 The primary function of importin is to mediate the translocation of proteins with nuclear localization signals into the nucleus, through nuclear pore complexes (NPC), in a process known as the nuclear protein import cycle.The first step of this cycle is the binding of cargo.Importin can perform this function as a monomeric importin-β protein, but usually requires the presence of importin-α, which acts as an adaptor to cargo proteins (via interactions with the NLS).