Protein transport across the nucleus is a selective, multi-step process involving several cytoplasmic factors. Proteins must be recognized as import substrates, dock at the nuclear pore complex and translocate across the nuclear envelope in an ATP-dependent fashion. Two cytosolic factors centrally involved in the recognition and docking process are the karyopherin alpha 1 and karyopherin beta1 subunits. Karyopherin alpha 1 functions in the recognition and targeting of substrates destined for nuclear import, while karyopherin beta 1 serves as an adapter, tethering the karyopherin alpha 1/substrate complex to docking proteins on the nuclear envelope, termed nucleoporins. Karyopherin alpha 2 has been shown to complex with Epstein-Barr virus nuclear antigen 1 (EBNA-1). Karyopherin beta 2 and karyopherin beta 2B (also designated transportin 1 and transportin 2) share 84% sequence identity at the amino acid level, however, they have been shown to have different substrate specificities. Karyopherin beta 2 mediates hnRNPA1 nuclear import while karyopherin beta 2B has been implicated in the export of cellular mRNAs through complexes formed with the mRNA export factor TAP.

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