Sexual hormones in yeast

Summary Hormone-like substances were isolated from culture media of haploid strains of Saccharomyces cerevisiae. The one excreted by cells of mating type a made cells of the type expand; the other, excreted by type cells, made cells of the a type expand. Tentatively we call the former a hormone and the latter hormone. The cell-expanding action of the a hormone was inhibited by actinomycin D, chloramphenicol and cycloheximide. The a hormone was shown to be heat-stable and dialyzable. Both hormones could be extracted with methylene chloride. The abilities of cells to produce these hormones and to respond to them are under control by the mating-type genes.     

Sexual conjugation in yeast: A paradigm to study G-protein-coupled receptor domain structure

The yeast Saccharomyces cerevisiae undergoes cell fusion during sexual conjugation to form diploid cells. The haploids participating in this process signal each other through secreted peptide-mating factors (alpha-factor and a-factor) that are recognized by G-protein-coupled receptors. The receptor (Ste2p) recognizing the tridecapeptide alpha-factor is used as a model system in our laboratory to understand various aspects of peptide-receptor interactions and receptor structure. Using chemical procedures we have synthesized peptides corresponding to the seven transmembrane domains of Ste2p and studied their structures in membrane mimetic environments. Extension of these studies requires preparation of longer fragments of Ste2p. This article discusses strategies used in our laboratory to prepare peptides containing multiple domains of Ste2p. Data are presented on the use of chemical synthesis, biosynthesis, and native chemical ligation. Using biosynthetic approaches fusion proteins have been expressed that contain single receptor domains, two transmembrane domains connected by the contiguous loop, and the tail connected to the seventh transmembrane domain. Tens of milligrams of fusion protein were obtained per liter, and multimilligram quantities of the isotopically labeled target peptides were isolated using such biosynthetic approaches. Initial circular dichroism results on a chemically synthesized 64-residue peptide containing a portion of the cytosolic tail and the complete seventh transmembrane domain showed that the tail portion and the hydrophobic core of this peptide maintained individual conformational preferences. Moreover, this peptide could be studied at near millimolar concentrations in the presence of micelles and did not aggregate under these conditions. Thus, these constructs can be investigated using high-resolution nuclear magnetic resonance techniques, and the cytosolic tail of Ste2p can be used as a hydrophilic template to improve solubility of transmembrane peptides for structural analysis.     

A filamentous growth response mediated by the yeast mating pathway.

Haploid cells of the budding yeast Saccharomyces cerevisiae respond to mating pheromones by arresting their cell-division cycle in G1 and differentiating into a cell type capable of locating and fusing with mating partners. Yeast cells undergo chemotactic cell surface growth when pheromones are present above a threshold level for morphogenesis; however, the morphogenetic responses of cells to levels of pheromone below this threshold have not been systematically explored. Here we show that MATa haploid cells exposed to low levels of the alpha-factor mating pheromone undergo a novel cellular response: cells modulate their division patterns and cell shape, forming colonies composed of filamentous chains of cells. Time-lapse analysis of filament formation shows that its dynamics are distinct from that of pseudohyphal growth; during pheromone-induced filament formation, daughter cells are delayed relative to mother cells with respect to the timing of bud emergence. Filament formation requires the RSR1(BUD1), BUD8, SLK1/BCK1, and SPA2 genes and many elements of the STE11/STE7 MAP kinase pathway; this response is also independent of FAR1, a gene involved in orienting cell polarization during the mating response. We suggest that mating yeast cells undergo a complex response to low levels of pheromone that may enhance the ability of cells to search for mating partners through the modification of cell shape and alteration of cell-division patterns.     

Cell fusion during yeast mating requires high levels of a-factor mating pheromone

During conjugation, two yeast cells fuse to form a single zygote. Cell fusion requires extensive remodeling of the cell wall, both to form a seal between the two cells and to remove the intervening material. The two plasma membranes then fuse to produce a continuous cytoplasm. We report the characterization of two cell fusion defective (Fus-) mutants, fus5 and fus8, isolated previously in our laboratory. Fluorescence and electron microscopy demonstrated that the fus5 and fus8 mutant zygotes were defective for cell wall remodeling/removal but not plasma membrane fusion. Strikingly, fus5 and fus8 were a specific; both mutations caused the mutant phenotype when present in the MATa parent but not in the MAT alpha parent. Consistent with an a-specific defect, the fus5 and fus8 mutants produced less a-factor than the isogenic wild-type strain. FUS5 and FUS8 were determined to be allelic to AXL1 and RAM1, respectively, two genes known to be required for biogenesis of a-factor. Several experiments demonstrated that the partial defect in a-factor production resulted in the Fus- phenotype. First, overexpression of a-factor in the fus mutants suppressed the Fus- defect. Second, matings to an MAT alpha partner supersensitive to mating pheromone (sst2 delta) suppressed the Fus- defect in trans. Finally, the gene encoding a-factor, MFA1, was placed under the control of a repressible promoter; reduced levels of wild-type a-factor caused an identical cell fusion defect during mating. We conclude that high levels of pheromone are required as one component of the signal for prezygotes to initiate cell fusion.     

Cell surface interactions in conjugation: Tetrahymena ciliary membrane vesicles.

Tetrahymena ciliary membrane vesicles are shown to interact with preconjugant cells in a mating type-specific way. When cells are treated with vesicles of a different mating type before mixing for conjugation, cell pairing is enhanced, and the normal prepairing period is partially eliminated. This enhancement is mating type specific since it is not observed after pretreatment of cells with vesicles of their own mating type. In contrast, when vesicles are added at the time of mixing of two starved cultures, cell pairing is delayed in a concentration-dependent manner. By varying the conditions, we demonstrated enhancement or inhibition, or both. These results are interpreted in terms of two independent interactions of cells with vesicles. We suggest that first, vesicles substitute for another cell in cell-cell prepairing interaction and second, vesicles compete for adhesion sites produced during the prepairing period. Finally, the data presented are summarized within a speculative framework that calls attention to potential analogies with hormone-receptor signaling in mammalian cells.     

A walk-through of the yeast mating pheromone response pathway

The intracellular signal transduction pathway by which the yeast Saccharomyces cerevisiae responds to the presence of peptide mating pheromone in its surroundings is one of the best understood signaling pathways in eukaryotes, yet continues to generate new surprises and insights. In this review, we take a brief walk down the pathway, focusing on how the signal is transmitted from the cell-surface receptor-coupled G protein, via a MAP kinase cascade, to the nucleus.     

Tyrosine phosphorylation of a yeast 40 kDa protein occurs in response to mating pheromone.

Tyrosine phosphorylation of proteins in the yeast Saccharomyces cerevisiae has been examined following exposure to the mating pheromone alpha-factor. When a cells are treated with alpha-factor a protein of approximately 40 kDa molecular weight is tyrosine phosphorylated. This tyrosine phosphorylation response requires an intact signal transduction pathway, is not restricted to a short interval of the cell division cycle, and requires protein synthesis for its maximal accumulation. Mating competent fus3 deletion strains fail to elaborate the phosphotyrosine response. The possibility that FUS3 encodes the 40 kDa protein is discussed.     

A walk-through of the yeast mating pheromone response pathway

The intracellular signal transduction pathway by which the yeast Saccharomyces cerevisiae responds to the presence of peptide mating pheromone in its surroundings is one of the best understood signaling pathways in eukaryotes, yet continues to generate new surprises and insights. In this review, we take a brief walk down the pathway, focusing on how the signal is transmitted from the cell-surface receptor-coupled G protein, via a MAP kinase cascade, to the nucleus.     

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

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

Sexual response of Saccharomyces cerevisiae: phosphorylation of yeast glyoxalase I by a cell extract of mating factor-treated cells

The phosphorylation of glyoxalase I was observed when the phosphatase-treated enzyme was incubated in the presence of [gamma-32P]ATP and a cell extract prepared from alpha-type yeast cells which had been treated with the culture supernatant of a-type yeast cells. The phosphorylated protein was identified as glyoxalase I by using anti-glyoxalase I rabbit immunoglobulin G.     

Sexual hormones in human skin.

The skin locally synthesizes significant amounts of sexual hormones with intracrine or paracrine actions. The local level of each sexual steroid depends upon the expression of each of the androgen- and estrogen-synthesizing enzymes in each cell type, with sebaceous glands and sweat glands being the major contributors. Sebocytes express very little of the key enzyme, cytochrome P450c17, necessary for synthesis of the androgenic prohormones dehydroepiandrosterone and androstenedione, however, these prohormones can be converted by sebocytes and sweat glands, and probably also by dermal papilla cells, into more potent androgens like testosterone and dihydrotestosterone. Five major enzymes are involved in the activation and deactivation of androgens in skin. Androgens affect several functions of human skin, such as sebaceous gland growth and differentiation, hair growth, epidermal barrier homeostasis and wound healing. Their effects are mediated by binding to the nuclear androgen receptor. Changes of isoenzyme and/or androgen receptor levels may have important implications in the development of hyperandrogenism and the associated skin diseases such as acne, seborrhoea, hirsutism and androgenetic alopecia. On the other hand, estrogens have been implicated in skin aging, pigmentation, hair growth, sebum production and skin cancer. Estrogens exert their actions through intracellular receptors or via cell surface receptors, which activate specific second messenger signaling pathways. Recent studies suggest specific site-related distribution of ERalpha and ERbeta in human skin. In contrast, progestins play no role in the pathogenesis of skin disorders. However, they play a major role in the treatment of hirsutism and acne vulgaris, where they are prescribed as components of estrogen-progestin combination pills and as anti-androgens. These combinations enhance gonadotropin suppression of ovarian androgen production. Estrogen-progestin treatment can reduce the need for shaving by half and arrest progression of hirsutism of various etiologies, but do not necessarily reverse it. However, they reliably reduce acne. Cyproterone acetate and spironolactone are similarly effective as anti-androgens in reducing hirsutism, although there is wide variability in individual responses.     

Relationship of iodide-induced increase in TSH response to TRH to changes in serum thyroid hormones.

Large doses of iodide (500 mg three times a day) administered to normal men for 10--12 days caused a rise in basal serum TSH and a concomitant rise in the peak TSH response to TRH. The basal and peak levels of TSH were highly correlated (p less than 0.001). However, the iodide-induced rise in the peak TSH after TRH was poorly correlated with concomitant changes in serum thyroid hormones. Serum T3 wa not lower after iodide and, while serum T4 was somewhat lower, the fall in serum T4 was unexpectedly inversely rather than directly correlated with the rise in the peak TSH response to TRH. Thus, increased TSH secretion after iodide need not always be directly correlated with decreased concentrations of circulating thyroid hormones even when large doses of iodide are used. Clinically, a patient taking iodide may have an increased TSH response in a TRH stimulation test even though there is little or no change in the serum level of T3 or T4.     

Sexual healing: mating induces a protective immune response in bumblebees

The prevalence of sexual, as opposed to clonal, reproduction given the many costs associated with sexual recombination has been an enduring question in evolutionary biology. In addition to these often discussed costs, there are further costs associated with mating, including the induction of a costly immune response, which leaves individuals prone to infection. Here, we test whether mating results in immune activation and susceptibility to a common, ecologically important, parasite of bumblebees. We find that mating does result in immune activation as measured by gene expression of known immune genes, but that this activation improves resistance to this parasite. We conclude that although mating can corrupt immunity in some systems, it can also enhance immunity in others.     

Cell type-specific chromatin organization of the region that governs directionality of yeast mating type switching.

Switching of mating type in Saccharomyces cerevisiae is directional; MAT alpha cells recombine to transfer information from HMRa while MATa cells switch using the silent cassette at HML alpha. Genetic analysis recently has defined a 700 bp recombination enhancer approximately 29 kb from the left end of chromosome III that is necessary for directionality. The chromatin structure of this region differs strikingly in a- and alpha-cells. Mat alpha2p organizes a 3.7 kb chromatin domain that opposes interaction of trans-acting proteins with the enhancer. In a-cells lacking the alpha2 repressor, two footprinted regions flank an approximately 100 bp section having a unique DNA structure. This structural signature probably reflects interactions of proteins that result in directional mating type switching.