway. We identify a point mutant in the Rho-insert domain of CDC42, called cdc42-138, which is specifically defective in cell fusion. The cell fusion defect is not a secondary consequence of ineffective signaling or polarization. Genetic and morphological data show that Cdc42p acts at a late stage in cell fusion in concert with a key cell fusion regulator, Fus2p, which contains a Dbl-homology domain. We find that Fus2p binds specifically with activated Cdc42p, and binding is blocked by the cdc42-138 mutation. Thus, in addition to signaling and morphogenetic roles in mating, Cdc42p plays a role late in cell fusion via activation of Fus2p.

Evolution of programmed cell fusion: common mechanisms and distinct functions

Eukaryotic cells have evolved diverged mechanisms to merge cells. Here, we discuss three types of cell fusion: (1) Non-self-fusion, cells with different genetic contents fuse to start a new organism and fusion between enveloped viruses and host cells; (2) Self-fusion, genetically identical cells fuse to form a multinucleated cell; and (3) Auto-fusion, a single cell fuses with itself by bringing specialized cell membrane domains into contact and transforming itself into a ring-shaped cell. This is a new type of selfish fusion discovered in C. elegans. We divide cell fusion into three stages: (1) Specification of the cell-fusion fate; (2) Cell attraction, attachment, and recognition; (3) Execution of plasma membrane fusion, cytoplasmic mixing and cytoskeletal rearrangements. We analyze cell fusion in diverse biological systems in development and disease emphasizing the mechanistic contributions of C. elegans to the understanding of programmed cell fusion, a genetically encoded pathway to merge specific 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.

Secretion is required for late events in the cell-fusion pathway of mating yeast

Secretory vesicles accumulate adjacent to the contact site between the two cells of a yeast mating pair before they fuse, but there is no direct evidence that secretion is required to complete fusion. In this study, temperature-sensitive secretion (sec(ts)) mutants were used to investigate the role of secretion in yeast cell fusion. Cell fusion arrested less than 5 minutes after inhibiting secretion. This rapid fusion arrest was not an indirect consequence of reduced mating pheromone signaling, mating-pair assembly or actin polarity. Furthermore, secretion was required to complete cell fusion when it was transiently inhibited by addition and removal of the lipophilic styryl dye, FM4-64. These results indicate that ongoing secretion is required for late events in the cell-fusion pathway, which include plasma-membrane fusion and the completion of cell-wall remodeling, and they demonstrate a just-in-time delivery mechanism for the cell-fusion machinery.

The yeast cell fusion protein Prm1p requires covalent dimerization to promote membrane fusion

Prm1p is a multipass membrane protein that promotes plasma membrane fusion during yeast mating. The mechanism by which Prm1p and other putative regulators of developmentally controlled cell-cell fusion events facilitate membrane fusion has remained largely elusive. Here, we report that Prm1p forms covalently linked homodimers. Covalent Prm1p dimer formation occurs via intermolecular disulfide bonds of two cysteines, Cys-120 and Cys-545. PRM1 mutants in which these cysteines have been substituted are fusion defective. These PRM1 mutants are normally expressed, retain homotypic interaction and can traffic to the fusion zone. Because prm1-C120S and prm1-C545S mutants can form covalent dimers when coexpressed with wild-type PRM1, an intermolecular C120-C545 disulfide linkage is inferred. Cys-120 is adjacent to a highly conserved hydrophobic domain. Mutation of a charged residue within this hydrophobic domain abrogates formation of covalent dimers, trafficking to the fusion zone, and fusion-promoting activity. The importance of intermolecular disulfide bonding informs models regarding the mechanism of Prm1-mediated cell-cell fusion.

Structure of sterol aliphatic chains affects yeast cell shape and cell fusion during mating

Under mating conditions, yeast cells adopt a characteristic pear-shaped morphology, called a "shmoo," as they project a cell extension toward their mating partners. Mating partners make contact at their shmoo tips, dissolve the intervening cell wall, and fuse their plasma membranes. We identified mutations in ERG4, encoding the enzyme that catalyzes the last step of ergosterol biosynthesis, that impair both shmoo formation and cell fusion. Upon pheromone treatment, erg4Delta mutants polarized growth, lipids, and proteins involved in mating but did not form properly shaped shmoos and fused with low efficiency. Supplementation with ergosterol partially suppressed the shmooing defect but not the cell fusion defect. By contrast, removal of the Erg4 substrate ergosta-5,7,22,24(28)-tetraenol, which accumulates in erg4Delta mutant cells and contains an extra double bond in the aliphatic chain of the sterol, restored both shmooing and cell fusion to wild-type levels. Thus, a two-atom change in the aliphatic moiety of ergosterol is sufficient to obstruct cell shape remodeling and cell fusion.

Prm1 functions as a disulfide-linked complex in yeast mating

Prm1 is a pheromone-induced membrane glycoprotein that promotes plasma membrane fusion in yeast mating pairs. HA-Prm1 migrates at twice its expected molecular weight on non-reducing SDS-PAGE gels and coprecipitates with Prm1-TAP, indicating that Prm1 is a disulfide-linked homodimer. The N terminus of a plasma membrane-localized GFP-Prm1 endocytic mutant projects into the cytoplasm, where it is protected from low pH quenching in live cells and from external protease in spheroplasts. In a revised topological map, Prm1 has four transmembrane domains and two large extracellular loops. Mutation of all four cysteines in the extracellular loops blocked disulfide bond formation and destabilized the Prm1 homodimer without preventing Prm1 transport to contact sites in mating pairs. Cys(120) in loop 1 and Cys(545) in loop 2 form disulfide cross-links in the Prm1 homodimer and are required for fusion activity. Cys(120) lies between a hydrophobic segment formerly thought to be a transmembrane domain and an amphipathic helix. An interaction between either of these regions and the opposing membrane could promote fusion.

Lipid mixing and content release in single-vesicle, SNARE-driven fusion assay with 1-5 ms resolution

A single-vesicle, fluorescence-based, SNARE-driven fusion assay enables simultaneous measurement of lipid mixing and content release with 5 ms/frame, or even 1 ms/frame, time resolution. The v-SNARE vesicles, labeled with lipid and content markers of different color, dock and fuse with a planar t-SNARE bilayer supported on glass. A narrow (<5 ms duration), intense spike of calcein fluorescence due to content release and dequenching coincides with inner-leaflet lipid mixing within 10 ms. The spike provides more sensitive detection of productive hemifusion events than do lipid labels alone. Consequently, many fast events previously thought to be prompt, full fusion events are now reclassified as productive hemifusion. Both full fusion and hemifusion occur with a time constant of 5-10 ms. At 60% phosphatidylethanolamine lipid composition, productive and dead-end hemifusion account for 65% of all fusion events. However, quantitative analysis shows that calcein is released into the space above the bilayer (vesicle bursting), rather than the thin aqueous space between the bilayer and glass. Evidently, at the instant of inner-leaflet mixing, flattening of the vesicle increases the internal pressure beyond the bursting point. This may be related to in vivo observations suggesting that membrane lysis often competes with membrane fusion.

The tetraspan protein Dni1p is required for correct membrane organization and cell wall remodelling during mating in Schizosaccharomyces pombe

In fungi, success of mating requires that both cells agglutinate, modify their extracellular envelopes, and fuse their plasma membranes and nuclei to produce a zygote. Here we studied the role of the Schizosaccharomyces pombe Dni1 protein in the cell fusion step of mating. Dni1p is a tetraspan protein bearing a conserved cystein motif similar to that present in fungal claudin-related proteins. Dni1p expression is induced during mating and Dni1p concentrates as discrete patches at the cell-cell contact area and along the mating bridge. Proper Dni1p localization depends on Fus1p, actin and integrity of lipid rafts. In dni1Delta mutants, cell differentiation and agglutination are as efficient as in the wild-type strain, but cell fusion is significantly reduced at temperatures above 25 degrees C. We found that the defect in cell fusion was not associated with an altered cytoskeleton, with an abnormal distribution of Fus1p, or with a defect in calcium accumulation, but with a severe disorganization of the plasma membrane and cell wall at the area of cell-cell contact. These results show that Dni1p plays a relevant role in co-ordinating membrane organization and cell wall remodelling during mating, a function that has not been described for other proteins in the fission yeast.

The Saccharomyces cerevisiae PRM1 homolog in Neurospora crassa is involved in vegetative and sexual cell fusion events but also has postfertilization functions

Cell-cell fusion is essential for a variety of developmental steps in many eukaryotic organisms, during both fertilization and vegetative cell growth. Although the molecular mechanisms associated with intracellular membrane fusion are well characterized, the molecular mechanisms of plasma membrane merger between cells are poorly understood. In the filamentous fungus Neurospora crassa, cell fusion events occur during both vegetative and sexual stages of its life cycle, thus making it an attractive model for studying the molecular basis of cell fusion during vegetative growth vs. sexual reproduction. In the unicellular yeast Saccharomyces cerevisiae, one of the few proteins implicated in plasma membrane merger during mating is Prm1p; prm1Delta mutants show an approximately 50% reduction in mating cell fusion. Here we report on the role of the PRM1 homolog in N. crassa. N. crassa strains with deletions of a Prm1-like gene (Prm1) showed an approximately 50% reduction in both vegetative and sexual cell fusion events, suggesting that PRM1 is part of the general cell fusion machinery. However, unlike S. cerevisiae, N. crassa strains carrying a Prm1 deletion exhibited complete sterility as either a male or female mating partner, a phenotype that was not complemented in a heterokaryon with wild type (WT). Crosses with DeltaPrm1 strains were blocked early in sexual development, well before development of ascogenous hyphae. The DeltaPrm1 sexual defect in N. crassa was not suppressed by mutations in Sad-1, which is required for meiotic silencing of unpaired DNA (MSUD). However, mutations in Sad-1 increased the number of progeny obtained in crosses with a DeltaPrm1 (Prm1-gfp) complemented strain. These data indicate multiple roles for PRM1 during sexual development.

Membrane lysis during biological membrane fusion: collateral damage by misregulated fusion machines

In the canonical model of membrane fusion, the integrity of the fusing membranes is never compromised, preserving the identity of fusing compartments. However, recent molecular simulations provided evidence for a pathway to fusion in which holes in the membrane evolve into a fusion pore. Additionally, two biological membrane fusion models-yeast cell mating and in vitro vacuole fusion-have shown that modifying the composition or altering the relative expression levels of membrane fusion complexes can result in membrane lysis. The convergence of these findings showing membrane integrity loss during biological membrane fusion suggests new mechanistic models for membrane fusion and the role of membrane fusion complexes.

Membrane fusion

Subcellular compartmentalization, cell growth, hormone secretion and neurotransmission require rapid, targeted, and regulated membrane fusion. Fusion entails extensive lipid rearrangements by two apposed (docked) membrane vesicles, joining their membrane proteins and lipids and mixing their luminal contents without lysis. Fusion of membranes in the secretory pathway involves Rab GTPases; their bound ‘effector’ proteins, which mediate downstream steps; SNARE proteins, which can ‘snare’ each other, in cis (bound to one membrane) or in trans (anchored to apposed membranes); and SNARE-associated proteins (SM proteins; NSF or Sec18p; SNAP or Sec17p; and others) cooperating with specific lipids to catalyze fusion. In contrast, mitochondrial and cell-cell fusion events are regulated by and use distinct catalysts.

Pheromone-regulated target genes respond differentially to MAPK phosphorylation of transcription factor Prf1

Pheromone signalling during mating is essential for pathogenicity of Ustilago maydis. The activity of the key transcription factor Prf1 is controlled at the transcriptional level and post-translationally by mitogen-activated protein kinase (MAPK) and protein kinase A (PKA) phosphorylation. However, the precise contribution of these regulatory mechanisms to the transcriptional output is unknown. Here, we genetically dissected the three levels of Prf1 regulation. We performed transcriptional profiling of respective mutants to identify and classify targets. This approach revealed that transcriptional regulation of prf1 had only minor influence on target gene expression stressing the importance of post-translational control. PKA regulation of Prf1 was sufficient to control expression of nine pheromone-responsive genes including the major transcription factor regulating pathogenicity. MAPK regulation was necessary for the pheromone response of a set of 57 genes. In 35 cases, pheromone responsiveness was completely lost, while in the remaining 22 cases regulation was alleviated. This indicated a novel level of complexity in MAPK signalling suggesting that target genes respond differentially to MAPK phosphorylation of the respective transcription factors.

Dynamic localization of yeast Fus2p to an expanding ring at the cell fusion junction during mating

Fus2p is a pheromone-induced protein associated with the amphiphysin homologue Rvs161p, which is required for cell fusion during mating in Saccharomyces cerevisiae. We constructed a functional Fus2p–green fluorescent protein (GFP), which exhibits highly dynamic localization patterns in pheromone-responding cells (shmoos): diffuse nuclear, mobile cytoplasmic dots and stable cortical patches concentrated at the shmoo tip. In mitotic cells, Fus2p-GFP is nuclear but becomes cytoplasmic as cells form shmoos, dependent on the Fus3p protein kinase and high levels of pheromone signaling. The rapid cytoplasmic movement of Fus2p-GFP dots requires Rvs161p and polymerized actin and is aberrant in mutants with compromised actin organization, which suggests that the Fus2p dots are transported along actin cables, possibly in association with vesicles. Maintenance of Fus2p-GFP patches at the shmoo tip cortex is jointly dependent on actin and a membrane protein, Fus1p, which suggests that Fus1p is an anchor for Fus2p. In zygotes, Fus2p-GFP forms a dilating ring at the cell junction, returning to the nucleus at the completion of cell fusion.

Ergosterol promotes pheromone signaling and plasma membrane fusion in mating yeast

Ergosterol depletion independently inhibits two aspects of yeast mating: pheromone signaling and plasma membrane fusion. In signaling, ergosterol participates in the recruitment of Ste5 to a polarized site on the plasma membrane. Ergosterol is thought to form microdomains within the membrane by interacting with the long acyl chains of sphingolipids. We find that although sphingolipid-free ergosterol is concentrated at sites of cell–cell contact, transmission of the pheromone signal at contact sites depends on a balanced ratio of ergosterol to sphingolipids. If a mating pair forms between ergosterol-depleted cells despite the attenuated pheromone response, the subsequent process of membrane fusion is retarded. Prm1 also participates in membrane fusion. However, ergosterol and Prm1 have independent functions and only prm1 mutant mating pairs are susceptible to contact-dependent lysis. In contrast to signaling, plasma membrane fusion is relatively insensitive to sphingolipid depletion. Thus, the sphingolipid-free pool of ergosterol promotes plasma membrane fusion.

Cell fusion assays for yeast mating pairs

Yeast mating provides an accessible genetic system for the discovery of fundamental mechanisms in eukaryotic cell fusion. Although aspects of yeast mating related to pheromone signaling and polarized growth have been intensively investigated, fusion itself is poorly understood. This chapter describes methods for measuring the overall efficiency of yeast cell fusion and for monitoring various stages of the fusion process including cell wall remodeling, plasma membrane fusion, and nuclear fusion.

Excess vacuolar SNAREs drive lysis and Rab bypass fusion

Although concentrated soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) drive liposome fusion and lysis, the fusion of intracellular membranes also requires Rab GTPases, Rab effectors, SM proteins, and specific regulatory lipids and is accompanied by little or no lysis. To rationalize these findings, we generated yeast strains that overexpress all four vacuolar SNAREs (4SNARE(++)). Although vacuoles with physiological levels of Rab, Rab effector/SM complex, and SNAREs support rapid fusion without Rab- and SNARE-dependent lysis, vacuoles from 4SNARE(++) strains show extensive lysis and a reduced need for the Rab Ypt7p or regulatory lipids for fusion. SNARE overexpression and the addition of pure homotypic fusion and vacuole protein sorting complex (HOPS), which bears the vacuolar SM protein, enables ypt7Delta vacuoles to fuse, allowing direct comparison of Rab-dependent and Rab-independent fusion. Because 3- to 40-fold more of each of the five components that form the SNARE/HOPS fusion complex are required for vacuoles from ypt7Delta strains to fuse at the same rate as vacuoles from wild-type strains, the apparent forward rate constant of 4SNARE/HOPS complex assembly is enhanced many thousand-fold by Ypt7p. Rabs function in normal membrane fusion by concentrating SNAREs, other proteins (e.g., SM), and key lipids at a fusion site and activating them for fusion without lysis.

Cell-cell fusion

Cell-cell fusion is a highly regulated and dramatic cellular event that is required for development and homeostasis. Fusion may also play a role in the development of cancer and in tissue repair by stem cells. While virus-cell fusion and the fusion of intracellular membranes have been the subject of intense investigation during the past decade, cell-cell fusion remains poorly understood. Given the importance of this cell-biological phenomenon, a number of investigators have begun analyses of the molecular mechanisms that mediate the specialized fusion events of a variety of cell types and species. We discuss recent genetic and biochemical studies that are beginning to yield exciting insights into the fusion mechanisms of Saccharomyces cerevisiae mating pairs, Caenorhabditis elegans epithelial cells and gametes, Drosophila melanogaster and mammalian myoblasts, and mammalian macrophages.

A role for a complex between activated G protein-coupled receptors in yeast cellular mating

Cell-cell fusion is a fundamental process that facilitates a wide variety of biological events in organisms ranging from yeast to humans. However, relatively little is actually understood with respect to fusion mechanisms. In the model organism Saccharomyces cerevisiae, mating of opposite-type cells is triggered by pheromone activation of the G protein-coupled receptors, alpha-factor receptor (Ste2p) and a-factor receptor (Ste3p), leading to mitogen-activated protein kinase signaling, growth arrest, and cellular fusion events. Herein we now provide evidence of a role for these receptors in the later cell fusion stage of mating. In vitro assays demonstrated the ability of the receptors to promote mixing of proteoliposomes containing phosphatidylserine, potentially based on a pheromone-dependent interaction between Ste2p and Ste3p that was confirmed by tandem affinity purification and cellular pull-down assays. The cellular mating activity of Ste2p was subsequently probed in vivo. Notably, a receptor-null yeast strain expressing N-terminally truncated Ste2p yielded a phenotype demonstrating wild-type signaling but arrested mating. The arrested prezygotes showed evidence of some cell wall erosion but no membrane juxtaposition at the fusion site. Further, in vitro analyses correlated this mutation with loss of the interaction between Ste2p and Ste3p and inhibition of related lipid mixing. Overall, these results support a role for a complex between activated yeast pheromone receptors in later cell fusion stages of mating, possibly mediating events at the level of cell wall digestion and membrane juxtaposition before membrane fusion.

Cell-cell membrane fusion during mammalian fertilization

The mechanism of sperm-egg fusion in mammals is a research area that has greatly benefited from the use of gene deletion technology. Because fertilization is internal in mammals and the gametes (particularly the eggs) are sparse in number, in vitro studies have considerable limitations. Using gene deletions, a few cell surface proteins in both gametes have been identified as essential for gamete fusion. Ongoing studies are directed at analysis of the function of these proteins and the search for additional proteins that may be involved in this process. So far, no mammalian proteins have been found that also function in sperm-egg fusion of non-mammalian species or in other types of cell-cell fusion.

The Golgi-resident protease Kex2 acts in conjunction with Prm1 to facilitate cell fusion during yeast mating

The molecular machines that mediate cell fusion are unknown. Previously, we identified a multispanning transmembrane protein, Prm1 (pheromone-regulated membrane protein 1), that acts during yeast mating (Heiman, M.G., and P. Walter. 2000. J. Cell Biol. 151:719-730). Without Prm1, a substantial fraction of mating pairs arrest with their plasma membranes tightly apposed yet unfused. In this study, we show that lack of the Golgi-resident protease Kex2 strongly enhances the cell fusion defect of Prm1-deficient mating pairs and causes a mild fusion defect in otherwise wild-type mating pairs. Lack of the Kex1 protease but not the Ste13 protease results in similar defects. Deltakex2 and Deltakex1 fusion defects were suppressed by osmotic support, a trait shared with mutants defective in cell wall remodeling. In contrast, other cell wall mutants do not enhance the Deltaprm1 fusion defect. Electron microscopy of Deltakex2-derived mating pairs revealed novel extracellular blebs at presumptive sites of fusion. Kex2 and Kex1 may promote cell fusion by proteolytically processing substrates that act in parallel to Prm1 as an alternative fusion machine, as cell wall components, or both.

The plasma membrane proteins Prm1 and Fig1 ascertain fidelity of membrane fusion during yeast mating

As for most cell-cell fusion events, the molecular details of membrane fusion during yeast mating are poorly understood. The multipass membrane protein Prm1 is the only known component that acts at the step of bilayer fusion. In its absence, mutant mating pairs lyse or arrest in the mating reaction with tightly apposed plasma membranes. We show that deletion of FIG 1, which controls pheromone-induced Ca(2+) influx, yields similar cell fusion defects. Although extracellular Ca(2+) is not required for efficient cell fusion of wild-type cells, cell fusion in prm1 mutant mating pairs is dramatically reduced when Ca(2+) is removed. This enhanced fusion defect is due to lysis. Time-lapse microscopy reveals that fusion and lysis events initiate with identical kinetics, suggesting that both outcomes result from engagement of the fusion machinery. The yeast synaptotagmin orthologue and Ca(2+) binding protein Tcb3 has a role in reducing lysis of prm1 mutants, which opens the possibility that the observed role of Ca(2+) is to engage a wound repair mechanism. Thus, our results suggest that Prm1 and Fig1 have a role in enhancing membrane fusion and maintaining its fidelity. Their absence results in frequent mating pair lysis, which is counteracted by Ca(2+)-dependent membrane repair.

Multiple signaling pathways regulate yeast cell death during the response to mating pheromones

Mating pheromones promote cellular differentiation and fusion of yeast cells with those of the opposite mating type. In the absence of a suitable partner, high concentrations of mating pheromones induced rapid cell death in approximately 25% of the population of clonal cultures independent of cell age. Rapid cell death required Fig1, a transmembrane protein homologous to PMP-22/EMP/MP20/Claudin proteins, but did not require its Ca2+ influx activity. Rapid cell death also required cell wall degradation, which was inhibited in some surviving cells by the activation of a negative feedback loop involving the MAP kinase Slt2/Mpk1. Mutants lacking Slt2/Mpk1 or its upstream regulators also underwent a second slower wave of cell death that was independent of Fig1 and dependent on much lower concentrations of pheromones. A third wave of cell death that was independent of Fig1 and Slt2/Mpk1 was observed in mutants and conditions that eliminate calcineurin signaling. All three waves of cell death appeared independent of the caspase-like protein Mca1 and lacked certain "hallmarks" of apoptosis. Though all three waves of cell death were preceded by accumulation of reactive oxygen species, mitochondrial respiration was only required for the slowest wave in calcineurin-deficient cells. These findings suggest that yeast cells can die by necrosis-like mechanisms during the response to mating pheromones if essential response pathways are lacking or if mating is attempted in the absence of a partner.

Cdc42p GDP/GTP cycling is necessary for efficient cell fusion during yeast mating

The highly conserved small Rho G-protein, Cdc42p plays a critical role in cell polarity and cytoskeleton organization in all eukaryotes. In the yeast Saccharomyces cerevisiae, Cdc42p is important for cell polarity establishment, septin ring assembly, and pheromone-dependent MAP-kinase signaling during the yeast mating process. In this study, we further investigated the role of Cdc42p in the mating process by screening for specific mating defective cdc42 alleles. We have identified and characterized novel mating defective cdc42 alleles that are unaffected in vegetative cell polarity. Replacement of the Cdc42p Val36 residue with Met resulted in a specific cell fusion defect. This cdc42[V36M] mutant responded to mating pheromone but was defective in cell fusion and in localization of the cell fusion protein Fus1p, similar to a previously isolated cdc24 (cdc24-m6) mutant. Overexpression of a fast cycling Cdc42p mutant suppressed the cdc24-m6 fusion defect and conversely, overexpression of Cdc24p suppressed the cdc42[V36M] fusion defect. Taken together, our results indicate that Cdc42p GDP-GTP cycling is critical for efficient cell fusion.

FUS1 regulates the opening and expansion of fusion pores between mating yeast

Mating yeast cells provide a genetically accessible system for the study of cell fusion. The dynamics of fusion pores between yeast cells were analyzed by following the exchange of fluorescent markers between fusion partners. Upon plasma membrane fusion, cytoplasmic GFP and DsRed diffuse between cells at rates proportional to the size of the fusion pore. GFP permeance measurements reveal that a typical fusion pore opens with a burst and then gradually expands. In some mating pairs, a sudden increase in GFP permeance was found, consistent with the opening of a second pore. In contrast, other fusion pores closed after permitting a limited amount of cytoplasmic exchange. Deletion of FUS1 from both mating partners caused a >10-fold reduction in the initial permeance and expansion rate of the fusion pore. Although fus1 mating pairs also have a defect in degrading the cell wall that separates mating partners before plasma membrane fusion, other cell fusion mutants with cell wall remodeling defects had more modest effects on fusion pore permeance. Karyogamy is delayed by >1 h in fus1 mating pairs, possibly as a consequence of retarded fusion pore expansion.