In the process of nucleocytoplasmic transport, the importation of proteins containing a nuclear localization signal requires a heterodimer of importin alpha, which binds the nuclear localization signal-containing cargo in the cytoplasm.
Nucleocytoplasmic transport, a signal- and energy-dependent process, takes place through nuclear pore complexes embedded in the nuclear envelope. The import of proteins containing a classical nuclear localization signal (NLS) requires the NLS import receptor, a heterodimer of importin alpha and beta subunits. Importin alpha binds the NLS-containing cargo in the cytoplasm and importin beta docks the complex at the cytoplasmic side of the nuclear pore complex. 
