Guerrero-Esteo Mercedes, Sanchez-Elsner Tilman, Letamendia Ainhoa, Bernabeu Carmelo
Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Cientificas, Velázquez 144, Madrid 28006, Spain.
J Biol Chem. 2002 Aug 9;277(32):29197-209. doi: 10.1074/jbc.M111991200. Epub 2002 May 15.
Endoglin is an auxiliary component of the transforming growth factor-beta (TGF-beta) receptor system, able to associate with the signaling receptor types I (TbetaRI) and II (TbetaRII) in the presence of ligand and to modulate the cellular responses to TGF-beta1. Endoglin cannot bind ligand on its own but requires the presence of the signaling receptors, supporting a critical role for the interaction between endoglin and TbetaRI or TbetaRII. This study shows that full-length endoglin interacts with both TbetaRI and TbetaRII, independently of their kinase activation state or the presence of exogenous TGF-beta1. Truncated constructs encoding either the extracellular or the cytoplasmic domains of endoglin demonstrated that the association with the signaling receptors occurs through both extracellular and cytoplasmic domains. However, a more specific mapping revealed that the endoglin/TbetaRI interaction was different from that of endoglin/TbetaRII. TbetaRII interacts with the amino acid region 437-558 of the extracellular domain of endoglin, whereas TbetaRI interacts not only with the region 437-558 but also with the protein region located between amino acid 437 and the N terminus. Both TbetaRI and TbetaRII interact with the cytoplasmic domain of endoglin, but TbetaRI only interacts when the kinase domain is inactive, whereas TbetaRII remains associated in its active and inactive forms. Upon association, TbetaRI and TbetaRII phosphorylate the endoglin cytoplasmic domain, and then TbetaRI, but not TbetaRII, kinase dissociates from the complex. Conversely, endoglin expression results in an altered phosphorylation state of TbetaRII, TbetaRI, and downstream Smad proteins as well as a modulation of TGF-beta signaling, as measured by the reporter gene expression. These results suggest that by interacting through its extracellular and cytoplasmic domains with the signaling receptors, endoglin might affect TGF-beta responses.
内皮糖蛋白是转化生长因子-β（TGF-β）受体系统的辅助成分，在配体存在的情况下能够与I型信号受体（TβRI）和II型信号受体（TβRII）结合，并调节细胞对TGF-β1的反应。内皮糖蛋白自身不能结合配体，但需要信号受体的存在，这支持了内皮糖蛋白与TβRI或TβRII之间相互作用的关键作用。本研究表明，全长内皮糖蛋白与TβRI和TβRII均相互作用，与它们的激酶激活状态或外源性TGF-β1的存在无关。编码内皮糖蛋白细胞外或细胞质结构域的截短构建体表明，与信号受体的结合通过细胞外和细胞质结构域发生。然而，更具体的定位显示，内皮糖蛋白/TβRI相互作用与内皮糖蛋白/TβRII相互作用不同。TβRII与内皮糖蛋白细胞外结构域的437-558氨基酸区域相互作用，而TβRI不仅与437-558区域相互作用，还与位于437氨基酸和N末端之间的蛋白质区域相互作用。TβRI和TβRII均与内皮糖蛋白的细胞质结构域相互作用，但TβRI仅在激酶结构域无活性时相互作用，而TβRII在其活性和无活性形式下均保持结合。结合后，TβRI和TβRII使内皮糖蛋白细胞质结构域磷酸化，然后TβRI激酶从复合物中解离，而TβRII激酶不发生解离。相反，内皮糖蛋白的表达导致TβRII、TβRI和下游Smad蛋白的磷酸化状态改变，以及TGF-β信号传导的调节，这通过报告基因表达来衡量。这些结果表明，内皮糖蛋白可能通过其细胞外和细胞质结构域与信号受体相互作用来影响TGF-β反应。
