title: Ser9 phosphorylation causes cytoplasmic detention of I2PP2A/SET in Alzheimer disease.
abstract: The nuclear protein I2(PP2A)/SET, an endogenous inhibitor of protein phosphatase-2A (PP2A), is increased and translocated to the cytoplasm in the neurons of Alzheimer's disease (AD) brains, and PP2A activity in cytoplasm is compromised. However, it is not fully understood how SET is retained in the cytoplasm. By generating a phosphorylation site-specific antibody, we found in the present study that SET is phosphorylated at Ser9, by which it is accumulated in the cytoplasm of the AD brains. Further studies demonstrate that both the phosphor-mimic and casein kinase (CK)II-mediated phosphorylation at Ser9 interferes with the formation of the SET/importin-α/importin-β complex, and thus inhibits SET nuclear import and induces the cytoplasmic detention of SET. Interestingly, Ser9 is nested in the center of the sequence (6)AKVSKK(11) of SET, which is consistent with a classical nuclear localization signal (NLS). To test whether (6)AKVSKK(11) is a new NLS of SET, we mutated SET lysine 7, lysine 10, and lysine 11 to alanine acid (K7A, K10A, K11A) respectively, and expressed these mutants in HEK293/tau cells. We found that expression of SET (K11A) led to a nuclear import defect of SET, and application of a synthesized peptide Tat-AAKVSKKE that can competitively bind to importin α/β resulted in cytoplasmic detention of SET. Finally, phosphorylation of SET aggravates PP2A inhibition and leads to tau hyperphosphorylation. In conclusion, the current study has identified a novel mechanism that causes cytoplasmic detention of SET with a new NLS-dependent CKII-associated phosphorylation of Ser9, suggesting that inhibition of CKII arrests cytoplasmic accumulation of SET and thus preserves PP2A activity in AD brains.
authors: Department of Pathophysiology, Key Laboratory of Neurological Disease of National Education Ministry, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China.
authors: Guang
authors: G
authors: Yu
authors: Tonghai
authors: T
authors: Yan
authors: Ye
authors: Y
authors: Feng
authors: Xinghua
authors: X
authors: Liu
authors: Yiyuan
authors: Y
authors: Xia
authors: Hongbin
authors: H
authors: Luo
authors: Jian-Zhi
authors: JZ
authors: Wang
authors: Xiaochuan
authors: X
authors: Wang
mesh_terms: D000544:Alzheimer Disease / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D003593:Cytoplasm / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins; D005260:Female; D057809:HEK293 Cells; D056488:Histone Chaperones / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010766:Phosphorylation / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012694:Serine / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
chemical_list: D004268:DNA-Binding Proteins; D056488:Histone Chaperones; C513473:SET protein, human; D014157:Transcription Factors; D012694:Serine