Phosphoproteomics--finally fulfilling the promise?

Networks of protein-protein and protein-metabolite interactions are commonly found in biological systems where signals must be passed from one location or component within a cell to another, such as from a receptor on the plasma membrane to a transcription factor in the nucleus. Regulation of such networks, or signal transduction pathways, is often achieved by transient, reversible modification of the components involved. Several types of post-translational modifications of proteins are employed in signal transduction including ubiquitylation of lysines and palmitoylation of cysteines, but by far the best appreciated and apparently the most important involves phosphorylation of serine, threonine and tyrosine residues. Whilst protein phosphorylation has long been recognized as functionally important, low stoichiometry has ultimately impeded global analyses (phosphoproteomics). Recent developments in the application of metal oxide chromatography and advanced mass spectrometric techniques have enabled phosphoproteomics to move beyond mere proof-of-principle experiments, to the stage where it can successfully address complex biological questions. Here we cover the development of phosphopeptide/protein analysis by mass spectrometry and the various techniques used to enrich phosphopeptides/proteins. We also speculate on the future of phosphoproteomic research, now that the goal of generating global phosphoproteomic datasets has been realized.