Signalling in the yeasts: an informational cascade with links to the filamentous fungi

Abstract

All cells, from bacteria and yeasts to mammalian cells, respond to cues from their environment. A variety of mechanisms exist for the transduction of these external signals to the interior of the cell, resulting in altered patterns of protein activity. Eukaryotic cells commonly transduce external cues via a conserved module composed of three protein kinases, the mitogen-activated protein kinase (MAPK) cascade. This module can then activate substrates, some of which include transcriptional activators. Multiple MAPK signalling pathways coexist in a cell. This review considers different MAPK cascade signalling pathways that govern several aspects of the life cycle of budding and fission yeasts: conjugation and meiosis by the pheromone response pathway, stress response by the high-osmolarity sensing pathway, cell wall biosynthesis in response to activation of the low-osmolarity and heat-sensing pathway, and pseudohyphal growth in response to activation of a subset of the components of the pheromone response pathway. Because the MAPK cascade components are highly conserved, a key question in studies of these pathways is the mechanism by which specificity of response is achieved. Several other issues to be addressed in this review concern the nature of the receptors used to sense the external signals and the mechanism by which the receptors communicate with other components leading to activation of the MAPK cascade. Recently, it has become apparent that MAPK cascades are important in governing the pathogenicity of filamentous fungi.

FIG. 1

MAPK module. (A) The MAPK module lies at the heart of many signalling pathways in eukaryotes. An input signal leads to activation of the MAPK cascade, which then generates an output response. (B) The MAPK cascade consists of MAPKKK (MEKK), MAPKK (MEK), and MAPK (ERK) (see the text for details). A receptor is activated in response to an extracellular signal, which leads to activation of the MAPK cascade (see the text for details). The activated MAPK phosphorylates substrates, some of which include transcriptional activators, resulting in altered patterns of gene expression and protein activity.

FIG. 2

Hyperosmotic stress response in S. cerevisiae and S. pombe. Response to hyperosmolarity is mediated by the HOG pathway in S. cerevisiae (left) and by the Sty pathway in S. pombe (right). Ptp2 is a phosphatase involved in down regulation of Hog1. Pyp2 is a phosphatase involved in down regulation of Sty1. Arrows indicate activation; lines with bars indicate inhibition. The dashed rectangle around Ypd1 indicates that the protein is present but not phosphorylated (see Fig. 5). See the text for details.

FIG. 3

Multiple roles of the MAPKKK Ste11. The MAPKKK Ste11 can be activated by different signals resulting in different outputs. It can be activated by pheromones in the pheromone response pathway, resulting in mating (stippled arrows); a signal generated by nitrogen starvation in the pseudohyphal growth pathway, leading to morphological changes and growth properties of the cells (black arrows); and hyperosmolarity via Sho1 in the HOG pathway, resulting in the stress response (gray arrows). The mechanism by which activated Ste11 is prevented from inappropriately activating other pathways may involve scaffold proteins (see the text for details).

FIG. 4

Phosphorelay system of the HOG pathway. A phosphorelay system consisting of three proteins, Sln1, Ypd1, and Ssk1, activates the HOG pathway. Sln1, the osmosensor, contains both a histidine kinase domain (black rectangle) and a receiver domain at its C terminus (hatched rectangle). Upon signal sensing, Sln1 autophosphorylates His576. Ssk1 contains a receiver domain (hatched rectangle) and is the activator of the MAPKKKs Ssk2 and Ssk22. Arrows indicate direction of phosphotransfer. Mutation of the His or Asp residues blocks phosphotransfer and results in constitutive activation of the pathway, suggesting that phosphorylation prevents activation of the HOG MAPK cascade. See the text for other details.

FIG. 5

High osmolarity inhibits phosphotransfer in the phosphorelay system and causes activation of the HOG MAPK cascade. Normal osmolarity conditions (left) stimulate phosphotransfer and result in phosphorylation of Ssk1. Phosphorylated Ssk1 cannot activate the MAPK cascade, and consequently there is no response. High osmolarity (right) inhibits Sln1 by an unknown mechanism. Phosphotransfer is blocked, and unphosphorylated Ssk1 is proposed to activate the MAPK cascade, resulting in the stress response. Arrows indicate activation; lines with bars indicate inhibition. The dashed rectangle around the MAPK cascade (left) indicates that it is inactive. The dashed rectangle around Ypd1 (right) indicates that the protein is present but not phosphorylated. See the text for details.

FIG. 6

Multiple stress conditions activate the Sty1 MAPK cascade, resulting in coordination of the stress response with mitosis and meiosis. The Sty pathway can be activated by osmotic, heat, and oxidative stress, by nutritional limitation (black arrows above rectangle), and by anisomycin (not shown). Osmotic stress not only activates the stress response (black arrow below rectangle) but also controls cell size at time of mitosis (open arrow below rectangle). In response to nutritional limitation, the stress response is activated, resulting in expression of ste11, which initiates sexual differentiation (stippled arrow below rectangle).

FIG. 7

The PKC pathway controls cell integrity. The PKC MAPK cascade is activated in response to heat and low osmotic stress and nutrient limitation. Hcs77 is a putative mechanosensor that is proposed to sense membrane stretch. Pkc1 is proposed to activate a branched pathway (one branch is the MAPK and the other is hypothetical). Rlm1 appears to be a target for Mpk1. Solid arrows indicate activation; the dashed arrow is speculative. See the text for details.

FIG. 8

Proposed activation of the Rho1 module by Hcs77. Hcs77, a putative mechanosensor, may activate Tor2, a phosphatidylinositol kinase homolog. Tor2 is proposed to activate Rom2, a GEF for Rho1-GDP. Alternatively, Hcs77 could activate Rom2 directly or could inhibit Sac7, a GAP for Rho1-GTP. This inhibition would promote the Rho1-GTP state. Rho1-GTP activates Pkc1, glucan synthase, and presumably other substrates. Solid arrows indicate activation; dashed arrows are highly speculative. See the text for details.

FIG. 9

Pheromone response pathway of S. cerevisiae. The pheromone response pathway is activated upon binding of a-factor or α-factor (solid sphere) the to serpentine receptors Ste3 and Ste2, respectively. Ste5 is shown as the shaded rectangle that holds Ste11, Ste7, and Fus3 or Kss1 together. Solid arrows indicate activation; dashed arrows indicate that evidence is not conclusive; lines with bars indicate inhibition. See the text for details.

FIG. 10

Pheromone response pathway of S. pombe. Ste11 is a key regulator of sexual differentiation in S. pombe. Its transcription is induced by the Sty pathway in response to nitrogen starvation (right) (also see Fig. 2). Its activity may be regulated by the Spk1 MAPK (middle). Ras1 and Gα-GTP are both necessary for activation of the pheromone response MAPK cascade. The target genes for this pathway include the pheromone precursor and receptor genes, the ste6 gene (encoding the GEF for Ras1), fus1 (required for cell fusion), mat1-Pm and mat1-Mm, and other genes (Table 1). Nitrogen starvation is indispensable for the pheromone response, since addition of pheromones to cells growing in rich medium does not elicit the mating response. Ras1 is proposed to regulate a morphogenetic cascade (left) by activation of Scd1 (a proposed GEF for Cdc42). Cdc42 in turn activates Shk1, a Ste20 homolog. See the text for details. Solid arrows indicate activation; dashed arrows indicate that evidence is limited or nonexistent.

FIG. 11

Stepwise induction of genes for mating and meiosis in S. pombe. The induction of genes necessary for mating and meiosis appears to occur in several steps (steps I to III), all of which require nitrogen-depleted conditions. Ste11 appears to be required for steps I and II. Symbols: +++, full-level induction; ++, induction but not to full level; +, low-level induction; +/−, slight induction; pher, pheromones; N-starv, nitrogen starvation. See the text for details.

FIG. 12

Signalling pathway for pseudohyphal growth. Nitrogen starvation activates the pseudohyphal pathway in a/α diploid cells, although the mechanism of activation is not known (dashed arrow from signal to Ras2). An alternative pathway may also regulate pseudohyphal growth, indicated by the dashed arrows originating from other inputs. X, unidentified MAPK, which may be Kss1. See the text for details.

FIG. 13

Multiple pheromones and multiple receptors govern filamentous growth in the mushrooms (Basidiomycete fungi). (A) Plasmogamy occurs between cells of hyphae in a mating-type-independent manner (step 1). Steps 2, 3, and 4 are regulated by the A mating complex, which encodes homeodomain proteins. Steps 1 (nuclear migration but not cell fusion) and 5 are regulated by the B mating complex. Step 5 may be viewed as analogous to cell fusion during mating in the yeasts. (B) The B complex encodes multiple pheromones and receptors. Bβ1 and Bβ2 are two of the nine alleles of the Bβ locus within the B complex (see the text for details). 1-1, 1-2, 1-3, 2-2, 2-2, and 2-3 are putative pheromone precursor genes; R1 and R2 are putative pheromone receptor genes.