A challenging case for protein crystal structure determination: the mating pheromone Er-1 from Euplotes raikovi.

Four different phasing methods have been applied to the determination of the crystal structure of the 40 amino-acid mating pheromone of the unicellular ciliated protozoan Euplotes raikovi. The difficulties, failures and successes in attempts to solve the structure by: (1) molecular replacement, (2) direct phasing using the 'Shake and Bake' algorithm, (3) isomorphous replacement, and (4) multiple-wavelength anomalous dispersion are described. The structure was first solved by molecular replacement, and then was the first successful structure determination by 'Shake and Bake' without the direct involvement of its authors. A description of the current status of the high-resolution refinement of the structure is also given. The model is refined against 1 A resolution data to an R factor of 12.9%, and includes H atoms and discretely disordered side chains.