Pheromone-mediated cell-cell signaling in Euplotes

In ciliates, intra-specific cell-cell signalling is functionally associated with the genetic mechanism of the mating types. Based essentially on studies on the “binary” mating-type systems of Paramecium and Blepharisma, this mechanism has traditionally been considered as a sexual mechanism, in which the “mating-type factor” of one type of cells would bind, and stimulate to mate, only cells of the second “complementary” mating type. However, this view does not appear to be supported by genetic and structural studies of families of the water-borne protein mating-type factors (pheromones) that are synthesized by species of Euplotes (i. e., E. raikovi, E. nobilii and E. crassus) characterized by mating-type systems of “high-multiple” type. These studies suggest that the pheromone sexual activity is secondary with respect to an autocrine pheromone activity, which promotes the vegetative (mitotic) growth of the same cells from which these signalling molecules are synthesized and secreted. In accord with the genetic determination of their structural specificities through multiple series of single-locus alleles, Euplotes pheromones have been shown to possess three-dimensional architectures that closely mimic one another. Therefore, they can bind their cell-membrane receptors in a competitive fashion and form either homologous, or heterologous protein-protein complexes. The homologous complexes have been shown to be responsible for signalling a cell growth response. They are internalized through endocytotic vesicles and activate a protein-kinase dependent transduction pathway. In contrast, the heterologous complexes appear destined to remain blocked on the cell surface, where they are probably involved in promoting the cell mating response.