Center for Pediatric Biomedical Research and Departments of Environmental Medicine and Biochemistry and Biophysics, University of Rochester Medical School, Rochester, NY 14642, USA-
Abstract: Saccharomyces cerevisiae is an ideal host from which to obtain high levels of posttranslationally modified eukaryotic proteins for x-ray crystallography- However, extensive replacement of methionine by selenomethionine for anomalous dispersion phasing has proven intractable in yeast- We report a general method to incorporate selenomethionine into proteins expressed in yeast based on manipulation of the appropriate metabolic pathways- sam1--- sam2--- mutants, in which the conversion of methionine to S-adenosylmethionine is blocked, exhibit reduced selenomethionine toxicity compared with wild-type yeast, increased production of protein during growth in selenomethionine, and efficient replacement of methionine by selenomethionine, based on quantitative mass spectrometry and x-ray crystallography- The structure of yeast tryptophanyl-tRNA synthetase was solved to 1-8 A by using multiwavelength anomalous dispersion phasing with protein that was expressed and purified from the sam1--- sam2--- strain grown in selenomethionine- Six of eight selenium residues were located in the structure-