title: A Rac GTPase-activating protein, MgcRacGAP, is a nuclear localizing signal-containing nuclear chaperone in the activation of STAT transcription factors.
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.
authors.affiliation: Division of Cellular Therapy, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo 108-8639, Japan. kkawa@ims.u-tokyo.ac.jp
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Kawashima
authors.forename: Ying Chun
authors.initials: YC
authors.lastname: Bao
authors.forename: Yukinori
authors.initials: Y
authors.lastname: Minoshima
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Nomura
authors.forename: Tomonori
authors.initials: T
authors.lastname: Hatori
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Hori
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Fukagawa
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Fukada
authors.forename: Noriko
authors.initials: N
authors.lastname: Takahashi
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Nosaka
authors.forename: Makoto
authors.initials: M
authors.lastname: Inoue
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Sato
authors.forename: Mutsuko
authors.initials: M
authors.lastname: Kukimoto-Niino
authors.forename: Mikako
authors.initials: M
authors.lastname: Shirouzu
authors.forename: Shigeyuki
authors.initials: S
authors.lastname: Yokoyama
authors.forename: Toshio
authors.initials: T
authors.lastname: Kitamura
mesh_terms: 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*
chemical_list: 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