Requirement for a nuclear function of beta-catenin in Wnt signaling.
Wnt signaling stabilizes beta-catenin, which in turn influences the transcription of Wnt-responsive genes in conjunction with T-cell factor (TCF) transcription factors. At present, there are two models for the actions of beta-catenin. The conventional nuclear model suggests that beta-catenin acts in the nucleus to form a heterodimeric transcriptional factor complex with TCF, with TCF providing DNA-specific binding and the C and N termini of beta-catenin stimulating transcription. The alternative cytoplasmic model postulates that beta-catenin exports TCF from the nucleus to relieve its repressive activity or activates it in the cytoplasm. We have generated modified forms of beta-catenin and used RNA interference against endogenous beta-catenin to distinguish between these models in cultured mammalian and Drosophila cells. We show that the VP16 transcriptional activation domain can replace the C terminus of beta-catenin without loss of function and that the function of beta-catenin is compromised by fusion to a transcriptional repressor domain from histone deacetylase, favoring the direct effects of beta-catenin in the nucleus. Furthermore, membrane-tethered beta-catenin requires interaction with the adenomatous polyposis coli protein but not with TCF for its function, whereas untethered beta-catenin requires binding to TCF for its signaling activity. Importantly, by using RNA interference, we show that the signaling activity of membrane-tethered beta-catenin, but not free beta-catenin, requires the presence of endogenous beta-catenin, which is able to accumulate in the nucleus when stabilized by the binding of the beta-catenin degradation machinery to the membrane-tethered form. All of these data support a nuclear model for the normal function of beta-catenin.
Cong
Feng
F
Program in Cell Biology, Sloan-Kettering Institute, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Schweizer
Liang
L
Chamorro
Mario
M
Varmus
Harold
H
D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D003598:Cytoskeletal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004247:DNA / Q000235:genetics; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004330:Drosophila; D006801:Humans; D051821:Lymphoid Enhancer-Binding Factor 1; D051379:Mice; D041681:NIH 3T3 Cells; D050809:POU Domain Factors; D017434:Protein Structure, Tertiary; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D034622:RNA Interference; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion; D015398:Signal Transduction; D015534:Trans-Activators / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D051153:Wnt Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin
C495270:CTNNB1 protein, human; C495267:CTNNB1 protein, mouse; D003598:Cytoskeletal Proteins; D004268:DNA-Binding Proteins; D051821:Lymphoid Enhancer-Binding Factor 1; D050809:POU Domain Factors; C082703:POU6F1 protein, human; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D051153:Wnt Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin; D004247:DNA