A Rac GTPase-activating protein, MgcRacGAP, is a nuclear localizing signal-containing nuclear chaperone in the activation of STAT transcription factors.
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.
Kawashima
Toshiyuki
T
Division of Cellular Therapy, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo 108-8639, Japan. kkawa@ims.u-tokyo.ac.jp
Bao
Ying Chun
YC
Minoshima
Yukinori
Y
Nomura
Yasushi
Y
Hatori
Tomonori
T
Hori
Tetsuya
T
Fukagawa
Tatsuo
T
Fukada
Toshiyuki
T
Takahashi
Noriko
N
Nosaka
Tetsuya
T
Inoue
Makoto
M
Sato
Tomohiro
T
Kukimoto-Niino
Mutsuko
M
Shirouzu
Mikako
M
Yokoyama
Shigeyuki
S
Kitamura
Toshio
T
D000818:Animals; D002460:Cell Line; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism*; D049109:Cell Proliferation / Q000187:drug effects; D002645:Chickens; D020690:GTPase-Activating Proteins / Q000737:chemistry* / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D018832:Molecular Chaperones / Q000737:chemistry / Q000378:metabolism*; D050505:Mutant Proteins / Q000378:metabolism; D019913:Nuclear Localization Signals / Q000378:metabolism*; D010641:Phenotype; D010750:Phosphoproteins / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding / Q000187:drug effects; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000187:drug effects; D050791:STAT Transcription Factors / Q000737:chemistry / Q000378:metabolism*; D012441:Saccharomyces cerevisiae; D017384:Sequence Deletion; D013752:Tetracycline / Q000494:pharmacology; D014158:Transcription, Genetic / Q000187:drug effects; D015533:Transcriptional Activation / Q000187:drug effects / Q000235:genetics; D028901:alpha Karyopherins / Q000378:metabolism; D020744:rac GTP-Binding Proteins / Q000737:chemistry / Q000378:metabolism*
D020690:GTPase-Activating Proteins; D018832:Molecular Chaperones; D050505:Mutant Proteins; D019913:Nuclear Localization Signals; D010750:Phosphoproteins; D050791:STAT Transcription Factors; D028901:alpha Karyopherins; C115995:mgcRacGAP; D006160:Guanosine Triphosphate; D020744:rac GTP-Binding Proteins; D013752:Tetracycline