Arginine/Lysine-rich Nuclear Localization Signals Mediate Interactions between Dimeric STATs and Importin α5

Riku Fagerlund,Krister Melén,Leena Kinnunen,Ilkka Julkunen

doi:10.1074/jbc.m202943200

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.

Highlights

Interferon stimulation results in tyrosine phosphorylation, dimerization, and nuclear import of STATs
Binding of STAT1 Homodimers or STAT1-STAT2 Heterodimers to Importin ␣5 Is Regulated by a Lysine-rich nuclear localization signals (NLSs) of STAT1—IFN-induced nuclear import of STAT1 has been suggested to be mediated by importin ␣5 [20]
We recently proposed that STAT proteins have a well conserved arginine/ lysine-rich NLS in their DNA binding domain that regulates their nuclear import [9]

Summary

Introduction

Interferon stimulation results in tyrosine phosphorylation, dimerization, and nuclear import of STATs (signal transducers and activators of transcription). Our results demonstrate that specific NLSs in STATs mediate direct interactions of STAT dimers with importin ␣, which activates the nuclear import process. In the nucleus STATs interact with IRF-9/p48 protein to form ISGF3 complexes, which bind to well conserved interferon-stimulated response elements in the promoter regions of IFN-␣/␤-responsive genes and activate transcription [5,6,7,8]. The N-terminal importin ␤ binding (IBB) domain of importin ␣ interacts with importin ␤ [15], which mediates the docking of NLS-containing cargo-importin ␣/␤ complex to the cytoplasmic side of the nuclear pore, and the complex translocates into the nucleus [16, 17]. We demonstrate by confocal microscopy that wild type STATs colocalize with importin ␣, whereas NLS-mutated STATs do not

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Conclusion