1. A Rac GTPase-activating protein, MgcRacGAP, is a nuclear localizing signal-containing nuclear chaperone in the activation of STAT transcription factors.
- Author
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Kawashima T, Bao YC, Minoshima Y, Nomura Y, Hatori T, Hori T, Fukagawa T, Fukada T, Takahashi N, Nosaka T, Inoue M, Sato T, Kukimoto-Niino M, Shirouzu M, Yokoyama S, and Kitamura T
- Subjects
- Animals, Cell Line, Cell Nucleus drug effects, Cell Proliferation drug effects, Chickens, Guanosine Triphosphate metabolism, Humans, Molecular Chaperones chemistry, Mutant Proteins metabolism, Phenotype, Phosphoproteins metabolism, Promoter Regions, Genetic genetics, Protein Binding drug effects, Protein Structure, Secondary, Protein Structure, Tertiary, Protein Transport drug effects, STAT Transcription Factors chemistry, Saccharomyces cerevisiae, Sequence Deletion, Tetracycline pharmacology, Transcription, Genetic drug effects, Transcriptional Activation drug effects, Transcriptional Activation genetics, alpha Karyopherins metabolism, rac GTP-Binding Proteins chemistry, Cell Nucleus metabolism, GTPase-Activating Proteins chemistry, GTPase-Activating Proteins metabolism, Molecular Chaperones metabolism, Nuclear Localization Signals metabolism, STAT Transcription Factors metabolism, rac GTP-Binding Proteins metabolism
- Abstract
In addition to their pleiotropic functions under physiological conditions, transcription factors STAT3 and STAT5 also have oncogenic activities, but how activated STATs are transported to the nucleus has not been fully understood. Here we show that an MgcRacGAP mutant lacking its nuclear localizing signal (NLS) blocks nuclear translocation of p-STATs both in vitro and in vivo. Unlike wild-type MgcRacGAP, this mutant did not promote complex formation of phosphorylated STATs (p-STATs) with importin alpha in the presence of GTP-bound Rac1, suggesting that MgcRacGAP functions as an NLS-containing nuclear chaperone. We also demonstrate that mutants of STATs lacking the MgcRacGAP binding site (the strand betab) are hardly tyrosine phosphorylated after cytokine stimulation. Intriguingly, mutants harboring small deletions in the C'-adjacent region (betab-betac loop region) of the strand betab became constitutively active with the enhanced binding to MgcRacGAP. The molecular basis of this phenomenon will be discussed, based on the computer-assisted tertiary structure models of STAT3. Thus, MgcRacGAP functions as both a critical mediator of STAT's tyrosine phosphorylation and an NLS-containing nuclear chaperone of p-STATs.
- Published
- 2009
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