title: Importin alpha/beta mediates nuclear transport of a mammalian circadian clock component, mCRY2, together with mPER2, through a bipartite nuclear localization signal.
abstract: Circadian rhythms, which period is approximately one day, are generated by endogenous biological clocks. These clocks are found throughout the animal kingdom, as well as in plants and even in prokaryotes. Molecular mechanisms for circadian rhythms are based on transcriptional oscillation of clock component genes, consisting of interwoven autoregulatory feedback loops. Among the loops, the nuclear transport of clock proteins is a crucial step for transcriptional regulation. In the present study, we showed that the nuclear entry of mCRY2, a mammalian clock component, is mediated by the importin alpha/beta system through a bipartite nuclear localization signal in its carboxyl end. In vitro transport assay using digitonin-permeabilized cells demonstrated that all three importin alphas, alpha1 (Rch1), alpha3 (Qip-1), and alpha7 (NPI-2), can mediate mCRY2 import. mCRY2 with the mutant nuclear localization signal failed to transport mPER2 into the nucleus of mammalian cultured cells, indicating that the nuclear localization signal identified in mCRY2 is physiologically significant. These results suggest that the importin alpha/beta system is involved in nuclear entry of mammalian clock components, which is indispensable to transcriptional oscillation of clock genes.
authors.affiliation: Osaka Bioscience Institute, Suita, Osaka 565-0874, Japan.
authors.forename: Yoko
authors.initials: Y
authors.lastname: Sakakida
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Miyamoto
authors.forename: Emi
authors.initials: E
authors.lastname: Nagoshi
authors.forename: Makoto
authors.initials: M
authors.lastname: Akashi
authors.forename: Takahiro J
authors.initials: TJ
authors.lastname: Nakamura
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Mamine
authors.forename: Megumi
authors.initials: M
authors.lastname: Kasahara
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Minami
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Yoneda
authors.forename: Toru
authors.initials: T
authors.lastname: Takumi
mesh_terms: D021581:Active Transport, Cell Nucleus / Q000502:physiology*; D000818:Animals; D001683:Biological Clocks / Q000502:physiology*; D018797:Cell Cycle Proteins; D002460:Cell Line; D002940:Circadian Rhythm / Q000502:physiology; D056931:Cryptochromes; D005420:Flavoproteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D051379:Mice; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D056950:Period Circadian Proteins; D011485:Protein Binding; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors; D028901:alpha Karyopherins / Q000378:metabolism*; D028961:beta Karyopherins / Q000378:metabolism*
chemical_list: D018797:Cell Cycle Proteins; D056931:Cryptochromes; D005420:Flavoproteins; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins; C421902:PER2 protein, human; C513312:Per2 protein, mouse; D056950:Period Circadian Proteins; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; D028901:alpha Karyopherins; D028961:beta Karyopherins