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

Microbial response to seasonal variation in arctic biocrusts with a focus on fungi and cyanobacteria

Biocrusts are crucial components of Arctic ecosystems, playing significant roles in carbon and nitrogen cycling, especially in regions where plant growth is limited. However, the microbial communities within Arctic biocrusts and their strategies for surviving the harsh conditions remain poorly understood. In this study, the microbial profiles of Arctic biocrusts across different seasons (summer, autumn, and winter) were investigated in order to elucidate their survival strategies in extreme conditions. Metagenomic and metatranscriptomic analyses revealed significant differences in microbial community composition among the sites located in different elevations. The bacterial communities were dominated by Actinobacteria and Proteobacteria, while the fungal communities were mainly represented by Ascomycota and Basidiomycota, with lichenized and saprotrophic traits prevailing. Cyanobacteria were primarily composed of heterocystous cyanobacteria. Furthermore, the study identified molecular mechanisms underlying cold adaptation, including the expression of heat shock proteins and cold-inducible RNA helicases in cyanobacteria and fungi. Overall, the microbial communities appear to be permanently well adapted to the extreme environment.

DhFIG_2, a gene of nematode-trapping fungus Dactylellina haptotyla that encodes a component of the low-affinity calcium uptake system, is required for conidiation and knob-trap formation

Calcium ion (Ca²⁺) is a universal second messenger involved in regulating diverse processes in animals, plants, and fungi. The low-affinity calcium uptake system (LACS) participates in acquiring Ca²⁺ from extracellular environments under high extracellular Ca²⁺ concentration. Unlike most fungi, which encode only one protein (FIG1) for LACS, nematode-trapping fungi (NTF) encode two related proteins. AoFIG_2, the NTF-specific LACS component encoded by adhesive network-trap forming Arthrobotrys oligospora, was shown to be required for conidiation and trap formation. We characterized the role of DhFIG_2, an AoFIG_2 ortholog encoded by knob-trap forming Dactylellina haptotyla, in growth and development to expand our understanding of the role of LACS in NTF. Because repeated attempts to disrupt DhFIG_2 failed, knocking down the expression of DhFIG_2 via RNA interference (RNAi) was used to study its function. RNAi of DhFIG_2 significantly decreased its expression, severely reduced conidiation and trap formation, and affected vegetative growth and stress responses, suggesting that this component of LACS is crucial for trap formation and conidiation in NTF. Our study demonstrated the utility of RNAi assisted by ATMT for studying gene function in D. haptotyla.

A focus on yeast mating: From pheromone signaling to cell-cell fusion

Cells live in a chemical environment and are able to orient towards chemical cues. Unicellular haploid fungal cells communicate by secreting pheromones to reproduce sexually. In the yeast models Saccharomyces cerevisiae and Schizosaccharomyces pombe, pheromonal communication activates similar pathways composed of cognate G-protein-coupled receptors and downstream small GTPase Cdc42 and MAP kinase cascades. Local pheromone release and sensing, at a mobile surface polarity patch, underlie spatial gradient interpretation to form pairs between two cells of distinct mating types. Concentration of secretion at the point of cell-cell contact then leads to local cell wall digestion for cell fusion, forming a diploid zygote that prevents further fusion attempts. A number of asymmetries between mating types may promote efficiency of the system. In this review, we present our current knowledge of pheromone signaling in the two model yeasts, with an emphasis on how cells decode the pheromone signal spatially and ultimately fuse together. Though overall pathway architectures are similar in the two species, their large evolutionary distance allows to explore how conceptually similar solutions to a general biological problem can arise from divergent molecular components.

A Velvet Transcription Factor Specifically Activates Mating through a Novel Mating-Responsive Protein in the Human Fungal Pathogen Cryptococcus deneoformans

Sexual reproduction facilitates infection by the production of both a lineage advantage and infectious sexual spores in the ubiquitous human fungal pathogen Cryptococcus deneoformans. However, the regulatory determinants specific for initiating mating remain poorly understood. Here, we identified a velvet family regulator, Cva1, that strongly promotes sexual reproduction in C. deneoformans. This regulation was determined to be specific, based on a comprehensive phenotypic analysis of cva1Δ under 26 distinct in vitro and in vivo growth conditions. We further revealed that Cva1 plays a critical role in the initiation of early mating events, including sexual cell-cell fusion, but is not important for the late sexual development stages or meiosis. Thus, Cva1 specifically contributes to mating activation. Importantly, a novel mating-responsive protein, Cfs1, serves as the key target of Cva1 during mating, since its absence nearly blocks cell-cell fusion in C. deneoformans and its sister species C. neoformans. Together, our findings provide insight into how C. deneoformans ensures the regulatory specificity of mating.

IMPORTANCE The human fungal pathogen C. deneoformans is a model organism for studying fungal sexual reproduction, which is considered to be important to infection. However, the specific regulatory determinants for activation of sexual reproduction remain poorly understood. In this study, by combining transcriptomic and comprehensive phenotypic analysis, we identified a velvet family regulator Cva1 that specifically and critically elicits early mating events, including sexual cell-cell fusion. Significantly, Cva1 induces mating through the novel mating-responsive protein Cfs1, which is essential for cell-cell fusion in C. deneoformans and its sister species C. neoformans. Considering that Cva1 and Cfs1 are highly conserved in species belonging to Cryptococcaeceae, they may play conserved and specific roles in the initiation of sexual reproduction in this important fungal clade, which includes multiple human fungal pathogens.

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.

The Sur7 cytoplasmic C terminus regulates morphogenesis and stress responses in Candida albicans

MCC/eisosome subdomains of the plasma membrane promote proper cell wall morphogenesis that is critical for the fungal pathogen Candida albicans to grow invasively and resist stressful environments in the host. Sur7 localizes to MCC/eisosomes and is needed for their function, so in this work, the role of this tetraspan membrane protein was studied by mutagenesis. Deletion mutant analysis showed that the N-terminal region containing the four transmembrane domains mediates Sur7 localization to MCC/eisosomes. Mutation of 32 conserved residues in the N-terminal region indicated that extracellular loop 1 is important, although these mutants generally displayed weak phenotypes. Surprisingly, two Cys residues in a conserved motif in extracellular loop 1 were not important. However, deletion of the entire 15 amino acid motif revealed that it was needed for proper membrane trafficking of Sur7. Deletion and substitution mutagenesis showed that the C terminus is important for resisting cell wall stress. This is significant as it indicates Sur7 carries out an important role in the cytoplasm. Altogether, these results indicate that the N-terminal region localizes Sur7 to MCC/eisosomes and that the C-terminal domain promotes responses in the cytoplasm needed for cell wall morphogenesis and stress resistance.

A zygote-based assay to evaluate intranuclear shuttling in S. cerevisiae

It is often necessary to learn whether macromolecules occupy a fixed place in cells. This protocol makes it possible to learn whether individual nucleolar proteins in S. cerevisiae remain in place or depart from and return to the nucleolus. The protocol uses early zygotes in which parental nucleoli are separate for at least one hour. The protocol demonstrates that the localization of many nucleolar proteins is in fact highly dynamic. Photobleaching is not required.

For complete details on the use and execution of this protocol, please refer to Tartakoff et al. (2021).

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The mobility of proteins within the yeast nucleus can be judged by constructing zygotes

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The assay requires minimal equipment other than a sensitive fluorescent microscope

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The protocol can be completed within one day once appropriate cells are available

Structural studies of the shortest extended synaptotagmin with only two C2 domains from Trypanosoma brucei

Extended synaptotagmins (E-Syts) localize at membrane contact sites between the endoplasmic reticulum (ER) and the plasma membrane to mediate inter-membrane lipid transfer and control plasma membrane lipid homeostasis. All known E-Syts contain an N-terminal transmembrane (TM) hairpin, a central synaptotagmin-like mitochondrial lipid-binding protein (SMP) domain, and three or five C2 domains at their C termini. Here we report an uncharacterized E-Syt from the protist parasite Trypanosoma brucei, namely, TbE-Syt. TbE-Syt contains only two C2 domains (C2A and C2B), making it the shortest E-Syt known by now. We determined a 1.5-Å-resolution crystal structure of TbE-Syt-C2B and revealed that it binds lipids via both Ca²⁺- and PI(4,5)P₂-dependent means. In contrast, TbE-Syt-C2A lacks the Ca²⁺-binding site but may still interact with lipids via a basic surface patch. Our studies suggest a mechanism for how TbE-Syt tethers the ER membrane tightly to the plasma membrane to transfer lipids between the two organelles.

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We identified a new type of extended synaptotagmin (E-Syt) in Trypanosoma brucei

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TbE-Syt is the shortest known E-Syt with only two C2 domains, C2A and C2B

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TbE-Syt-C2B binds lipids via both Ca²⁺- and PI(4,5)P₂-dependent means

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Unlike all other known E-Syts, TbE-Syt-C2A and C2B are connected by a flexible loop

A fluorescence-based yeast sensor for monitoring acetic acid

Accumulation of acetic acid indicates an imbalance of the process due to a disturbed composition of the microorganisms. Hence, monitoring the acetic acid concentration is an important parameter to control the biogas process. Here, we describe the generation and validation of a fluorescence-based whole cell sensor for the detection of acetic acid based on the yeast Saccharomyces cerevisiae. Acetic acid induces the transcription of a subset of genes. The 5´-regulatory sequences (5´ URS) of these genes were cloned into a multicopy plasmid to drive the expression of a red fluorescent reporter gene. The 5´ URS of YGP1, encoding a cell wall-related glycoprotein, led to a 20-fold increase of fluorescence upon addition of 30 mM acetic acid to the media. We show that the system allows estimating the approximate concentration of acetic acid in condensation samples from a biogas plant. To avoid plasmid loss and increase the long-term stability of the sensor, we integrated the reporter construct into the yeast genome and tested the suitability of spores for long-term storage of sensor cells. Lowering the reporter gene's copy number resulted in a significant drop of the fluorescence, which can be compensated by applying a yeast pheromone-based signal amplification system.

Cation Transporters of Candida albicans-New Targets to Fight Candidiasis?

Candidiasis is the wide-spread fungal infection caused by numerous strains of yeast, with the prevalence of Candida albicans. The current treatment of candidiasis is becoming rather ineffective and costly owing to the emergence of resistant strains; hence, the exploration of new possible drug targets is necessary. The most promising route is the development of novel antibiotics targeting this pathogen. In this review, we summarize such candidates found in C. albicans and those involved in the transport of (metal) cations, as the latter are essential for numerous processes within the cell; hence, disruption of their fluxes can be fatal for C. albicans.

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.

Role of Low-Affinity Calcium System Member Fig1 Homologous Proteins in Conidiation and Trap-Formation of Nematode-trapping Fungus Arthrobotrys oligospora

Arthrobotrys oligospora is a typical nematode-trapping fungus capturing free-living nematodes by adhesive networks. Component of the low-affinity calcium uptake system (LACS) has been documented to involve in growth and sexual development of filamentous fungi. Bioassay showed incapacity of trap formation in A. oligospora on Water Agar plate containing 1 mM ethylene glycol tetraacetic acid (EGTA) due to Ca²⁺ absorbing block. The functions of homologous proteins (AoFIG_1 and AoFIG_2) of LACS were examined on conidiation and trap formation of A. oligospora. Compared with wild type, ΔAoFIG_1 (AOL_s00007g566) resulted in 90% of trap reduction, while ΔAoFIG_2 (AOL_s00004g576) reduced vegetative growth rate up to 44% and had no trap and conidia formed. The results suggest that LACS transmembrane protein fig1 homologs play vital roles in the trap-formation and is involved in conidiation and mycelium growth of A. oligospora. Our findings expand fig1 role to include development of complex trap device and conidiation.

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.

The Claudin Family Protein FigA Mediates Ca2+ Homeostasis in Response to Extracellular Stimuli in Aspergillus nidulans and Aspergillus fumigatus

The claudin family protein Fig1 is a unique fungal protein that is involved in pheromone-induced calcium influx and membrane fusion during the mating of Saccharomyces cerevisiae and Candida albicans. Whether and how Fig1 regulates Ca²⁺ homeostasis in response to extracellular stimuli is poorly understood. Previously, we found Aspergillus nidulans FigA, a homolog of Fig1 in S. cerevisiae, similar to the high-affinity calcium uptake system, is required for normal growth under low-Ca²⁺ minimal medium. In this study, using the calcium-sensitive photoprotein aequorin to monitor cytosolic free calcium concentration ([Ca²⁺]_c) in living cells, we found that the FigA dysfunction decreases the transient [Ca²⁺]_c induced by a high extracellular calcium stress. Furthermore, FigA acts synergistically with CchA (a high-affinity Ca²⁺ channel) to coordinate cytoplasmic Ca²⁺ influx in response to an extracellular Ca²⁺ stimulus. Moreover, FigA mediates ER stress-induced transient [Ca²⁺]_c in the presence or absence of extracellular calcium. Most importantly, these [Ca²⁺]_c responses mediated by FigA are closely related to its conserved claudin superfamily motif, which is also required for hyphal growth and asexual development in A. nidulans. Finally, the function of FigA in Aspergillus fumigatus, the most common airborne human fungal pathogen was studied. The result showed that the two FigA homologous in A. nidulans and A. fumigatus have a large degree of functional homology not only in asexual development but also in regulating transient [Ca²⁺]_c. Our study expands the knowledge of claudin family protein FigA in Ca²⁺ homeostasis in response to extracellular stimuli.

The ancient claudin Dni2 facilitates yeast cell fusion by compartmentalizing Dni1 into a membrane subdomain

Dni1 and Dni2 facilitate cell fusion during mating. Here, we show that these proteins are interdependent for their localization in a plasma membrane subdomain, which we have termed the mating fusion domain. Dni1 compartmentation in the domain is required for cell fusion. The contribution of actin, sterol-dependent membrane organization, and Dni2 to this compartmentation was analysed, and the results showed that Dni2 plays the most relevant role in the process. In turn, the Dni2 exit from the endoplasmic reticulum depends on Dni1. These proteins share the presence of a cysteine motif in their first extracellular loop related to the claudin GLWxxC(8-10 aa)C signature motif. Structure-function analyses show that mutating each Dni1 conserved cysteine has mild effects, and that only simultaneous elimination of several cysteines leads to a mating defect. On the contrary, eliminating each single cysteine and the C-terminal tail in Dni2 abrogates Dni1 compartmentation and cell fusion. Sequence alignments show that claudin trans-membrane helixes bear small-XXX-small motifs at conserved positions. The fourth Dni2 trans-membrane helix tends to form homo-oligomers in Escherichia plasma membrane, and two concatenated small-XXX-small motifs are required for efficient oligomerization and for Dni2 export from the yeast endoplasmic reticulum. Together, our results strongly suggest that Dni2 is an ancient claudin that blocks Dni1 diffusion from the intercellular region where two plasma membranes are in close proximity, and that this function is required for Dni1 to facilitate cell fusion.

PRM1 and KAR5 function in cell-cell fusion and karyogamy to drive distinct bisexual and unisexual cycles in the Cryptococcus pathogenic species complex

Sexual reproduction is critical for successful evolution of eukaryotic organisms in adaptation to changing environments. In the opportunistic human fungal pathogens, the Cryptococcus pathogenic species complex, C. neoformans primarily undergoes bisexual reproduction, while C. deneoformans undergoes both unisexual and bisexual reproduction. During both unisexual and bisexual cycles, a common set of genetic circuits regulates a yeast-to-hyphal morphological transition, that produces either monokaryotic or dikaryotic hyphae. As such, both the unisexual and bisexual cycles can generate genotypic and phenotypic diversity de novo. Despite the similarities between these two cycles, genetic and morphological differences exist, such as the absence of an opposite mating-type partner and monokaryotic instead of dikaryotic hyphae during C. deneoformans unisexual cycle. To better understand the similarities and differences between these modes of sexual reproduction, we focused on two cellular processes involved in sexual reproduction: cell-cell fusion and karyogamy. We identified orthologs of the plasma membrane fusion protein Prm1 and the nuclear membrane fusion protein Kar5 in both Cryptococcus species, and demonstrated their conserved roles in cell fusion and karyogamy during C. deneoformans α-α unisexual reproduction and C. deneoformans and C. neoformans a-α bisexual reproduction. Notably, karyogamy occurs inside the basidum during bisexual reproduction in C. neoformans, but often occurs earlier following cell fusion during bisexual reproduction in C. deneoformans. Characterization of these two genes also showed that cell fusion is dispensable for solo unisexual reproduction in C. deneoformans. The blastospores produced along hyphae during C. deneoformans unisexual reproduction are diploid, suggesting that diploidization occurs early during hyphal development, possibly through either an endoreplication pathway or cell fusion-independent karyogamy events. Taken together, our findings suggest distinct mating mechanisms for unisexual and bisexual reproduction in Cryptococcus, exemplifying distinct evolutionary trajectories within this pathogenic species complex.

Comparative transcriptome analysis of the calcium signaling and expression analysis of sodium/calcium exchanger in Aspergillus cristatus

Aspergillus cristatus develops into various stages under different Na concentrations: the sexual stage in 0.5 M NaCl and asexual development stage in 3 M NaCl. In order to explore whether the Ca²⁺ signaling pathway in A. cristatus responded to the changes in the salt stress, we analyzed the gene expression levels in A. cristatus respectively cultured in 0.5 M NaCl and 3 M NaCl. According to the BLAST analysis results, we identified 25 Ca²⁺ -signaling proteins in A. cristatus. The expression levels of most genes involved in the Ca²⁺ -signaling pathway in A. cristatus cultured in different salt concentrations showed significant differences, indicating that the Ca²⁺ signaling pathway was involved in the response to the changes in the salt stress. In yeasts, only calcium ion influx proteins were reported to be involved in the response to the changes in the salt stress. So far, the protein for the exchanger of calcium/sodium ions has not been reported. Therefore, we obtained the sodium/calcium exchanger (termed NCX) proteins from the KEGG Database. The ncx gene of A. cristatus was cloned and characterized. The full length of ncx gene is 3055 bp, including a 2994-bp open reading frame encoding 994 amino acids. The expression levels of ncx in the sexual development stage and asexual development stage were respectively ∼8.94 times and ∼2.57 times of that in the hyphal formation stage. Therefore, we suggested that ncx gene was up-regulated to resist the sodium stress. The study results provide the basis for further exploring the Ca²⁺ -signaling mechanism and ion exchanger mechanism.

Endoplasmic Reticulum-Plasma Membrane Contact Sites

The endoplasmic reticulum (ER) has a broad localization throughout the cell and forms direct physical contacts with all other classes of membranous organelles, including the plasma membrane (PM). A number of protein tethers that mediate these contacts have been identified, and study of these protein tethers has revealed a multiplicity of roles in cell physiology, including regulation of intracellular Ca²⁺ dynamics and signaling as well as control of lipid traffic and homeostasis. In this review, we discuss the cross talk between the ER and the PM mediated by direct contacts. We review factors that tether the two membranes, their properties, and their dynamics in response to the functional state of the cell. We focus in particular on the role of ER-PM contacts in nonvesicular lipid transport between the two bilayers mediated by lipid transfer proteins.

Endoplasmic Reticulum - Plasma Membrane Crosstalk Mediated by the Extended Synaptotagmins

The endoplasmic reticulum (ER) possesses multiplicity of functions including protein synthesis, membrane lipid biogenesis, and Ca²⁺ storage and has broad localization throughout the cell. While the ER and most other membranous organelles are highly interconnected via vesicular traffic that relies on membrane budding and fusion reactions, the ER forms direct contacts with virtually all other membranous organelles, including the plasma membrane (PM), without membrane fusion. Growing evidence suggests that these contacts play major roles in cellular physiology, including the regulation of Ca²⁺ homeostasis and signaling and control of cellular lipid homeostasis. Extended synaptotagmins (E-Syts) are evolutionarily conserved family of ER-anchored proteins that tether the ER to the PM in PM PI(4,5)P₂-dependent and cytosolic Ca²⁺-regulated manner. In addition, E-Syts possess a cytosolically exposed lipid-harboring module that confers the ability to transfer/exchange glycerolipids between the ER and the PM at E-Syts-mediated ER-PM contacts. In this chapter, the functions of ER-PM contacts and their role in non-vesicular lipid transport with special emphasis on the crosstalk between the two bilayers mediated by E-Syts will be discussed.

Sexual pheromone modulates the frequency of cytosolic Ca2+ bursts in Saccharomyces cerevisiae

Transient and highly regulated elevations of cytosolic Ca²⁺ control a variety of cellular processes. Bulk measurements using radioactive Ca²⁺ and the luminescent sensor aequorin have shown that in response to pheromone, budding yeast cells undergo a rise of cytosolic Ca²⁺ that is mediated by two import systems composed of the Mid1-Cch1-Ecm7 protein complex and the Fig1 protein. Although this response has been widely studied, there is no treatment of Ca²⁺ dynamics at the single-cell level. Here, using protein calcium indicators, we show that both vegetative and pheromone-treated yeast cells exhibit discrete and asynchronous Ca²⁺ bursts. Most bursts reach maximal amplitude in 1-10 s, range between 7 and 30 s, and decay in a way that fits a single-exponential model. In vegetative cells, bursts are scarce but preferentially occur when cells are transitioning G1 and S phases. On pheromone presence, Ca²⁺ burst occurrence increases dramatically, persisting during cell growth polarization. Pheromone concentration modulates burst frequency in a mechanism that depends on Mid1, Fig1, and a third, unidentified, import system. We also show that the calcineurin-responsive transcription factor Crz1 undergoes nuclear localization bursts during the pheromone response.

High Transmembrane Voltage Raised by Close Contact Initiates Fusion Pore

Membrane fusion lies at the heart of neuronal communication but the detailed mechanism of a critical step, fusion pore initiation, remains poorly understood. Here, through atomistic molecular dynamics simulations, a transient pore formation induced by a close contact of two apposed bilayers is firstly reported. Such a close contact gives rise to a high local transmembrane voltage that induces the transient pore formation. Through simulations on two apposed bilayers fixed at a series of given distances, the process in which two bilayers approaching to each other under the pulling force from fusion proteins for membrane fusion was mimicked. Of note, this close contact induced fusion pore formation is contrasted with previous reported electroporation under ad hoc applied external electric field or ionic charge in-balance. We show that the transmembrane voltage increases with the decrease of the distance between the bilayers. Below a critical distance, depending on the lipid composition, the local transmembrane voltage can be sufficiently high to induce the transient pores. The size of these pores is approximately 1~2 nm in diameter, which is large enough to allow passing of neurotransmitters. A resealing of the membrane pores resulting from the neutralization of the transmembrane voltage by ions through the pores was then observed. We also found that the membrane tension can either prolong the lifetime of transient pores or cause them to dilate for full collapse. This result provides a possible mechanism for fusion pore formation and regulation of pathway of fusion process.

Control of plasma membrane lipid homeostasis by the extended synaptotagmins

Acute metabolic changes of plasma membrane (PM) lipids, such as those mediating signaling reactions, are rapidly compensated by homeostatic responses whose molecular basis is poorly understood. Here we show that the Extended-Synaptotagmins (E-Syts), ER proteins which function as PI(4,5)P₂ and Ca²⁺-regulated tethers to the PM, participate in these responses. E-Syts transfer glycerolipids between bilayers in vitro and such transfer requires Ca²⁺ and their SMP domain, a lipid-harboring module. Genome edited cells lacking E-Syts do not exhibit abnormalities in the major glycerolipids at rest, but display enhanced and sustained accumulation of PM diacylglycerol (DAG) upon PI(4,5)P₂ hydrolysis by PLC activation, which can be rescued by expression of E-Syt1, but not by mutant E-Syt1 lacking the SMP domain. The formation of E-Syts-dependent ER-PM tethers in response to stimuli that cleave PI(4,5)P₂ and elevate Ca²⁺ may help reverse accumulation of DAG in the PM by transferring it to the ER for metabolic recycling.

Extended synaptotagmins are Ca2+-dependent lipid transfer proteins at membrane contact sites

Organelles are in constant communication with each other through exchange of proteins (mediated by trafficking vesicles) and lipids [mediated by both trafficking vesicles and lipid transfer proteins (LTPs)]. It has long been known that vesicle trafficking can be tightly regulated by the second messenger Ca(2+), allowing membrane protein transport to be adjusted according to physiological demands. However, it remains unclear whether LTP-mediated lipid transport can also be regulated by Ca(2+) In this work, we show that extended synaptotagmins (E-Syts), poorly understood membrane proteins at endoplasmic reticulum-plasma membrane contact sites, are Ca(2+)-dependent LTPs. Using both recombinant and endogenous mammalian proteins, we discovered that E-Syts transfer glycerophospholipids between membrane bilayers in the presence of Ca(2+) E-Syts use their lipid-accommodating synaptotagmin-like mitochondrial lipid binding protein (SMP) domains to transfer lipids. However, the SMP domains themselves cannot transport lipids unless the two membranes are tightly tethered by Ca(2+)-bound C2 domains. Strikingly, the Ca(2+)-regulated lipid transfer activity of E-Syts was fully recapitulated when the SMP domain was fused to the cytosolic domain of synaptotagmin-1, the Ca(2+)sensor in synaptic vesicle fusion, indicating that a common mechanism of membrane tethering governs the Ca(2+)regulation of lipid transfer and vesicle fusion. Finally, we showed that microsomal vesicles isolated from mammalian cells contained robust Ca(2+)-dependent lipid transfer activities, which were mediated by E-Syts. These findings established E-Syts as a novel class of LTPs and showed that LTP-mediated lipid trafficking, like vesicular transport, can be subject to tight Ca(2+)regulation.

Genetic interactions between the Golgi Ca2+/H+ exchanger Gdt1 and the plasma membrane calcium channel Cch1/Mid1 in the regulation of calcium homeostasis, stress response and virulence in Candida albicans

The Golgi-localized Saccharomyces cerevisiae ScGdt1 is a member of the cation/Ca(2+) exchanger superfamily. We show here that Candida albicans CaGdt1 is the functional homolog of ScGdt1 in calcium sensitivity, and shows genetic interactions with CaCch1 or CaMid1 in response to ER stresses. In addition, similar to ScCCH1 and ScMID1, deletion of either CaCCH1 or CaMID1 leads to a growth sensitivity of cells to cold stress, which can be suppressed by deletion of CaGDT1. Furthermore, deletion of CaCCH1 leads to a severe delay in filamentation of C. albicans cells, and this defect is abolished by deletion of CaGDT1. In contrast, CaGDT1 does not show genetic interaction with CaMID1 in filamentation. Interestingly, C. albicans cells lacking both CaMID1 and CaGDT1 exhibit an intermediate virulence between C. albicans cells lacking CaCCH1 (non-virulent) and C. albicans cells lacking CaGDT1 (partially virulent), while C. albicans cells lacking both CaCCH1 and CaGDT1 are not virulent in a mouse model of systemic candidiasis. Therefore, CaGdt1 genetically interacts with the plasma membrane calcium channel, CaCch1/CaMid1, in the response of C. albicans cells to cold and ER stresses and antifungal drug challenge as well as in filamentation and virulence.

Unisexual versus bisexual mating in Cryptococcus neoformans: Consequences and biological impacts

Cryptococcus neoformans is an opportunistic human fungal pathogen and can undergo both bisexual and unisexual mating. Despite the fact that one mating type is dispensable for unisexual mating, the two sexual cycles share surprisingly similar features. Both mating cycles are affected by similar environmental factors and regulated by the same pheromone response pathway. Recombination takes place during unisexual reproduction in a fashion similar to bisexual reproduction and can both admix pre-existing genetic diversity and also generate diversity de novo just like bisexual reproduction. These common features may allow the unisexual life cycle to provide phenotypic and genotypic plasticity for the natural Cryptococcus population, which is predominantly α mating type, and to avoid Muller's ratchet. The morphological transition from yeast to hyphal growth during both bisexual and unisexual mating may provide increased opportunities for outcrossing and the ability to forage for nutrients at a distance. The unisexual life cycle is a key evolutionary factor for Cryptococcus as a highly successful global fungal pathogen.

Cell biology of yeast zygotes, from genesis to budding

The zygote is the essential intermediate that allows interchange of nuclear, mitochondrial and cytosolic determinants between cells. Zygote formation in Saccharomyces cerevisiae is accomplished by mechanisms that are not characteristic of mitotic cells. These include shifting the axis of growth away from classical cortical landmarks, dramatically reorganizing the cell cortex, remodeling the cell wall in preparation for cell fusion, fusing with an adjacent partner, accomplishing nuclear fusion, orchestrating two steps of septin morphogenesis that account for a delay in fusion of mitochondria, and implementing new norms for bud site selection. This essay emphasizes the sequence of dependent relationships that account for this progression from cell encounters through zygote budding. It briefly summarizes classical studies of signal transduction and polarity specification and then focuses on downstream events.

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.

FigA, a putative homolog of low-affinity calcium system member Fig1 in Saccharomyces cerevisiae, is involved in growth and asexual and sexual development in Aspergillus nidulans

Calcium-mediated signaling pathways are widely employed in eukaryotes and are implicated in the regulation of diverse biological processes. In Saccharomyces cerevisiae, at least two different calcium uptake systems have been identified: the high-affinity calcium influx system (HACS) and the low-affinity calcium influx system (LACS). Compared to the HACS, the LACS in fungi is not well known. In this study, FigA, a homolog of the LACS member Fig1 from S. cerevisiae, was functionally characterized in the filamentous fungus Aspergillus nidulans. Loss of figA resulted in retardant hyphal growth and a sharp reduction of conidial production. Most importantly, FigA is essential for the homothallic mating (self-fertilization) process; further, FigA is required for heterothallic mating (outcrossing) in the absence of HACS midA. Interestingly, in a figA deletion mutant, adding extracellular Ca(2+) rescued the hyphal growth defects but could not restore asexual and sexual reproduction. Furthermore, quantitative PCR results revealed that figA deletion sharply decreased the expression of brlA and nsdD, which are known as key regulators during asexual and sexual development, respectively. In addition, green fluorescent protein (GFP) tagging at the C terminus of FigA (FigA::GFP) showed that FigA localized to the center of the septum in mature hyphal cells, to the location between vesicles and metulae, and between the junctions of metulae and phialides in conidiophores. Thus, our findings suggest that FigA, apart from being a member of a calcium uptake system in A. nidulans, may play multiple unexplored roles during hyphal growth and asexual and sexual development.

Temporal and spatial properties of a yeast multi-cellular amplification system based on signal molecule diffusion

We report on the spatial and temporal signaling properties of a yeast pheromone-based cell communication and amplifier system. It utilizes the Saccharomyces cerevisiae mating response pathway and relies on diffusion of the pheromone α-factor as key signaling molecule between two cell types. One cell type represents the α-factor secreting sensor part and the other the reporter part emitting fluorescence upon activation. Although multi-cellular signaling systems promise higher specificity and modularity, the complex interaction of the cells makes prediction of sensor performance difficult. To test the maximum distance and response time between sensor and reporter cells, the two cell types were spatially separated in defined compartments of agarose hydrogel (5 x 5 mm) and reconnected by diffusion of the yeast pheromone. Different ratios of sensor to reporter cells were tested to evaluate the minimum amount of sensor cells required for signal transduction. Even the smallest ratio, one α-factor-secreting cell to twenty reporter cells, generated a distinct fluorescence signal. When using a 1:1 ratio, the secreted pheromone induced fluorescence in a distance of up to four millimeters after six hours. We conclude from both our experimental results and a mathematical diffusion model that in our approach: (1) the maximum dimension of separated compartments should not exceed five millimeters in gradient direction; and (2) the time-limiting step is not diffusion of the signaling molecule but production of the reporter protein.

Regulation of cation balance in Saccharomyces cerevisiae

All living organisms require nutrient minerals for growth and have developed mechanisms to acquire, utilize, and store nutrient minerals effectively. In the aqueous cellular environment, these elements exist as charged ions that, together with protons and hydroxide ions, facilitate biochemical reactions and establish the electrochemical gradients across membranes that drive cellular processes such as transport and ATP synthesis. Metal ions serve as essential enzyme cofactors and perform both structural and signaling roles within cells. However, because these ions can also be toxic, cells have developed sophisticated homeostatic mechanisms to regulate their levels and avoid toxicity. Studies in Saccharomyces cerevisiae have characterized many of the gene products and processes responsible for acquiring, utilizing, storing, and regulating levels of these ions. Findings in this model organism have often allowed the corresponding machinery in humans to be identified and have provided insights into diseases that result from defects in ion homeostasis. This review summarizes our current understanding of how cation balance is achieved and modulated in baker's yeast. Control of intracellular pH is discussed, as well as uptake, storage, and efflux mechanisms for the alkali metal cations, Na(+) and K(+), the divalent cations, Ca(2+) and Mg(2+), and the trace metal ions, Fe(2+), Zn(2+), Cu(2+), and Mn(2+). Signal transduction pathways that are regulated by pH and Ca(2+) are reviewed, as well as the mechanisms that allow cells to maintain appropriate intracellular cation concentrations when challenged by extreme conditions, i.e., either limited availability or toxic levels in the environment.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

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.

Many Saccharomyces cerevisiae Cell Wall Protein Encoding Genes Are Coregulated by Mss11, but Cellular Adhesion Phenotypes Appear Only Flo Protein Dependent

The outer cell wall of the yeast Saccharomyces cerevisiae serves as the interface with the surrounding environment and directly affects cell-cell and cell-surface interactions. Many of these interactions are facilitated by specific adhesins that belong to the Flo protein family. Flo mannoproteins have been implicated in phenotypes such as flocculation, substrate adhesion, biofilm formation, and pseudohyphal growth. Genetic data strongly suggest that individual Flo proteins are responsible for many specific cellular adhesion phenotypes. However, it remains unclear whether such phenotypes are determined solely by the nature of the expressed FLO genes or rather as the result of a combination of FLO gene expression and other cell wall properties and cell wall proteins. Mss11 has been shown to be a central element of FLO1 and FLO11 gene regulation and acts together with the cAMP-PKA-dependent transcription factor Flo8. Here we use genome-wide transcription analysis to identify genes that are directly or indirectly regulated by Mss11. Interestingly, many of these genes encode cell wall mannoproteins, in particular, members of the TIR and DAN families. To examine whether these genes play a role in the adhesion properties associated with Mss11 expression, we assessed deletion mutants of these genes in wild-type and flo11Δ genetic backgrounds. This analysis shows that only FLO genes, in particular FLO1/10/11, appear to significantly impact on such phenotypes. Thus adhesion-related phenotypes are primarily dependent on the balance of FLO gene expression.

Acidic calcium stores of Saccharomyces cerevisiae

Fungi and animals constitute sister kingdoms in the eukaryotic domain of life. The major classes of transporters, channels, sensors, and effectors that move and respond to calcium ions were already highly networked in the common ancestor of fungi and animals. Since that time, some key components of the network have been moved, altered, relocalized, lost, or duplicated in the fungal and animal lineages and at the same time some of the regulatory circuitry has been dramatically rewired. Today the calcium transport and signaling networks in fungi provide a fresh perspective on the scene that has emerged from studies of the network in animal cells. This review provides an overview of calcium signaling networks in fungi, particularly the model yeast Saccharomyces cerevisiae, with special attention to the dominant roles of acidic calcium stores in fungal cell physiology.

Fig1 facilitates calcium influx and localizes to membranes destined to undergo fusion during mating in Candida albicans

Few mating-regulated genes have been characterized in Candida albicans. C. albicans FIG1 (CaFIG1) is a fungus-specific and mating-induced gene encoding a putative 4-transmembrane domain protein that shares sequence similarities with members of the claudin superfamily. In Saccharomyces cerevisiae, Fig1 is required for shmoo fusion and is upregulated in response to mating pheromones. Expression of CaFIG1 was also strongly activated in the presence of cells of the opposite mating type. CaFig1-green fluorescent protein (GFP) was visible only during the mating response, when it localized predominantly to the plasma membrane and perinuclear zone in mating projections and daughter cells. At the plasma membrane, CaFig1-GFP was visualized as discontinuous zones, but the distribution of perinuclear CaFig1-GFP was homogeneous. Exposure to pheromone induced a 5-fold increase in Ca(2+) uptake in mating-competent opaque cells. Uptake was reduced substantially in the fig1Δ null mutant. CaFig1 is therefore involved in Ca(2+) influx and localizes to membranes that are destined to undergo fusion during mating.

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.