Krieghoff Eva, Behrens Jürgen, Mayr Bernhard
Nikolaus-Fiebiger-Center for Molecular Medicine, University of Erlangen-Nürnberg, Glückstr. 6, 91054 Erlangen, Germany.
J Cell Sci. 2006 Apr 1;119(Pt 7):1453-63. doi: 10.1242/jcs.02864.
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.
β-连环蛋白是经典Wnt信号通路的核心信号分子,在细胞核中它与LEF/TCF转录因子形成复合物激活靶基因。β-连环蛋白活性的调节主要发生在蛋白质降解水平,但有研究表明,β-连环蛋白的核定位及其转录活性可能还会通过TCF4和BCL9介导的核输入以及APC和axin介导的核输出进行调节。利用活细胞显微镜技术和光漂白后荧光恢复(FRAP)技术,我们直接分析了这些因子对β-连环蛋白亚细胞定位、核质穿梭及其在细胞核和细胞质中移动性的影响。我们发现,TCF4和BCL9/矮脚鸡蛋白将β-连环蛋白招募到细胞核,而APC、axin和axin2则使β-连环蛋白在细胞质中富集。然而,重要的是,这些因子均未加速β-连环蛋白的核质穿梭,即未提高β-连环蛋白的核输入或输出速率。此外,抑制CRM-1依赖性核输出并不能消除APC和axin导致的β-连环蛋白在细胞质中的富集。TCF4、APC、axin和axin2在各自区域内的移动速度均比β-连环蛋白慢,并且会相应降低β-连环蛋白的移动性。综上所述,这些数据表明,β-连环蛋白相互作用伙伴主要通过将其保留在自身所在区域来调节β-连环蛋白的亚细胞定位,而非通过主动转运使其进出细胞核。
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