Sexual pheromones and mating responses in fungi.

Abstract

Sexual development in fungi is initiated by the fusion of haploid cells that are morphologically indistinguishable. However, they can fuse only if they are of different mating types. In general, the term “mating type” defines all specific activities of a cell that are necessary to go from the haploid to the diploid stage and to continue on to meiosis. The mating type is determined by the genetic information present at the mating-type (mar) locus. Diverse systems have evolved to ensure the maintenance of different sexes of either hrvo or multiple mating types. In many fungal species, specific cell recognition and fusion are mediated by diffusible peptide mating factors. These secreted peptides have been termed pheromones by analogy with the secreted chemical substances of insects and mammals, which can elicit specific responses in very low doses. In this review, we will focus on some of these fungal peptide pheromones and their functions within the sexual cycle. The study of these pheromones has allowed insights into fields as diverse as protein modification and trafficking , receptor-ligand interactions, signal transduction, and cell cycle regulation. By far the best-understood system with respect to the genetic and molecular mechanisms of pheromone production and pheromone response is that of the yeast Saccharomyces cerevisiae. The first hint of the existence of peptide pheromones in this system came from the observation that a diffusible substance can act at a distance and that this effect is cell type specific (Levi, 1956). The mating reaction of two haploid cells involves a complicated set of reactions to ensure the coordinated expression of a number of genes that are necessary to mediate the controlled fusion of cells and the subsequent karyogamy of nuclei. Being in the same phase of the cell cycle is a prerequisite for nuclear fusion; therefore, cell cycle arrest in G1 is one of the primary responses to pheromones in yeast. This response provides the opportunity to investigate how a signal elicited by extracellular pheromones is passed into the nucleus, where it affects the cell cycle. The pheromone receptors belong to the large family of G-protein-coupled seven-transmembrane receptors, and the yeast system has proved to be a useful model system for studying these receptors and their signal transduction chain. The enzymes involved in modification and translocation of the prenylated a-factor of yeast have attracted interest because