A Minimalist's Approach to the Phase Problem – Phasing Selenomethionyl Protein Structures Using Cu Kα Data

The feasibility of phasing protein structures through the use of the isomorphous and anomalous signal of selenomethionyl (Se-Met) derivative and diffraction data collected with a standard laboratory Cu Kα X-ray source has been investigated. Interpretable electron-density maps were obtained for the core domain of avian sarcoma virus integrase, a typical medium-sized protein having four Met residues in a sequence of 156 amino acids. The r.m.s. difference between 3.1 Å experimental phases obtained from Se-Met Cu Kα data and the final phases calculated from the refined model is 55°. A procedure combining single isomorphous replacement/single anomalous scattering phasing and solvent flattening for data based on a single Se-Met derivative and Cu Kα radiation has been tested on this and another protein. The results are encouraging enough to indicate that such procedures might be recommended when a synchrotron source is not readily available.