Function
Importins (IPO) mediate the import of cellular proteins into the nucleus by binding to the Nuclear Localization Signal (NLS). IPO has 2 subunits: α and β. IPO-α is an adaptor protein which binds the NLS[1]. IPOs can contain one or two NLS motifs. IPO-α contains several armadillo repeats with two NLS-binding sites and an IPO-β binding (IBB) site. IPO-β contains 19 HEAT (solenoid domain) repeats. Many nuclear proteins contain Phe-Gly sequences which can bind to HEAT repeats and thus be transported. See also Protein Transport Membrane Protein.
Disease
IPO-α isoform expression is altered in many kinds of cancer.
Relevance
Specific IPO-α isoforms are critical in nerve regenerative response after injury. Loss of IPO-α expression in aging myocardial cells could make recovery after heart disease in elderly patients more difficult[2].
Structural highlights
3D structures of importin
Importin 3D structures
