title: Phosphorylation by casein kinase 1 regulates tonicity-induced osmotic response element-binding protein/tonicity enhancer-binding protein nucleocytoplasmic trafficking.
abstract: The osmotic response element-binding protein (OREBP), also known as tonicity enhancer-binding protein (TonEBP) or NFAT5, is the only known osmo-sensitive transcription factor that mediates cellular adaptations to extracellular hypertonic stress. Although it is well documented that the subcellular localization and transactivation activity of OREBP/TonEBP are tightly regulated by extracellular tonicity, the molecular mechanisms involved remain elusive. Here we show that nucleocytoplasmic trafficking of OREBP/TonEBP is regulated by the dual phosphorylation of Ser-155 and Ser-158. Alanine scanning mutagenesis revealed that Ser-155 is an essential residue that regulates OREBP/TonEBP nucleocytoplasmic trafficking. Tandem mass spectrometry revealed that Ser-155 and Ser-158 of OREBP/TonEBP are both phosphorylated in living cells under hypotonic conditions. In vitro phosphorylation assays further suggest that phosphorylation of the two serine residues proceeds in a hierarchical manner with phosphorylation of Ser-155 priming the phosphorylation of Ser-158 and that these phosphorylations are essential for nucleocytoplasmic trafficking of the transcription factor. Finally, we have shown that the pharmacological inhibition of casein kinase 1 (CK1) abolishes the phosphorylation of Ser-158 and impedes OREBP/TonEBP nuclear export and that recombinant CK1 phosphorylates Ser-158. Knockdown of CK1alpha1L, a novel isoform of CK1, inhibits hypotonicity-induced OREBP/TonEBP nuclear export. Together these data highlight the importance of Ser-155 and Ser-158 in the nucleocytoplasmic trafficking of OREBP/TonEBP and indicate that CK1 plays a major role in regulating this process.
authors.affiliation: The State Key Laboratory in Oncology in South China, Chinese University of Hong Kong, Hong Kong, China.
authors.forename: SongXiao
authors.initials: S
authors.lastname: Xu
authors.forename: Catherine C L
authors.initials: CC
authors.lastname: Wong
authors.forename: Edith H Y
authors.initials: EH
authors.lastname: Tong
authors.forename: Stephen S M
authors.initials: SS
authors.lastname: Chung
authors.forename: John R
authors.initials: JR
authors.lastname: Yates
authors.forename: YiBing
authors.initials: Y
authors.lastname: Yin
authors.forename: Ben C B
authors.initials: BC
authors.lastname: Ko
mesh_terms: D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D047389:Casein Kinase I / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D009995:Osmosis; D010766:Phosphorylation; D011485:Protein Binding; D020033:Protein Isoforms; D017386:Sequence Homology, Amino Acid; D014157:Transcription Factors / Q000378:metabolism*
chemical_list: C118125:NFAT5 protein, human; D020033:Protein Isoforms; D014157:Transcription Factors; D047389:Casein Kinase I