Tracing a protein at 5.5 Å resolution

In a paper published in Nature 26 March 2009 (p. 475), Pomeranz Krummel et al. describe the structure of human spliceosomal U1 snRNP at 5.5 Å resolution. what struck me is how they traced the structure of one of the component proteins: via mutating individually all methionine residues of the Se-Met versions of the protein, crystallising and collecting data of all variant complexes and locating the said methionines in anomalous difference maps. A real tour-de-force!