Determination of protein phosphorylation by polyacrylamide gel electrophoresis

Phosphorylation is the most important modification for protein regulation; it controls many signal transduction pathways in all organisms. While several tools to detect phosphorylated proteins have been developed to study a variety of basic cellular processes involving protein phosphorylation, these methods have several limitations. Many proteins exhibit a phosphorylation-dependent electrophoretic mobility shift (PDEMS) in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and the molecular mechanism responsible for this phenomenon has been elucidated recently. The method for detecting phosphorylated proteins can be simplified by the application of the PDEMS. Herein, we present a novel simple method to detect protein phosphorylation, which is based on the construction of a variant protein displaying a PDEMS. The PDEMS of proteins is caused by the distribution of negatively charged amino acids around the phosphorylation site, i.e. an electrophoretic mobility shift (EMS)-related motif (ΘX_1-3ΘX_1-3Θ, where Θ corresponds to an acidic or phosphorylated amino acid and X represents any amino acid). The EMS-related motif can be constructed by the introduction of a negative charge by phosphorylation; it results in the decreased binding of SDS to the proteins, consequently inducing the retardation of the mobility of the protein during SDS-PAGE. Based on these molecular analyses of the PDEMS, a protein with the EMSrelated motif is designed and used to determine the in vivo phosphorylation state of the protein. This method may be used as a general strategy to easily measure the ratio of protein phosphorylation in cells.