Selenocysteine: Wherefore Art Thou?

Selenocysteine is a naturally occurring proteogenic amino acid that is encoded in the genomic sequence of relatively abundant proteins in many of the model species commonly used for biomedical research. On the basis of an analysis of publicly available proteomics information, it was discovered that peptides containing selenocysteine were not being identified in tandem mass spectrometry proteomics data. Once the chemical basis for this exclusion was understood, a simple alteration in search parameters led to the confident identification of selenocysteine containing peptides from existing proteomics data, with no change in experimental protocols required.