Nucleo-cytoplasmic distribution of beta-catenin is regulated by retention.
beta-catenin is the central signalling molecule of the canonical Wnt pathway, where it activates target genes in a complex with LEF/TCF transcription factors in the nucleus. The regulation of beta-catenin activity is thought to occur mainly on the level of protein degradation, but it has been suggested that beta-catenin nuclear localization and hence its transcriptional activity may additionally be regulated via nuclear import by TCF4 and BCL9 and via nuclear export by APC and axin. Using live-cell microscopy and fluorescence recovery after photobleaching (FRAP), we have directly analysed the impact of these factors on the subcellular localization of beta-catenin, its nucleo-cytoplasmic shuttling and its mobility within the nucleus and the cytoplasm. We show that TCF4 and BCL9/Pygopus recruit beta-catenin to the nucleus, and APC, axin and axin2 enrich beta-catenin in the cytoplasm. Importantly, however, none of these factors accelerates the nucleo-cytoplasmic shuttling of beta-catenin, i.e. increases the rate of beta-catenin nuclear import or export. Moreover, the cytoplasmic enrichment of beta-catenin by APC and axin is not abolished by inhibition of CRM-1-dependent nuclear export. TCF4, APC, axin and axin2 move more slowly than beta-catenin in their respective compartment, and concomitantly decrease beta-catenin mobility. Together, these data indicate that beta-catenin interaction partners mainly regulate beta-catenin subcellular localization by retaining it in the compartment in which they are localized, rather than by active transport into or out of the nucleus.
Krieghoff
Eva
E
Nikolaus-Fiebiger-Center for Molecular Medicine, University of Erlangen-Nürnberg, Glückstr. 6, 91054 Erlangen, Germany.
Behrens
Jürgen
J
Mayr
Bernhard
B
D025601:Adenomatous Polyposis Coli Protein / Q000378:metabolism; D060466:Axin Protein; D015153:Blotting, Western; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism*; D003598:Cytoskeletal Proteins / Q000378:metabolism; D036681:Fluorescence Recovery After Photobleaching; D005455:Fluorescent Antibody Technique; D017930:Genes, Reporter; D006801:Humans; D008156:Luciferases / Q000378:metabolism; D018715:Microscopy, Video; D009363:Neoplasm Proteins / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism; D051820:TCF Transcription Factors / Q000378:metabolism; D058118:Transcription Factor 7-Like 2 Protein; D014157:Transcription Factors; D020798:Two-Hybrid System Techniques; D051176:beta Catenin / Q000378:metabolism*
C491761:AXIN2 protein, human; D025601:Adenomatous Polyposis Coli Protein; D060466:Axin Protein; C111180:BCL9 protein, human; D003598:Cytoskeletal Proteins; D009363:Neoplasm Proteins; D012097:Repressor Proteins; D051820:TCF Transcription Factors; C544894:TCF7L2 protein, human; D058118:Transcription Factor 7-Like 2 Protein; D014157:Transcription Factors; D051176:beta Catenin; D008156:Luciferases