Quantitative mass spectrometry of posttranslational modifications: Keys to confidence

Posttranslational modifications (PTMs) of proteins represent an important level of cellular control. They participate in the efficient transduction of signals and form the basis of long-term cellular memory, allowing cells to adapt to a rapidly changing environment. More than 200 different PTMs have been described that affect many aspects of protein functions, and the importance of these modifications is evident from the number of diseases that arise from their deregulation. The proteome-wide analysis of certain PTMs, such as phosphorylation, acetylation, glycosylation, methylation, ubiquitination, and sumoylation, has become a standard procedure in many laboratories. We highlight and discuss some important aspects of systems-wide PTM analyses using mass spectrometry-based methods.