title: Nuclear protein transport pathways.
abstract: Nuclear proteins like transcription factors and ribosomal proteins are synthesized in the cytoplasm and have to be transported into the nucleus to fulfill their functions. The transport of proteins >20-60 kD through the nuclear pore complex (NPC) into the nucleus is an active, energy-requiring process. Transport substrates are recognized by their transport proteins via certain signals. The best-characterized protein import pathway is the 'classical' nuclear localization signal-dependent pathway with importin alpha and beta carrying the substrate to the NPC. The transport of the importin-substrate complex into the nucleus is regulated by the small GTPase Ran/TC4. During the last years more than ten proteins have been discovered which have already been proven or are very likely to be nuclear transport factors of distinct import pathways: members of the importin alpha protein family are very similar and transport in complex with importin beta nuclear localization signal-bearing proteins into the nucleus. Members of the Ran-binding protein family show some weak similarity to importin beta. Sharing a common domain at the amino terminus, they are able to bind RanGTP, a prerequisite for their function as nuclear import or export factors for distinct proteins or RNAs. However, Ran/TC4 seems to play a key regulatory role in all nuclear transport pathways described so far, although the molecular mechanism of the translocation step through the NPC is still unclear.
authors.affiliation: Charité, Franz Volhard Clinic, Humboldt University, Berlin, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Köhler
authors.forename: H
authors.initials: H
authors.lastname: Haller
authors.forename: E
authors.initials: E
authors.lastname: Hartmann
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D006801:Humans; D009685:Nuclear Envelope / Q000378:metabolism; D009687:Nuclear Proteins / Q000378:metabolism*; D020931:ran GTP-Binding Protein
chemical_list: D009687:Nuclear Proteins; D020931:ran GTP-Binding Protein