title: Major nucleolar proteins shuttle between nucleus and cytoplasm.
abstract: Nucleolin is a 92 kd nucleolar protein implicated in regulating polymerase I transcription and binding of preribosomal RNA. Another abundant nucleolar protein of 38 kd (B23/No38) is thought to be involved in intranuclear packaging of preribosomal particles. Although both proteins have previously been detected only in nuclei, we conclude that they shuttle constantly between nucleus and cytoplasm. This conclusion is based on monitoring the equilibration of these proteins between nuclei present in interspecies heterokaryons, and on observing the antigen-mediated nuclear accumulation of cytoplasmically injected antibodies. Our unexpected results suggest a role for these major nucleolar proteins in the nucleocytoplasmic transport of ribosomal components. Moreover, they suggest that transient exposure of shuttling proteins to the cytoplasm may provide a mechanism for cytoplasmic regulation of nuclear activities.
authors.affiliation: Institut für Zellbiologie, ETH Hönggerberg, Zürich, Switzerland.
authors.forename: R A
authors.initials: RA
authors.lastname: Borer
authors.forename: C F
authors.initials: CF
authors.lastname: Lehner
authors.forename: H M
authors.initials: HM
authors.lastname: Eppenberger
authors.forename: E A
authors.initials: EA
authors.lastname: Nigg
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D002466:Cell Nucleolus / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism*; D002642:Chick Embryo; D002645:Chickens; D003001:Cloning, Molecular; D003593:Cytoplasm / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism; D000090243:Nucleophosmin; D010750:Phosphoproteins / Q000235:genetics* / Q000378:metabolism; D016601:RNA-Binding Proteins; D013045:Species Specificity; D014981:Xenopus; D000097796:Nucleolin
chemical_list: D009687:Nuclear Proteins; D000090243:Nucleophosmin; D010750:Phosphoproteins; D016601:RNA-Binding Proteins; D000097796:Nucleolin; C000715570:Npm1 protein, mouse