The NMR solution structure of the pheromone Er‐2 from the ciliated protozoan Euplotes raikovi

The NMR structure of the pheromone Er‐2 from the ciliated protozoan Euplotes raikovi has been determined in aqueous solution. The structure of this 40‐residue protein was calculated with the distance geometry program DIANA from 621 distance constraints and 89 dihedral angle constraints; the program OPAL was employed for the energy minimization. For a group of 20 conformers used to characterize the solution structure, the average pairwise RMS deviation from the mean structure calculated for the backbone heavy atoms N, C^α, and C′ of residues 3–37 was 0.31 Å. The molecular architecture is dominated by an up‐down‐up bundle of 3 short helices of residues 5–11, 14–20, and 23–33, which is similar to the structures of the homologous pheromones Er‐1 and Er‐10. Novel structural features include a well‐defined N‐cap on the first helix, a 1‐residue deletion in the second helix resulting in the formation of a 3₁₀‐helix rather than an α‐helix as found in Er‐1 and Er‐10, and the simultaneous presence of 2 different conformations for the C‐terminal tetrapeptide segment, i.e., a major conformation with the Leu 39‐Pro 40 peptide bond in the trans form and a minor conformation with this peptide bond in the cis form.