Prieve M G, Waterman M L
Department of Microbiology and Molecular Genetics, College of Medicine, University of California, Irvine, Irvine, California 92697-4025, USA.
Mol Cell Biol. 1999 Jun;19(6):4503-15. doi: 10.1128/MCB.19.6.4503.
In response to activation of the Wnt signaling pathway, beta-catenin accumulates in the nucleus, where it cooperates with LEF/TCF (for lymphoid enhancer factor and T-cell factor) transcription factors to activate gene expression. The mechanisms by which beta-catenin undergoes this shift in location and participates in activation of gene transcription are unknown. We demonstrate here that beta-catenin can be imported into the nucleus independently of LEF/TCF binding, and it may also be exported from nuclei. We have introduced a small deletion within beta-catenin (Delta19) that disrupts binding to LEF-1, E-cadherin, and APC but not axin. This Delta19 beta-catenin mutant localizes to the nucleus because it may not be efficiently sequestered in the cytoplasm. The nuclear localization of Delta19 definitively demonstrates that the mechanisms by which beta-catenin localizes in the nucleus are completely independent of LEF/TCF factors. beta-Catenin and LEF-1 complexes can activate reporter gene expression in a transformed T-lymphocyte cell line (Jurkat) but not in normal T lymphocytes, even though both factors are nuclear. Thus, localization of both factors to the nucleus is not sufficient for activation of gene expression. Excess beta-catenin can squelch reporter gene activation by LEF-1-beta-catenin complexes but not activation by the transcription factor VP16. Taken together, these data suggest that a third component is necessary for gene activation and that this third component may vary with cell type.
作为对Wnt信号通路激活的反应，β-连环蛋白在细胞核中积累，在那里它与淋巴增强因子（LEF）/T细胞因子（TCF）转录因子协同作用以激活基因表达。β-连环蛋白发生这种位置变化并参与基因转录激活的机制尚不清楚。我们在此证明，β-连环蛋白可以独立于LEF/TCF结合而被导入细胞核，并且它也可能从细胞核中输出。我们在β-连环蛋白内引入了一个小的缺失（Delta19），该缺失破坏了与LEF-1、E-钙黏蛋白和腺瘤性息肉病（APC）的结合，但不影响与axin的结合。这种Delta19β-连环蛋白突变体定位于细胞核，因为它可能无法有效地被隔离在细胞质中。Delta19的核定位明确表明，β-连环蛋白定位于细胞核的机制完全独立于LEF/TCF因子。β-连环蛋白和LEF-1复合物可以在转化的T淋巴细胞系（Jurkat）中激活报告基因表达，但在正常T淋巴细胞中则不能，尽管这两种因子都位于细胞核中。因此，这两种因子定位于细胞核并不足以激活基因表达。过量的β-连环蛋白可以抑制LEF-1-β-连环蛋白复合物对报告基因的激活，但不能抑制转录因子VP16对报告基因的激活。综上所述，这些数据表明基因激活需要第三种成分，并且这种第三种成分可能因细胞类型而异。
