Phosphatase Protection Assay: 14-3-3 Binding Protects the Phosphate group of RSG from λ Protein Phosphatase

Abstract

14-3-3 proteins regulate diverse cellular processes in eukaryotes by binding to phospho-serine or threonine of target proteins. One of the physiological functions of 14-3-3 is to bind and protect phosphate groups of the target proteins against phosphatases. REPRESSION OF SHOOT GROWTH (RSG) is a tobacco (Nicotiana tabacum) transcription factor that is involved in the feedback regulation of biosynthetic genes of plant hormone gibberellin. 14-3-3 binds to phospho-Ser-114 in RSG. Ca2+-dependent protein kinase NtCDPK1 was identified as a kinase that phosphorylates Ser-114 of RSG. Our recent study revealed that NtCDPK1 forms a heterotrimer with RSG and 14-3-3 and that 14-3-3 was transferred from NtCDPK1 to phosphorylated RSG (Ito et al., 2014). In the course of the study, we found that 14-3-3 protects the phosphate group of RSG from λ protein phosphatase in vitro. Here, we describe a protocol for in vitro phosphatase protection assay. To detect the phosphorylation state of proteins, we used Phos-tag SDS-PAGE and autoradiography. This protocol can be adapted for the examinations whether the phosphoprotein-binding proteins protect the phosphate group of target proteins from phosphatases although protein kinases may be required for the phosphorylation of target proteins.