Nuclear Localization Signal Receptor Importin α Associates with the Cytoskeleton

Abstract
Importin α is the nuclear localization signal (NLS) receptor that is involved in the nuclear import of proteins containing basic NLSs. Using importin α as a tool, we were interested in determining whether the cytoskeleton could function in the transport of NLS-containing proteins from the cytoplasm to the nucleus. Double-labeling immunofluorescence studies showed that most of the cytoplasmic importin α coaligned with microtubules and microfilaments in tobacco protoplasts. Treatment of tobacco protoplasts with microtubule- or microfilament-depolymerizing agents disrupted the strands of importin α in the cytoplasm, whereas a microtubule-stabilizing agent had no effect. Biochemical analysis showed that importin α associated with microtubules and microfilaments in vitro in an NLS-dependent manner. The interaction of importin α with the cytoskeleton could be an essential element of protein transport from the cytoplasm to the nucleus in vivo.