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Karyopherins are a group of proteins involved in transporting molecules between the cytoplasm and the nucleus of a eukaryotic cell. The inside of the nucleus is called the karyoplasm (or nucleoplasm). Generally, karyopherin-mediated transport occurs through the nuclear pore, which acts as a gateway into and out of the nucleus. Most proteins require karyopherins to traverse the nuclear pore.
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Karyopherins can act as importins (i.e. helping proteins get into the nucleus) or exportins (i.e. helping proteins get out of the nucleus). They belong to The Nuclear Pore Complex Family in the transporter classification database (TCDB).
Energy for transport is derived from the Ran gradient. See Ran for further details.
Upon stress, several karyopherins stop shuttling between the nucleus and the cytoplasm and are sequestered in stress granules, cytoplasmic aggregates of ribonucleoprotein complexes.
Importin beta
Importin beta is a specific type of karyopherin that facilitates the transport of cargo proteins into the nucleus. First, it is binding importin alpha - another type of karyopherin that binds the cargo protein in the cytoplasm - before the cargo protein is imported into the nucleus through the nuclear pore using energy derived from the Ran gradient. Once inside the nucleus, the cargo dissociates from the karyopherins.
Importin beta can also carry proteins into the nucleus without the aid of the importin alpha adapter protein.