Quantitative Analysis of Protein Phosphorylation Using Two-Dimensional Difference Gel Electrophoresis

Posttranslational modifications of proteins, especially phosphorylation and dephosphorylation, play an important role in signal transduction and cellular regulation in plants. Both 2-DE gel-based and non-gel-based proteomic technologies can monitor the changes in phosphorylation state of proteins. In this chapter, we describe two protocols for discovery and validation of differential protein phosphorylation using affinity enrichment of phosphoproteins by immobilized metal affinity chromatography (IMAC) or protein immunoprecipitation (IP) followed by two-dimensional difference gel electrophoresis (2-D DIGE). We name these methods IMAC-DIGE and IP-DIGE. For IMAC-DIGE, phosphoproteins are enriched from tissue extract using GaCl₃ -based IMAC and then analyzed by 2-D DIGE, which reveals changes of protein phosphorylation as protein spot shifts. IMAC enrichment improves detection of low-abundance regulatory phosphoproteins. For IP-DIGE, proteins of interest can be immunopurified and then analyzed by 2-D DIGE to confirm changes of posttranslational modifications that alter the charge or size of the proteins.