title: Arginine/lysine-rich nuclear localization signals mediate interactions between dimeric STATs and importin alpha 5.
abstract: Interferon stimulation results in tyrosine phosphorylation, dimerization, and nuclear import of STATs (signal transducers and activators of transcription). Proteins to be targeted into the nucleus usually contain nuclear localization signals (NLSs), which interact with importin alpha. Importin alpha binds to importin beta, which docks the protein complex to nuclear pores, and the complex translocates into the nucleus. Here we show that baculovirus-produced and -activated STAT1 homodimers and STAT1-STAT2 heterodimers directly interacted with importin alpha 5 (NPI-1). This interaction was very stable and was dependent on lysines 410 and 413 of STAT1. Only STAT dimers that had two intact NLS elements, one in each monomer, were able to bind to importin alpha 5. STAT-importin alpha 5 complexes apparently consisted of two STAT and two importin alpha molecules. STAT NLS-dependent colocalization of importin alpha 5 with STAT1 or STAT2 was seen in the nucleus of transfected cells. gamma-Activated sequence DNA elements efficiently inhibited STAT binding to importin alpha 5 suggesting that the DNA and importin alpha binding sites are close to each other in STAT dimers. Our results demonstrate that specific NLSs in STATs mediate direct interactions of STAT dimers with importin alpha, which activates the nuclear import process.
authors.affiliation: Laboratory of Infectious Disease Immunology, Department of Microbiology, National Public Health Institute, FIN-00300 Helsinki, Finland. riku.fagerlund@ktl.fi
authors.forename: Riku
authors.initials: R
authors.lastname: Fagerlund
authors.forename: Krister
authors.initials: K
authors.lastname: Mélen
authors.forename: Leena
authors.initials: L
authors.lastname: Kinnunen
authors.forename: Ilkka
authors.initials: I
authors.lastname: Julkunen
mesh_terms: D000818:Animals; D001120:Arginine / Q000378:metabolism*; D016367:Baculoviridae / Q000235:genetics; D001483:Base Sequence; D002460:Cell Line; D002850:Chromatography, Gel; D017931:DNA Primers; D004268:DNA-Binding Proteins / Q000378:metabolism*; D019281:Dimerization; D006801:Humans; D008239:Lysine / Q000378:metabolism*; D019913:Nuclear Localization Signals; D010766:Phosphorylation; D011485:Protein Binding; D050794:STAT1 Transcription Factor; D050795:STAT2 Transcription Factor; D018411:Spodoptera; D015534:Trans-Activators / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism; D028901:alpha Karyopherins / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D004268:DNA-Binding Proteins; C423142:KPNA1 protein, human; D019913:Nuclear Localization Signals; D050794:STAT1 Transcription Factor; C494079:STAT1 protein, human; D050795:STAT2 Transcription Factor; C494084:STAT2 protein, human; D015534:Trans-Activators; D028901:alpha Karyopherins; D014443:Tyrosine; D001120:Arginine; D008239:Lysine