S-SAD, Se-SAD and S/Se-SIRAS using Cu Kα radiation: why wait for synchrotron time?

The structure of Escherichia coli argininosuccinate synthetase (EAS) has been determined using S-SAD, Se-SAD and S/Se-­SIRAS data measured with Cu Kα radiation. EAS contains 16 methionines and three cysteines in 455 amino acids. At a wavelength of 1.54 Å (Cu Kα), the native (S-Met) and derivative (Se-Met) proteins yield anomalous signals of approximately 0.86 and 1.6%, respectively. Highly redundant data were measured to 2.0 Å from native and derivative EAS crystals. All three structure determinations were carried out in a highly automated manner using SnB and SOLVE/RESOLVE. Despite the minute Bijvoet differences at 1.54 Å, the signal was sufficient to determine the heavy-atom substructure and produce high-quality electron-density maps in all three cases. These maps were readily interpretable by the RESOLVE automated building algorithm, which modeled greater than 75% of all three structures. The success of these methods has profound implications for crystallographers experiencing difficulty with heavy-atom incorporation or with limited access to a synchrotron source.