8月28日，国际知名学术期刊The Journal of Biological Chemistry刊登了上海生命科学研究院生物化学与细胞生物学研究所丁小燕实验室的研究论文，研究人员通过免疫共沉淀等方法发现了转录因子Tbx6的一个新的相互作用蛋白--Smad6。进一步的研究发现Smad6通过增加Tbx6泛素化修饰，进而抑制Tbx6的转录活性。
T-box基因家族是近十年来新发现的一类在胚胎早期发育和遗传性疾病中起重要作用的转录调控因子。近十年的研究表明，作为T-box基因家族成员之一的Tbx6参与了中胚层细胞的特化和肌肉的发生过程。Myf-5和MyoD是成肌调控因子（myogenic regulatory factors，MRFs）家族中的两个重要成员，它们的活化是肌肉前体细胞分化的作用。BMP（bone morphogenetic protein）信号通过负向调控Myf-5和MyoD的转录，有效抑制肌肉前体细胞的分化，但具体的分子机制尚不清楚。
生化与细胞所博士生陈跃磊等人发现Tbx6蛋白与BMP信号的下游分子Smad6之间存在直接的蛋白相互作用。缺失突变实验发现，Smad6通过其C端MH2结构域与Tbx6的第90-180位氨基酸发生蛋白相互作用。Smad6招募E3连接酶Smurf1后能有效增强Tbx6蛋白的泛素化降解，从而实现对Tbx6下游基因的转录抑制。同时，他们还建立了小鼠TT-D6间充质干细胞系体外诱导分化形成软骨细胞的模型，发现TT-D6细胞中内源的Smad6和Tbx6蛋白也存在着相互作用。利用siRNA降低内源Smad6蛋白的表达后，能增加细胞内Tbx6的蛋白含量，并且解除了BMP信号对成肌调控因子的抑制作用。他们的研究提出了一个全新的假说，即BMP信号通过下游分子Smad6与Tbx6相互作用后降解Tbx6蛋白，进而实现对成肌调控因子Myf-5的转录抑制。这也为更好地阐释BMP信号如何调控细胞分化过程提供了一个新的分子机制。上述研究成果得到国际同行评审专家和杂志编辑的好评，
该项研究工作得到了国家科技部、国家自然基金委 、中国科学院的经费支持。（生物谷Bioon.com）
生物谷推荐原始出处：
J. Biol. Chem., Vol. 284, Issue 35, 23481-23490, August 28, 2009
Smad6 Inhibits the Transcriptional Activity of Tbx6 by Mediating Its Degradation*
Yue-Lei Chen, Bin Liu, Zhen-Ning Zhou, Rui-Ying Hu, Cong Fei, Zhi-Hui Xie, and Xiaoyan Ding1
From the From the Laboratory of Molecular Cell Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Shanghai 200031, China
ABSTRACT
Members of the bone morphogenetic protein (BMP) and T-box gene families play several critical roles in the early embryonic development and tissue homeostasis. Although BMP proteins are the upstream regulators of T-box genes, few studies have investigated the molecular mechanisms between these two protein families. Here, we report that Tbx6 interacts directly with Smad6, an inhibitory Smad that antagonizes the BMP signal. This interaction is mediated through the Mad homology 2 (MH2) domain of Smad6 and residues 90–180 of Tbx6. We demonstrate that Smad6 facilitates the degradation of Tbx6 protein through recruitment of Smurf1, a ubiquitin E3 ligase. Consequently, Smad6 reduces Tbx6-mediated Myf-5 gene activation. Furthermore, specific knockdown of endogenous Smad6 and Smurf1 by small interfering RNA increases the protein levels of Tbx6 and enhance the expression of Tbx6 target genes. Collectively, these findings reveal that Smad6 serves as a critical mediator of BMP signal via a functional interaction with Tbx6, thus regulating the activation of Tbx6 downstream genes during cell differentiation.
