The protein pheromone family of the ciliate Euplotes petzi, the earliest branching species in the Euplotes phylogentic tree

Self/non-self recognition in ciliates relies on signaling proteins (pheromones) synthesized in association with a genetic mating-type mechanism, that regulates the cell switching between the growth (mitotic) and sexual (mating) stages of the life cycle. In Euplotes species, these pheromones are freely released into the medium from where they can be purified in relative abundance. The knowledge of their molecular structures has so far been limited to four species, namely E. raikovi, E. octocarinatus, E. nobilii and E. crassus, that occupy varied positions in the Euplotes phylogenetic tree. Most research interest has now been focused on the pheromone family of E. petzi because of a major distinctive, phylogenetic trait of this species. Together with E. sinicus, E. petzi forms the earliest branching clade in Euplotes evolution. Four structurally distinct E. petzi pheromones have so far been structurally characterized together with their coding genes. With respect to the other known Euplotes pheromones, they show smaller dimensions (only 32 amino acids vs. up to 108 in E. octocarinatus), a higher density of disulfide bonds (four), and a folding in which molecular districts with no regular structures equal in extension districts with regular structures represented by one extended and two single-turn alpha-helices. Considering that in the other Euplotes species pheromones have structures dominated by a bundle of three regular alpha-helices, the minimal dimensions and the relatively simple architecture of E. petzi pheromones thus indicates that the structural evolution of Euplotes pheromones involves a progressive increase of architectural complexity. And the finding that the E. petzi pheromone genes are practically half in dimensions the pheromone genes of the other Euplotes species reinforced this indication.