Generating isomorphous heavy-atom derivatives by a quick-soak method. Part II: phasing of new structures

A quick-soak method has been applied to generate de novo heavy-atom phasing to solve two new protein structures, a type II transforming growth factor β receptor (TBRII) and a natural killer cell receptor–ligand complex, NKG2D–ULBP3. In the case of TBRII, a crystal derivatized for only 10 min in saturated HgCl₂ provided adequate phasing for structure determination. Comparison between HgCl₂ derivatives generated by 10 min soaking and by 12 h soaking revealed similar phasing statistics. The shorter soak, however, resulted in a derivative more isomorphous to the native than the longer soak as judged by changes in the unit-cell parameter a upon derivatization as well as by the quality of a combined SIRAS electron-density map. In the case of the NKG2D–ULBP3 structure, all overnight soaks in heavy-atom solutions resulted in crystal lattice disorder and only the quick soaks preserved diffraction. Despite fragile lattice packing, the quick-soaked K₂PtCl₄ derivative was isomorphous with the native crystal and the electron-density map calculated from combined SIR and MAD phases is better than that calculated from MAD phases alone. Combined with mass-spectrometry-assisted solution heavy-atom derivative screening and the use of synchrotron radiation, the quick-soak derivatization has the potential to transform the time-consuming conventional heavy-atom search into a real-time `on-the-fly' derivatization process that will benefit high-throughput structural genomics.