부산대학교 도서관

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 α. Importin α binds to importin β, 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 α5 (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 α5. STAT-importin α5 complexes apparently consisted of two STAT and two importin α molecules. STAT NLS-dependent colocalization of importin α5 with STAT1 or STAT2 was seen in the nucleus of transfected cells. γ-Activated sequence DNA elements efficiently inhibited STAT binding to importin α5 suggesting that the DNA and importin α 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 α, which activates the nuclear import process.