THE 1.0Å RESOLUTION REFINED STRUCTURE OF MATING PHEROMONE Er-1 FROM EUPLOTES RAIKOVI. Daniel H. Anderson, Manfred S. Weiss, David Eisenberg, Molecular Biology Institute and Department of Chemistry and Biochemistry, University of California, Los Angeles. Los Angeles, California, 90095-1570, USA

Er-1 is one of a family of protein pheromones that modulate self-non-self recognition for cell aggregation and mating in the 1-cell ciliated protozoan Euplotes raikovi. We summarize the course and results of the refinement of the Er-1 model against atomic resolution X-ray data. The model was refined with anisotropic thermal parameters, to a R-factor of 12.92%, and includes 4 disordered sidechains, 22 water molecules, a disordered ethanol, and "riding" hydrogen atoms. There is almost no net libration of the molecule, but the sidechains vibrate relative to the backbone. Post-refinement analysis of the model revealed that this dense crystal is perfused by hydrogen bonding networks of solvent and protein atoms. The ends of helices are capped by hydrogen bonding to solvent and symmetry related molecules. Water molecules mediate almost all of the interhelical hydrogen bonding, and many of the lattice interactions. Indirect evidence is presented that motions at the sites of discrete disorder may be correlated, and that protonation of acidic sidechains may switch the conformations.