Nuclear localization signal and protein context both mediate importin α specificity of nuclear import substrates


  • B. Friedrich
  • C. Quensel
  • T. Sommer
  • E. Hartmann
  • M. Koehler


  • Molecular and Cellular Biology


  • Mol Cell Biol 26 (23): 8697-8709


  • The 'classical' nuclear protein import pathway depends on importin alpha and importin beta. Importin alpha binds nuclear localization signal (NLS)-bearing proteins and functions as an adapter to access the importin beta dependent import pathway. In humans only one importin beta is known to interact with importin alpha, while six alpha importins have been described. Various experimental approaches provided evidence that several substrates are transported specifically by particular alpha importins. Whether or not the NLS is sufficient to mediate importin alpha specificity is unclear. To address this question, we exchanged the NLS of two well-characterized import substrates, the seven-bladed propeller protein RCC1, preferentially transported into the nucleus by importin alpha3, and the less specifically imported substrate nucleoplasmin. In vitro binding studies and nuclear import assays revealed that both NLS and protein context contribute to specificity of importin alpha binding and transport.