Nuclear localization signal receptor importin alpha associates with the cytoskeleton

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