Crystal structure of the pheromone Er-13 from the ciliate Euplotes raikovi, with implications for a protein–protein association model in pheromone/receptor interactions

•The crystal structure of E. raikovi pheromone Er-13 was determined at 1.36 Å of resolution.•Er-13 molecules associate cooperatively into linear chains taking opposite orientations.•Helix-3 is the key functional domain of the three-helical pheromone structure.•The Er-13 crystal structure compared with the known Er-1 structure provides insights to seek into pheromone/receptor interactions.