Blocking S-Adenosylmethionine Synthesis in Yeast Allows Selenomethionine Incorporation And Multiwavelength Anomalous Dispersion Phasing

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{sup -} sam2{sup -} 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 {angstrom} by using multiwavelength anomalous dispersion phasing with protein that was expressed and purified from the sam1{sup -} sam2{sup -} strain grown in selenomethionine. Six of eight selenium residues were located in the structure.