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Molecular and Cellular Biology, April 2009, p. 1796-1813, Vol. 29, No. 7
0270-7306/09/$08.00+0 doi:10.1128/MCB.01423-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
A Rac GTPase-Activating Protein, MgcRacGAP, Is a Nuclear Localizing Signal-Containing Nuclear Chaperone in the Activation of STAT Transcription Factors
,
Toshiyuki Kawashima,1,
*
Ying Chun Bao,1,
Yukinori Minoshima,1
Yasushi Nomura,1
Tomonori Hatori,1
Tetsuya Hori,2
Tatsuo Fukagawa,2
Toshiyuki Fukada,3,4
Noriko Takahashi,1
Tetsuya Nosaka,1,5
Makoto Inoue,6
Tomohiro Sato,6,7
Mutsuko Kukimoto-Niino,6
Mikako Shirouzu,6
Shigeyuki Yokoyama,6,7 and
Toshio Kitamura1*
Division of Cellular Therapy, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo 108-8639, Japan,1 Department of Molecular Genetics, National Institute of Genetics and the Graduate University for Advanced Studies, Mishima, Shizuoka 411-8540, Japan,2 Laboratory for Cytokine Signaling, RIKEN Research Center for Allergy and Immunology,3 Protein Research Group, Genomic Sciences Center, Yokohama Institute, RIKEN, 1-7-22 Suehiro-cho, Tsurumi, Yokohama 230-0045, Japan,6 Department of Allergy and Immunology, Osaka University Graduate School of Medicine, Osaka University, 2-2 Yamada-oka, Suita, Osaka 565-0871, Japan,4 Department of Microbiology, Mie University Graduate School of Medicine, 2-174 Edobashi, Tsu 514-8507, Japan,5 Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan7
Received 10 September 2008/ Returned for modification 1 November 2008/ Accepted 12 January 2009
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 
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 βb) are hardly tyrosine phosphorylated after cytokine stimulation. Intriguingly, mutants harboring small deletions in the C'-adjacent region (βb-βc loop region) of the strand βb 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.
* Corresponding author. Mailing address: Division of Cellular Therapy, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo 108-8639, Japan. Phone: 81-3-5449-5758. Fax: 81-3-5449-5453. E-mail for Toshiyuki Kawashima: kkawa{at}ims.u-tokyo.ac.jp. E-mail for Toshio Kitamura: kitamura{at}ims.u-tokyo.ac.jp
Published ahead of print on 21 January 2009.
Supplemental material for this article may be found at http://mcb.asm.org/.
T.K. and Y.C.B. contributed equally to this work.
Molecular and Cellular Biology, April 2009, p. 1796-1813, Vol. 29, No. 7
0270-7306/09/$08.00+0 doi:10.1128/MCB.01423-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
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