John Davey

Pheromone communication in the fission yeast Schizosaccharomyces pombe

Conjugation between two haploid yeast cells is generally controlled by the reciprocal action of diffusible mating pheromones, cells of each mating type releasing pheromones that induce mating-specific changes in cells of the opposite type. Recent studies into pheromone signalling in the fission yeast Schizosaccharomyces pombe have revealed significant parallels with processes in higher eukaryotes and could provide the opportunity for investigating communication in an organism that is amenable to both biochemical and genetic manipulation.

Mutations in cyr1 and pat1 reveal pheromone-induced G1 arrest in the fission yeast Schizosaccharomyces pombe.

Investigations into sexual differentiation and pheromone response in the fission yeast Schizosaccharomyces pombe are complicated by the need to first starve the cells of nitrogen. Most mating-related experiments are therefore performed on non-dividing cells. Here we overcome this problem by using two mutants that bypass the nutritional requirements and respond to the M-factor mating pheromone in rich medium. The first mutant lacks the cyr1 gene which encodes adenylate cyclase and these cells contain no measurable amounts of cAMP. When M-factor is added to a growing h+ cyr1− strain it causes a transient G1 arrest of cell division, transcription of mat1-Pm, and elongation of the cells to form shmoos. The second mutant contains the temperature-sensitive pat1-114 allele. At 30°C this mutant was previously shown not only to bypass the nutritional signal but also to stop growing in a state derepressed for pheromone-controlled functions. We now report that an h+pat1-114 strain growing mitotically at 23°C responds to M-factor. This shows that the pat1 protein kinase can be tuned to derepress nutritional signalling while repressing the other stages in the differentiation process.

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.

[13] Pheromone procedures in fission yeast

Publisher Summary Comparative analyses of signal transduction in mammalian cells and lower eukaryotes, such as yeast, have revealed a striking conservation of modular components and regulatory cascades, and it is likely that further investigations into the yeast systems could provide valuable new information into the molecular interactions involved in cell-to-cell communication. The pheromone response pathway of yeast cells has received particular attention and this chapter reviews the similarities between the budding yeast, Saccharomyces cerevisiae, and the fission yeast, Schizosaccharomyces pombe . Although the two yeast are remarkably similar, they are sufficiently different to prevent the direct transfer of methods from one to the other and the well-characterized methodology of S. cerevisiae must often be modified for S. pombe . The chapter reviews several current methods for studying pheromones and signal transduction in S. pombe and describes the biological background for these procedures. P-factor is an unmodified peptide of 23 amino acids that is released by P cells. It is encoded by the map 2 gene and the primary translation product contains an aminoterminal signal sequence and four repetitive units of the pheromone. There are potentially three different species of P-factor, but one apparently dominates the others in quantity and has been demonstrated to have pheromone activity.

Sequence of ptb1, a Gene for the β subunit of the type-II geranylgeranyltransferase from the fission yeast Schizosaccharomyces pombe

We have isolated and sequenced the ptb1 gene from the fission yeast Schizosaccharomyces pombe. Sequence analysis suggests that Ptb1 is the β subunit of the type‐II geranylgeranyltransferase that is responsible for geranylgeranylation of the Rab‐like YPT proteins in this yeast. The sequence has been deposited in the EMBL data library under the Accession Number X92183.

Processing Proteases in S. pombe

A simple search of available databases reveals far more proteases in Schizosaccharomyces pombe than could be included in a review of this type. Limiting the search to proteases that generate a biologically active product reduced the number of candidates to an almost manageable level, while focusing on substrates within the secretory pathway introduced just the right amount of trimming to produce our final list. Many of the enzymes described are involved in processing of the mating pheromones, which are crucial in intercellular signalling during the sexual phase of the life cycle (Chap. 18). This focusing reflects not only our own research interests but also provides a structure for introducing the various processing events. Many readers will no doubt disagree with these decisions and some would have made other choices, but we hope that everyone sees this review as a starting point from which they can explore what is an ever expanding topic of research in the fission yeast.

Membrane Flow and Endocytosis

It is not my intention for this to be an exhaustive review of membrane transport or indeed of endocytosis. These topics have been dealt with extensively elsewhere and the reader is encouraged to browse through them at their leisure (Anderson and Kaplan, 1983; Goldstein et al., 1985; Silverstein et al., 1977; Steinman et al., 1983; Wileman et al., 1985). What I have tried to achieve is a representation of my personal views of membrane transport: highlighting those aspects of transport that are of particular interest to myself. Since my research interests are largely concerned with endocytosis it is from this pathway of membrane transport that I will draw the majority of examples and evidence. I will however include examples from other transport pathways where they are relevant or where they provide insights into transport events that are not observable on the endocytic pathway.