title: Molecular control of nuclear and subnuclear targeting of the plant CDK inhibitor ICK1 and ICK1-mediated nuclear transport of CDKA.
abstract: ICK1 is the first member of a family of plant cyclin-dependent kinase (CDK) inhibitors. It has been shown that ICK1 is localized in the nuclei of transgenic Arabidopsis plants. Since cellular localization is important for the functions of cell cycle regulators, a comprehensive analysis was undertaken to identify specific sequences regulating the cellular localization of ICK1. Deletion and site-specific mutants fused to the green fluorescent protein (GFP) were used in transgenic Arabidopsis plants and transfected tobacco cells. Surprisingly, three separate sequences in the N-terminal, central and C-terminal regions of ICK1 could independently confer nuclear localization of the GFP fusion proteins. The central nuclear localization signal NLS(ICK1) could transport the much larger GUS (beta-glucuronidase)-GFP fusion protein into nuclei, while the other two sequences were unable to. These results suggest that NLS(ICK1) is a strong NLS that actively transports the fusion protein into nuclei, while the other two sequences are either a weaker NLS or confer the nuclear localization of GFP indirectly. It was further observed that the N-terminal sequence specifies a punctate pattern of subnuclear localization, while the C-terminal sequence suppresses it. Furthermore, co-expression of ICK1 and Arabidopsis CDKA, tagged with different GFP variants, showed that ICK1 could mediate the transport of CDKA into nuclei while a mutant ICK1(1-162) that does not interact with CDKA lost this ability. These results illustrate how the nuclear localization of ICK1 is regulated and also suggest a possible role of ICK1 in regulating the cellular distribution of CDKA.
authors.affiliation: Department of Biology, University of Saskatchewan, Saskatoon, Canada S7N 5E2.
authors.forename: Yongming
authors.initials: Y
authors.lastname: Zhou
authors.forename: Hesheng
authors.initials: H
authors.lastname: Niu
authors.forename: Federica
authors.initials: F
authors.lastname: Brandizzi
authors.forename: Larry C
authors.initials: LC
authors.lastname: Fowke
authors.forename: Hong
authors.initials: H
authors.lastname: Wang
mesh_terms: D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism*; D001483:Base Sequence; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000235:genetics*; D002467:Cell Nucleus / Q000201:enzymology; D002874:Chromosome Mapping; D050756:Cyclin-Dependent Kinase Inhibitor Proteins / Q000235:genetics*; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D017931:DNA Primers; D005784:Gene Amplification; D017354:Point Mutation
chemical_list: D029681:Arabidopsis Proteins; D018797:Cell Cycle Proteins; D050756:Cyclin-Dependent Kinase Inhibitor Proteins; D017931:DNA Primers; C114853:ICK1 protein, Arabidopsis; C407546:ICK2 protein, Arabidopsis; D016203:CDC2 Protein Kinase; C434472:CDKA1 protein, Arabidopsis; D018844:Cyclin-Dependent Kinases