Nuclear localization signal-independent and importin/karyopherin-independent nuclear import of beta-catenin.
BACKGROUND
Control of the nuclear localization of specific proteins is an important mechanism for regulating many signal transduction pathways. Upon activation of the Wnt signaling pathway, beta-catenin localizes into the nucleus and interacts with TCF/LEF-1 (T-cell factor/lymphocyte enhancer factor-1) transcription factors, triggering activation of downstream genes. The role of regulated nuclear localization in beta-catenin signaling is still unclear. Beta-catenin has no nuclear localization sequence (NLS). Although it has been reported that beta-catenin can piggyback into the nucleus by binding to TCF/LEF-1, there is evidence that its import is independent of TCF/LEF-1 in vivo. Therefore, the mechanism for beta-catenin nuclear localization remains to be established.


RESULTS
We have analyzed beta-catenin nuclear import in an in vitro assay using permeabilized cells. Beta-catenin docks specifically onto the nuclear envelope in the absence of other cytosolic factors. Docking is not inhibited by an NLS peptide and does not require importins/karyopherins, the receptors for classical NLS substrates. Rather, docking is specifically competed by importin-beta/beta-karyopherin, indicating that beta-catenin and importin-beta/beta-karyopherin both interact with common nuclear pore components. Nuclear translocation of beta-catenin is energy dependent and is inhibited by nonhydrolyzable GTP analogs and by a dominant-negative mutant form of the Ran GTPase. Cytosol preparations contain inhibitory activities for beta-catenin import that are distinct from the competition by importin-beta/beta-karyopherin and may be involved in the physiological regulation of the pathway.


CONCLUSIONS
Beta-catenin is imported into the nucleus by binding directly to the nuclear pore machinery, similar to importin-beta/beta-karyopherin or other importin-beta-like import factors, such as transportin. These findings provide an explanation for how beta-catenin localizes to the nucleus without an NLS and independently of its interaction with TCF/LEF-1. This is a new and unusual mechanism for the nuclear import of a signal transduction protein. The lack of beta-catenin import activity in the presence of normal cytosol suggests that its import may be regulated by upstream events in the Wnt signaling pathway.
Fagotto
F
F
Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York 10021, USA.
Glück
U
U
Gumbiner
B M
BM
D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport; D002467:Cell Nucleus / Q000378:metabolism*; D003598:Cytoskeletal Proteins / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D028884:Karyopherins; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D015534:Trans-Activators; D014981:Xenopus; D029867:Xenopus Proteins; D028901:alpha Karyopherins; D051176:beta Catenin
C495266:CTNNB1 protein, Xenopus; D003598:Cytoskeletal Proteins; D028884:Karyopherins; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins; D011994:Recombinant Proteins; D015534:Trans-Activators; D029867:Xenopus Proteins; D028901:alpha Karyopherins; D051176:beta Catenin