Cells Of Opposite Mating Type Research Articles

Gamete fusion and cytokinesis preceding zygote establishment in the sexual process of Dictyostelium discoideum

Cells of Dictyostelium discoideum become sexually mature under submerged and dark conditions, and fuse with opposite mating-type cells to form zygote giant cells, which gather surrounding cells and finally develop into dormant structures called macrocysts. In the present study, we found that the multinuclear fused cells formed during this process frequently underwent cytokinesis driven by random local movements. The split cells were capable of re-fusion, and repeated cytokinesis. These radical behaviors continued until the extensive cell aggregation started around the giant cells. Thus, gamete fusion and initiation of zygote development do not coincide in the mating of D. discoideum. Analyses by confocal microscopy and flow cytometry indicated that the cessation of the random movement followed pronuclear fusion, and that microtubule organizing centers (MTOC), abundant in the fused cells at the beginning, gradually decreased and only one of them remained within the developed macrocyst. Some of the genes known to control cell movement, such as rasGEFB and rasS, increased shortly before the cessation of repeated fusion-cytokinesis and initiation of phagocytosis. These results suggest that the sequential molecular events are necessary in D. discoideum after gamete fusion to establish a new individuality of zygotes.

Activation of the pheromone-responsive MAP kinase drives haploid cells to undergo ectopic meiosis with normal telomere clustering and sister chromatid segregation in fission yeast.

Meiosis is a process of importance for sexually reproducing eukaryotic organisms. In the fission yeast Schizosaccharomyces pombe, meiosis normally proceeds in a diploid zygote which is produced by conjugation of haploid cells of opposite mating types. We demonstrate that activation of the pheromone-responsive MAPK, Spk1, by the ectopic expression of a constitutively active form of Byr1 (MAPKK for Spk1) induced the cells to undergo meiosis while in the haploid state. Moreover, the induction of meiosis required Mei2 (a key positive regulator of meiosis), but did not require Mei3; Mei3 is normally required to inactivate the Pat1 kinase (a negative regulator of Mei2) thereby allowing Mei2 to drive meiosis. Therefore, expression of a constitutively active form of Byr1 activates Mei2 without the need of Mei3. In cells induced to undergo meiosis by activating the Spk1 MAPK signaling pathway, telomeres clustered at the spindle pole body (SPB) and centromeres detached normally from the SPB during meiotic prophase, and the cells showed the correct segregation of sister chromatids during meiotic divisions. In contrast, in meiosis induced by inactivation of Pat1, sister chromatids segregate precociously during the first meiotic division. Thus, these results suggest that activation of Spk1 drives meiosis in S. pombe.

A yeast under cover: the capsule of Cryptococcus neoformans.

Few fungi are pathogenic to humans. Of these, Cryptococcus neoformans has emerged as an important cause of mortality in immunocompromised patients, especially those with AIDS. As a result, extensive research efforts have addressed the pathogenesis and virulence of this organism. C. neoformans is a basidiomycetous fungus that is ubiquitous in the environment, where it is found in soil, in association with certain trees, and in bird guano (16). Because of its ubiquity, it has been suggested that most people are exposed to C. neoformans early in life (41). The fungus is heterothallic, with mating types MATa and MAT. Asexual reproduction takes place either by budding or, in the case of MAT cells, by haploid fruiting in response to nutrient deprivation or exposure to the mating pheromone a factor (106). Sexual reproduction occurs when cells of opposite mating types come together to form a heterokaryon that ultimately leads to the production of basidia and basidiospores (64). Desiccated cells and the spores formed by haploid fruiting or sexual reproduction have all been suggested to serve as infective particles, which must be less than 2 m in diameter to penetrate the lung parenchyma (44, 86). Infection occurs when the fungal particles are inhaled and enter the alveolar space. In most immunocompetent individuals, this infection is either cleared or remains dormant until an immune imbalance leads to further development. In the setting of compromised immune function, however, the fungus disseminates, with particular tropism for the central nervous system. In severe cases, cryptococcal infection progresses to a meningoencephalitis that is fatal if left untreated. C. neoformans virulence is mediated predominantly by a polysaccharide capsule that surrounds its cell wall and has multiple effects on the host immune system. This structure provides a physical barrier that interferes with normal phagocytosis and clearance by the immune system. Capsule components inhibit the production of proinflammatory cytokines, deplete complement components (by efficiently binding them), and reduce leukocyte migration to sites of inflammation (11). The capsule also constitutes the major diagnostic feature of cryptococcosis, because its components can be detected in the bloodstream and it can be visualized with light microscopy by using India ink staining. The capsule excludes the ink particles and forms characteristic halos (Fig. 1A) whose diameters are often several times that of the cell. The elaborate structure of the capsule may also be appreciated by electron microscopy (Fig. 1B and C). Given the importance of the capsule in cryptococcal disease, tremendous effort has been applied in recent years to understanding its biology. This review focuses on the resulting advances in our understanding of the structure and synthesis of the capsular components, the incorporation of these components into the existing capsular network, the association between the capsule and the cell wall, and the regulation of capsule growth.

Kinesin II Hauls Signaling Cargo

The unicellular green alga Chlamydomonas uses a flagellum not only for locomotion, but also for presentation of flagellar adhesion molecules that promote interaction between gamete cells and initiate signaling events important to fertilization. The flagella lack the apparatus to make new proteins, so the cells use an intraflagellar transport system to transport particles containing proteins made in the cell body to the flagella. Such transport occurs constitutively, but regulated transport has also been described. For example, the Chlamydomonas aurora protein kinase (CALK) is transported to the flagellum only in response to signals generated when receptors on flagella of cells of opposite mating types interact. (Interestingly, aurora protein kinases are implicated in cell division in other systems.) Pan and Snell used mutants of the motor protein kinesin II to show that kinesin II is required for such activated transport of CALK to the flagellum. Kinesin II is also the motor responsible for the constitutive transport of proteins to flagella. However, effects of the kinesin II mutation in cells maintained at different temperatures could distinguish between the two roles of kinesin II, and the regulated transport of CALK appeared to result in association with the flagellar membrane whereas unregulated transport delivers proteins to a soluble cytoplasmic location. Schnapp reviews other evidence for active transport of signaling molecules, which in some cases involves interaction of motor proteins with scaffold proteins, allowing whole signaling complexes to be delivered to the appropriate site of action. J. Pan, W. J. Snell, Kinesin II and regulated intraflagellar transport of Chlamydomonas aurora protein kinase. J. Cell Sci. 116 , 2179-2186 (2003). [Abstract] [Full Text] B. J. Schnapp, Trafficking of signaling modules by kinesin motors. J. Cell Sci. 116 , 2125-2135 (2003). [Abstract] [Full Text]

Kinesin II Hauls Signaling Cargo

The unicellular green alga Chlamydomonas uses a flagellum not only for locomotion, but also for presentation of flagellar adhesion molecules that promote interaction between gamete cells and initiate signaling events important to fertilization. The flagella lack the apparatus to make new proteins, so the cells use an intraflagellar transport system to transport particles containing proteins made in the cell body to the flagella. Such transport occurs constitutively, but regulated transport has also been described. For example, the Chlamydomonas aurora protein kinase (CALK) is transported to the flagellum only in response to signals generated when receptors on flagella of cells of opposite mating types interact. (Interestingly, aurora protein kinases are implicated in cell division in other systems.) Pan and Snell used mutants of the motor protein kinesin II to show that kinesin II is required for such activated transport of CALK to the flagellum. Kinesin II is also the motor responsible for the constitutive transport of proteins to flagella. However, effects of the kinesin II mutation in cells maintained at different temperatures could distinguish between the two roles of kinesin II, and the regulated transport of CALK appeared to result in association with the flagellar membrane whereas unregulated transport delivers proteins to a soluble cytoplasmic location. Schnapp reviews other evidence for active transport of signaling molecules, which in some cases involves interaction of motor proteins with scaffold proteins, allowing whole signaling complexes to be delivered to the appropriate site of action.J. Pan, W. J. Snell, Kinesin II and regulated intraflagellar transport of Chlamydomonas aurora protein kinase. J. Cell Sci. 116, 2179-2186 (2003). [Abstract] [Full Text]B. J. Schnapp, Trafficking of signaling modules by kinesin motors. J. Cell Sci. 116, 2125-2135 (2003). [Abstract] [Full Text]

Molecular analysis of the pheromone and pheromone receptor genes of Ustilago hordei.

Cell-cell signaling is an integral part of the sexual and disease cycles of the smut fungi, which must mate to be pathogenic. This study reports the cloning and characterization of the pheromone genes Uhmfa1 and Uhmfa2 from MAT-1 and MAT-2 mating types of U. hordei, respectively, and the pheromone receptor gene Uhpra2 from MAT-2 cells. Similar to other fungal pheromone genes, Uhmfa1 and Uhmfa2 encode precursor peptides. Uhpra2 encodes a protein with sequence similarity to the 7-transmembrane class of G-protein coupled receptors. Deletion of Uhmfa1 and Uhpra1, and their subsequent replacement, confirmed the role of these genes in initiation of the sexual cycle. Uhmfa1 and Uhmfa2 were differentially expressed in various cell types and when opposite mating-type cells were grown together. The predicted mature pheromones of each mating type were synthesized, and each specifically induced conjugation tube formation in cells of the opposite mating type.

Microtubule dynamics from mating through the first zygotic division in the budding yeast Saccharomyces cerevisiae.

We have used time-lapse digital imaging microscopy to examine cytoplasmic astral microtubules (Mts) and spindle dynamics during the mating pathway in budding yeast Saccharomyces cerevisiae. Mating begins when two cells of opposite mating type come into proximity. The cells arrest in the G1 phase of the cell cycle and grow a projection towards one another forming a shmoo projection. Imaging of microtubule dynamics with green fluorescent protein (GFP) fusions to dynein or tubulin revealed that the nucleus and spindle pole body (SPB) became oriented and tethered to the shmoo tip by a Mt-dependent search and capture mechanism. Dynamically unstable astral Mts were captured at the shmoo tip forming a bundle of three or four astral Mts. This bundle changed length as the tethered nucleus and SPB oscillated toward and away from the shmoo tip at growth and shortening velocities typical of free plus end astral Mts (approximately 0.5 micrometer/min). Fluorescent fiduciary marks in Mt bundles showed that Mt growth and shortening occurred primarily at the shmoo tip, not the SPB. This indicates that Mt plus end assembly/disassembly was coupled to pushing and pulling of the nucleus. Upon cell fusion, a fluorescent bar of Mts was formed between the two shmoo tip bundles, which slowly shortened (0.23 +/- 0.07 micrometer/min) as the two nuclei and their SPBs came together and fused (karyogamy). Bud emergence occurred adjacent to the fused SPB approximately 30 min after SPB fusion. During the first mitosis, the SPBs separated as the spindle elongated at a constant velocity (0.75 micrometer/min) into the zygotic bud. There was no indication of a temporal delay at the 2-micrometer stage of spindle morphogenesis or a lag in Mt nucleation by replicated SPBs as occurs in vegetative mitosis implying a lack of normal checkpoints. Thus, the shmoo tip appears to be a new model system for studying Mt plus end dynamic attachments and much like higher eukaryotes, the first mitosis after haploid cell fusion in budding yeast may forgo cell cycle checkpoints present in vegetative mitosis.

Selfing versus outcrossing propensity of the fungal pathogen Microbotryum violaceum across Silene latifolia host plants: O. KALTZ AND J. A. SHYKOFF

Abstract In the fungal pathogen Microbotryumviolaceum mating (i.e. conjugation between cells of opposite mating type) is indispensable for infection of its host plant Silenelatifolia. Since outcrossing opportunities are potentially rare, selfing may be appropriate to ensure reproduction. On the other hand, outcrossing may create genetic variability necessary in the coevolutionary arms race with its host. We investigated the propensity of M. violaceum to outcross vs. self in different host environments. We used haploid sporidia from each of three strains from five fungal populations for pairwise mixtures of opposite mating type, representing either selfing or outcrossing combinations. Mixtures were exposed to leaf extract from seven S. latifolia plants. The proportion of conjugated sporidia quantified mating propensity. The identity of both fungal strains and host influenced conjugation. First, individual strains differed in conjugation frequency by up to 30%, and strains differed in their performance across the different hosts. Second, selfing combinations produced, on average, more conjugations than did outcrossing combinations. Selfing appears to be the predominant mode of reproduction in this fungus, and selfing preference may have evolved as a mechanism of reproductive assurance. Third, individual strains varied considerably in conjugation frequency in selfing and outcrossing combinations across different hosts. This indicates that conjugation between outcrossing partners could be favoured at least in some hosts. Since the dikaryon resulting from conjugation is the infectious unit, conjugation frequency may correspond with infection probability. This assumption was supported by an inoculation experiment, where high infectious sporidial dosage resulted in higher infections success than did low dosage. We therefore predict that sexual recombination can provide this pathogen with novel genotypes able to infect local resistant hosts.

Mitochondrial transmission during mating in Saccharomyces cerevisiae is determined by mitochondrial fusion and fission and the intramitochondrial segregation of mitochondrial DNA.

To gain insight into the process of mitochondrial transmission in yeast, we directly labeled mitochondrial proteins and mitochondrial DNA (mtDNA) and observed their fate after the fusion of two cells. To this end, mitochondrial proteins in haploid cells of opposite mating type were labeled with different fluorescent dyes and observed by fluorescence microscopy after mating of the cells. Parental mitochondrial protein markers rapidly redistributed and colocalized throughout zygotes, indicating that during mating, parental mitochondria fuse and their protein contents intermix, consistent with results previously obtained with a single parentally derived protein marker. Analysis of the three-dimensional structure and dynamics of mitochondria in living cells with wide-field fluorescence microscopy indicated that mitochondria form a single dynamic network, whose continuity is maintained by a balanced frequency of fission and fusion events. Thus, the complete mixing of mitochondrial proteins can be explained by the formation of one continuous mitochondrial compartment after mating. In marked contrast to the mixing of parental mitochondrial proteins after fusion, mtDNA (labeled with the thymidine analogue 5-bromodeoxyuridine) remained distinctly localized to one half of the zygotic cell. This observation provides a direct explanation for the genetically observed nonrandom patterns of mtDNA transmission. We propose that anchoring of mtDNA within the organelle is linked to an active segregation mechanism that ensures accurate inheritance of mtDNA along with the organelle.

The Schizosaccharomyces pombe mam1 gene encodes an ABC transporter mediating secretion of M-factor.

In the fission yeast Schizosaccharomyces pombe, cells of opposite mating type communicate via diffusible peptide pheromones prior to mating. We have cloned the S. pombe mam1 gene, which encodes a 1336-amino acid protein belonging to the ATP-binding cassette (ABC) superfamily. The mam1 gene is only expressed in M cells and the gene product is responsible for the secretion of the mating pheromone. M-factor, a nonapeptide that is S-farnesylated and carboxy-methylated on its C-terminal cysteine residue. The predicted Mam1 protein is highly homologous to mammalian multiple drug-resistance proteins and to the Saccharomyces cerevisiae STE6 gene product, which mediates export of a-factor mating pheromone. We show that STE6 can also mediate secretion of M-factor in S. pombe.

The sxa2-dependent inactivation of the P-factor mating pheromone in the fission yeast Schizosaccharomyces pombe.

Haploid cells of the fission yeast Schizosaccharomyces pombe exist in one of two mating types, referred to as M and P. Conjugation occurs between cells of opposite mating type and is controlled by the reciprocal action of diffusible pheromones. Loss of function of the sxa2 gene in M cells causes hypersensitivity to the P-factor mating pheromone and a reduction in mating efficiency. Here we demonstrate the secretion of an sxa2-dependent carboxypeptidase that inactivates P-factor by removal of the C-terminal leucine residue.

Chapter 76 Mating and Tetrad Analysis in Chlamydomonas reinhardtii

Publisher Summary The ease of detecting mutations in haploid organisms is straightforward; any newly arising recessive mutation is apparent because there is no wild-type allele to mask the mutant phenotype. Thus, large-scale screens and/or selections can be performed for the isolation of mutations affecting the flagella of Chlamydomonas . Two different mating types exist in C. reinhardtii : mating type plus (mt + ) and mating type minus (mt - ). These mating types behave as alleles at a genetic locus on linkage group VI. Chlamydomonas reinhardtii is a haploid organism. Haploid organisms have several advantages over diploid organisms when dissecting a process by a mutational approach. Mating is initiated when the cells are starved for nutrients and specifically when they are starved for nitrogen. Recognition occurs between cells of opposite mating types at two levels. The initial recognition occurs by interactions between hydroxyprolinerich glycoproteins, agglutinins, in the membrane of the flagella of the two cell types. This process is known as tipping. It leads to an increase in the intracellular levels of cyclic AMP, which stimulates the formation of a fertilization tube on the surface of the mt + cell. Mutations that affect flagellar function, length, or number do not block the ability of C. reinhardtii strains to mate. Short flagella or paralyzed flagella may reduce the efficiency of mating, but do not block mating. Flagella are required for conventional mating procedures. Mutations that produce cells that lack flagella require additional steps to ensure mating. Because the first recognition events involve the tipping of flagella of the two mating types, this step must be bypassed. When meiosis is completed and the meiotic progeny have been collected, they can be analyzed as the progeny from individual meiotic events (a tetrad) or individually (random progeny). One advantage of tetrad analysis is the ability to map a gene with respect to its centromere as well as with respect to other genes.

Chapter 77 Strategies for Isolation of Flagellar Motility and Assembly Mutants in Chlamydomonas

Publisher Summary Chlarnydornonas is uniquely suited for genetic studies because its flagellated vegetative cells are haploid and its mutants can be genetically analyzed by sexual crosses between different strains. More than 40 mutants are known that are deficient in internal structures of the flagella such as inner and outer dynein arms, central pair microtubules, or radial spoke. This chapter demonstrates a simple method for the isolation of various motility mutants. It is based on enrichment for cells that grow on the bottom of test tube cultures after mutagenesis with UV light. The enrichment methods can similarly be used for the selection of paralyzed or slow-swimming mutants generated by insertional mutagenesis. Using TAP as the medium the mutagenesis initiated the process to culture wild-type Chlamydomonas reinhardtii (plus or minus mating type) to a density of 1–2 x 10 6 /ml in liquid TAP medium under continuous illumination. While enriching of Motility-Deficient Cells, during the second light phase, it is reqired to carefully transfer cells on the bottom of each test tube to another test tube containing 4 ml of fresh medium. The volume of transferred suspension is 50–200 μ I. It is necessary to use a new, sterilized Pasteur pipette for each transfer. When single colonies are apparent, use sterile toothpicks to transfer the colonies to 96-well culture plates, each well containing 200 μ I of liquid medium. When mutants of interest are obtained, mate them with wild type to obtain daughter cells of opposite mating types. This step is necessary because the first-generation mutants often carry a second, undesirable mutation.

Pheromones trigger filamentous growth in Ustilago maydis.

Cell recognition and mating in the smut fungus Ustilago maydis have been proposed to involve specific pheromones and pheromone receptors. The respective structural genes are located in the a mating type locus that exists in the alleles a1 and a2. We demonstrate that binding of pheromone to the receptor can induce a morphological switch from yeast-like to filamentous growth in certain strains. Using this as biological assay we were able to purify both the a1 and a2 pheromone. The structure of the secreted pheromones was determined to be 13 amino acids for a1 and nine amino acids for a2. Both pheromones are post-translationally modified by farnesylation and carboxyl methyl esterification of the C-terminal cysteine. An unmodified a1 peptide exhibits dramatically reduced activity. The pheromone alone is able to induce characteristic conjugation tubes in cells of opposite mating type and confers mating competence; even cells of the same mating type undergo fusion. We discuss the role of pheromones in initiating filamentous growth and pathogenic development.

Double staining of mitochondria and mitochondrial nucleoids in the living yeast during the life cycle.

Mitochondria and mitochondrial nucleoids (mt-nucleoids) of the yeast, Saccharomyces cerevisiae were vitally double-stained with two fluorochromes, 3, 3′-dihexyloxacarbocyanine iodide [DiOC6(3)] and 4′, 6-diamidino-2-phenylindole (DAPI), respectively in different types of cells during the life cycle. Budded cells at logarithmic phase had a single, branched, giant mitochondrion, a part of which migrated into the daughter cell prior to nuclear division. Mt-nucleoids appeared as a string-of-beads within a giant mitochondrion. Single cells at stationary phase had numerous fragmented mitochondria with dot-like mt-nucleoids. When stationary-phase cells were transferred to the sporulation medium, fragmented mitochondria fused with each other and a network of mitochondrion changed its configuration as the meiotic division proceeded. Finally, mitochondria formed four rings which enclosed each daughter nucleus in premature spores. Mitochondria were hardly stained with DiOC6(3) in mature spores. Fused mitochondria were clearly observed again in cells at germination and outgrowth. When cells of opposite mating type were mixed in mating medium, zygote formation and first bud emergence occurred in 4h. Just after mating, string-like and fragmented mitochondria transmitted from each parent cell were separately visible on both sides of a dumbbell-shaped zygote. In contrast, extensive fusion of mitochondria occurred in zygotes with a first bud.

Morphological and Mutational Analysis of Mating in Ustilago hordei

Martinez-Espinoza, A. D., Gerhardt, S. A., and Sherwood, J. E. 1993. Morphological and mutational analysis of mating in Ustilago hordei. Experimental Mycology 17, 200-214. Ustilago hordei is a basidiomycete that causes covered smut on barley. Mating in U. hordei, which is controlled by a single locus with two alleles, results in the conversion of haploid, nonpathogenic yeast-like sporidia to dikaryotic, pathogenic mycelia. When sporidia of the opposite mating type were mixed and placed on water agar, both cell types produced conjugation tubes within 2 h at 21°C. Growth of conjugation tubes was directed toward the tip of tubes arising from cells of the opposite mating type. These tubes fused and the dikaryotic mycelium emerged from the conjugation bridge. Sporidia separated by a dialysis membrane were still capable of inducing conjugation tube formation by cells of the opposite mating type, indicating the involvement of diffusible small-molecular-weight mating factors (pheromones). Numerous nutritional and environmental variables were examined in order to optimize conjugation tube induction. Twenty-six mutants that fail to form dikaryotic mycelium have been isolated and characterized. These mutants were arranged into classes based on their ability to form conjugation tubes, the ability to induce conjugation tube formation by opposite mating-type cells, and cell morphology. These mutants provide an indication of the genetic complexity involved in this critical phase of the U. hordei life cycle.

Zygote formation in the homothallic green alga Chlamydomonas monoica Strehlow

Mating between cells of opposite mating type within a clonal population of Chlamydomonas monoica results in thick-walled zygotes. Zygote formation was studied in cells from continuous cultures fed with culture medium containing nitrate concentrations sufficient or limiting for growth. The factors that were considered were cell density and nitrate content of the medium. The following results were obtained: (i) Zygotes were only formed by cells that had experienced a relatively low nitrogen level that did not limit cell division. (ii) Cells were competent to mate only during a limited period of time after their release from the mother cell wall. (iii) There was a correlation between zygote yields and the number of low-nitrogen cells that were able to execute a cell division under the conditions being tested. (iv) The zygote yield per cell division was independent of the cell density. These findings indicate that the strategy used by C. monoica cells to find a mate is not dependent on random encounters. A possible explanation is that at least a large proportion of zygotes is formed by matings between cells originating from the same mother cell (siblings).

Transmission of mitochondrial DNA in Ustilago violacea.

Mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs) were used as genetic markers for following mitochondrial transmission in the basidiomycete Ustilago violacea. Yeast-like cells of opposite mating types (a1 and a2) were mated on 2% water agar and were treated with alpha-tocopherol to induce formation of dikaryotic hyphae. Upon depletion of the alpha-tocopherol, the hyphae budded off haploid cells with parental nuclear genotypes. These cells were examined for mitochondrial RFLP phenotype. In progeny expressing the a1 mating type, mitochondria from either parent were observed equally frequently. In progeny with the a2 mating type, mitochondria were almost exclusively (94%) from the a2 parent.

The a mating type locus of U. maydis specifies cell signaling components

The a mating type locus of the phytopathogenic fungus U. maydis controls fusion of haploid cells and filamentous growth of the dikaryotic mycelium. The a locus exists in two alleles, termed a1 and a2, which are defined by nonhomologous DNA regions comprising 4.5 kb for a1 and 8 kb for a2, flanked by identical sequences. Based on functional assays, mutants, and sequencing, we demonstrate that the mating type in each allele is determined by a set of two genes. One encodes a precursor for a lipopeptide mating factor, and the other specifies the receptor for the pheromone secreted by cells of opposite mating type. Thus, U. maydis employs a novel strategy to determine its mating type by providing the primary determinants of cell-cell recognition directly from the mating type locus.

Sexual agglutination in budding yeasts: structure, function, and regulation of adhesion glycoproteins.

The sexual agglutinins of the budding yeasts are cell adhesion proteins that promote aggregation of cells during mating. In each yeast species, complementary agglutinins are expressed by cells of opposite mating type that interact to mediate aggregation. Saccharomyces cerevisiae alpha-agglutinin and its analogs from other yeasts are single-subunit glycoproteins that contain N-linked and O-linked oligosaccharides. The N-glycosidase-sensitive carbohydrate is not necessary for activity. The proposed binding domain of alpha-agglutinin has features characteristic of the immunoglobulin fold structures of cell adhesion proteins of higher eukaryotes. The C-terminal region of alpha-agglutinin plays a role in anchoring the glycoprotein to the cell surface. The S. cerevisiae alpha-agglutinin and its analogs from other species contain multiple subunits; one or more binding subunits, which interact with the opposite agglutinin, are disulfide bonded to a core subunit, which mediates cell wall anchorage. The core subunits are composed of 80 to 95% O-linked carbohydrate. The binding subunits have less carbohydrate, and both carbohydrate and peptide play roles in binding. The alpha-agglutinin and alpha-agglutinin genes from S. cerevisiae have been cloned and shown to be regulated by the mating-type locus, MAT, and by pheromone induction. The agglutinins are necessary for mating under conditions that do not promote cell-cell contact. The role of the agglutinins therefore is to promote close interactions between cells of opposite mating type and possibly to facilitate the response to phermone, thus increasing the efficiency of mating. We speculate that they mediate enhanced response to sex pheromones by providing a synapse at the point of cell-cell contact, at which both pheromone secretion and cell fusion occur.