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Phosphorylation is a posttranslational modification of proteins that regulates many cellular processes, such as communication between cells, cell proliferation, cell movements, and gene expression. Therefore, many studies have been conducted to determine the significance and function of phosphorylation. These studies involve the identification of phosphorylation site(s), kinases and phosphatases, and regulatory mechanisms. Recently, phosphorylation sites were identified using mass spectrometry and detected by immunoblotting with phosphorylation site-specific antibodies. However, the in vivo phosphorylation profile of the target protein is not easy to grasp, and the quantification of site-specific phosphorylation is challenging if the protein is phosphorylated at multiple sites. Phos-tag is a phospho-affinity SDS-PAGE approach in which phosphorylated proteins are separated depending on the number and sites of phosphorylation during electrophoresis, which overcomes the aforementioned problems. We applied this technique to perform an in vivo analysis of the phosphorylation of many proteins. In this article, we show our results for the phosphorylation of tau protein, p35 Cdk5 activator and GSK3β to reveal the utility and power of this technique in protein phosphorylation analyses in vivo. SIGNIFICANT: We show the in vivo phosphorylation of tau and two tau kinases analysed by using Phos-tag SDS-PAGE. Tau represents about 12 different phosphoisotypes when expressed in cultured cells. Tau is differently phosphorylated in patients with different tauopathy. Phosphorylation of p35 Cdk5 activator, which suppress the abnormal activation of Cdk5 by cleavage with calpain, is regulated developmentally. The Ser9 phosphorylation is not a proper marker of the GSK3β activity in vivo.

Glycogen synthase kinase 3β (GSK3β) is a multifunctional protein kinase involved in many cellular activities including development, differentiation and diseases. GSK3β is thought to be constitutively activated by autophosphorylation at Tyr216 and inactivated by phosphorylation at Ser9. The GSK3β activity has previously been evaluated by inhibitory Ser9 phosphorylation, but it does not necessarily indicate the kinase activity itself. Here, we applied the Phos-tag SDS-PAGE technique to the analysis of GSK3β phosphoisotypes in cells and brains. There were three phosphoisotypes of GSK3β; double phosphorylation at Ser9 and Tyr216, single phosphorylation at Tyr216 and the nonphosphorylated isotype. Active GSK3β with phosphorylation at Tyr216 represented half or more of the total GSK3β in cultured cells. Although levels of phospho-Ser9 were increased by insulin treatment, Ser9 phosphorylation occurred only in a minor fraction of GSK3β. In mouse brains, GSK3β was principally in the active form with little Ser9 phosphorylation, and the phosphoisotypes of GSK3β changed depending on the regions of the brain, age, sex and disease conditions. These results indicate that the Phos-tag SDS-PAGE method provides a simple and appropriate measurement of active GSK3β in vivo, and the activity is regulated by the mechanism other than phosphorylation on Ser9.