Methodologies for characterizing phosphoproteins by mass spectrometry

Posttranslational regulation of proteins via protein phosphorylation is one of the major means of protein regulation. Phosphorylation is a very rapid and reversible method of changing the function of proteins. Detection of phosphorylated proteins and the identification of phosphorylation sites are necessary to molecularly link specific phosphorylated events with change in phosphoprotein function. Mass Spectrometry (MS) has become the methodology of choice for phosphosite identification. Here we review current approaches including sample separation and enrichment techniques (SDS-PAGE, immunoprecipitation, metal-assisted enrichment, strong cation exchange, dendrimer capture), quantitative MS analysis methods (SILAC, iTRAQ, AQUA), and the application of recently developed methods including electron transfer dissociation ionization and "top-down" proteomics to phosphoprotein analysis.