Liu Wei, Rui Hongliang, Wang Jifeng, Lin Shuyong, He Ying, Chen Mingliang, Li Qinxi, Ye Zhiyun, Zhang Suping, Chan Siu Chiu, Chen Ye-Guang, Han Jiahuai, Lin Sheng-Cai
Department of Biochemistry, Hong Kong University of Science and Technology, Kowloon, Hong Kong, China.
EMBO J. 2006 Apr 19;25(8):1646-58. doi: 10.1038/sj.emboj.7601057. Epub 2006 Apr 6.
TGF-beta signaling involves a wide array of signaling molecules and multiple controlling events. Scaffold proteins create a functional proximity of signaling molecules and control the specificity of signal transduction. While many components involved in the TGF-beta pathway have been elucidated, little is known about how those components are coordinated by scaffold proteins. Here, we show that Axin activates TGF-beta signaling by forming a multimeric complex consisting of Smad7 and ubiquitin E3 ligase Arkadia. Axin depends on Arkadia to facilitate TGF-beta signaling, as their small interfering RNAs reciprocally abolished the stimulatory effect on TGF-beta signaling. Specific knockdown of Axin or Arkadia revealed that Axin and Arkadia cooperate with each other in promoting Smad7 ubiquitination. Pulse-chase experiments further illustrated that Axin significantly decreased the half-life of Smad7. Axin also induces nuclear export of Smad7. Interestingly, Axin associates with Arkadia and Smad7 independently of TGF-beta signal, in contrast to its transient association with inactive Smad3. However, coexpression of Wnt-1 reduced Smad7 ubiquitination by downregulating Axin levels, underscoring the importance of Axin as an intrinsic regulator in TGF-beta signaling.
转化生长因子-β（TGF-β）信号传导涉及大量信号分子和多个调控事件。支架蛋白使信号分子在功能上相互靠近，并控制信号转导的特异性。虽然TGF-β信号通路中的许多组分已被阐明，但对于这些组分如何由支架蛋白进行协调却知之甚少。在此，我们表明轴蛋白（Axin）通过形成由Smad7和泛素E3连接酶Arkadia组成的多聚体复合物来激活TGF-β信号传导。Axin依赖于Arkadia来促进TGF-β信号传导，因为它们的小干扰RNA相互消除了对TGF-β信号传导的刺激作用。对Axin或Arkadia的特异性敲低表明，Axin和Arkadia在促进Smad7泛素化方面相互协作。脉冲追踪实验进一步表明，Axin显著缩短了Smad7的半衰期。Axin还诱导Smad7的核输出。有趣的是，与它与无活性的Smad3的瞬时结合不同，Axin独立于TGF-β信号与Arkadia和Smad7结合。然而，Wnt-1的共表达通过下调Axin水平降低了Smad7的泛素化，强调了Axin作为TGF-β信号传导中一种内在调节因子的重要性。
