title: Identification of nuclear import mechanisms for the neuronal Cdk5 activator.
abstract: The activation of Cdk5 by p35 plays a pivotal role in a multitude of nervous system activities ranging from neuronal differentiation to degeneration. A fraction of Cdk5 and p35 localizes in the nucleus where Cdk5-p35 exerts its functions via protein phosphorylation, and p35 displays a dynamic localization between the cytoplasm and the nucleus. Here, we examined the nuclear import properties of p35. In nuclear import assays, p35 was actively transported into the nuclei of digitonin-permeabilized HeLa cells and cortical neurons by cytoplasmic carrier-mediated mechanisms. Importin-beta, importin-5, and importin-7 were identified to import p35 into the nuclei via a direct interaction with it. An N-terminal region of p35 was defined to interact with the above importins, serving as a nuclear localization signal. Finally, we show that the nuclear localization of p35 does not require the association of Cdk5. Furthermore, Cdk5 and importin-beta/5/7 are mutually exclusive in binding to p35. These results suggest that p35 employs pathways distinct from that used by Cdk5 for transport to the nucleus.
authors.affiliation: Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
authors.forename: Xinrong
authors.initials: X
authors.lastname: Fu
authors.forename: Yuk-Kwan
authors.initials: YK
authors.lastname: Choi
authors.forename: Dianbo
authors.initials: D
authors.lastname: Qu
authors.forename: Yan
authors.initials: Y
authors.lastname: Yu
authors.forename: Nam Sang
authors.initials: NS
authors.lastname: Cheung
authors.forename: Robert Z
authors.initials: RZ
authors.lastname: Qi
mesh_terms: D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D002467:Cell Nucleus / Q000378:metabolism; D002522:Chlorocebus aethiops; D051360:Cyclin-Dependent Kinase 5 / Q000378:metabolism / Q000502:physiology*; D003593:Cytoplasm / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D009474:Neurons / Q000378:metabolism*; D010766:Phosphorylation; D017434:Protein Structure, Tertiary
chemical_list: D051360:Cyclin-Dependent Kinase 5; C495905:CDK5 protein, human