STRUCTURE OF DIMERIC AND MONOMERIC ERABUTOXIN A REFINED AT 1.5 ANGSTROM RESOLUTION

Erabutoxin a has been crystallized in its monomeric and dimeric forms. The structures were refined at 1.50 and 1.49 Angstrom resolution, res pectively, using synchrotron radiation data. The crystals belong to sp ace group P2(1)2(1)2(1), with cell dimensions a = 49.84, b = 46.62, c = 21.22 Angstrom for the monomer and a = 55.32, b = 53.54, c = 40.76 A ngstrom for the dimer. Using starting models from earlier structure de terminations, the monomeric structure refined to an R value of 16.7% ( 8004 unique reflections, 17.0-1.50 Angstrom resolution range), while t he dimeric structure has been solved by the molecular-replacement meth od with a final R value of 16.9% (19 444 unique reflections, 17.4-1.49 Angstrom resolution range). The high-resolution electron-density maps clearly revealed significant discrete disorder in the proteins and al lowed an accurate determination of the solvent structure. For the mono mer, the side chains of six residues were modelled with alternate conf ormers and 106 sites fur water molecules and one site for a sulfate io n were included in the final model, whereas for the dimer, 206 sites f or water molecules were included and both C-terminal residues together with the side chains of II residues adopted alternative conformations . A comparison was made with earlier structure determinations. The fea tures of the solvent structure of the erabutoxin molecules are discuss ed in detail.