Functional analysis on follistatin domain-containing proteins and activin signaling

1.卵泡抑素相关蛋白是一种含有卵泡抑素结构域的蛋白，最初是由转化生长因子-β上调小鼠成骨细胞MC3T3E1表达的。为了阐明非洲爪哇在早期胚胎发育中的功能，我们从非洲爪哇中分离出了该基因的同源基因(XFRP)。在非洲爪哇胚胎中，xFRP基因的表达始于原肠胚形成，并持续到尾芽期。MRNAs定位于Spemann组织器、脊索、神经底板、下索和体节。与非洲爪哇卵泡蛋白mRNA表达模式的相似性表明，xFRP可能在神经化过程中起作用。卵泡抑素是一种激活素结合蛋白，与细胞表面蛋白多糖上的肝素/硫酸肝素碳水化合物部分结合。因此，Folistain对神经化的作用可能是通过抑制激活素的作用来实现的。我们还发现，在不同的培养细胞系中，卵泡抑素可以通过表面硫酸肝素蛋白多糖内化后，介导激活素的代谢。我们已经鉴定出一种与激活素IIA受体(ActRIIA)相互作用的小鼠PDZ蛋白，我们将其命名为激活素受体相互作用蛋白1(ARIP1)。AR1P1在NH(2)末端有1个鸟氨酸激酶区，其次是2个WW区和5个PDZ区(PDZ1-5)，并通过PDZ5与ActRIIA相互作用。在转化生长因子β家族的受体中，ARIP1与ActRIIA特异性地相互作用。有趣的是，ARIP1还与Smad3相互作用，Smad3是一种激活素/转化生长因子β细胞内信号分子。ARIPI基因在脑中的表达比在其他组织中更为丰富。这些发现表明，ARIP1在特定的亚细胞位置组装激活素信号分子和调节神经细胞中的信号转导方面具有重要作用。

1. Follistatin-related protein (FRP, TSC-36), a follistatin domain-containing protein was originally isolated as a cDNA that was up-regulated by TGF-β in mouse osteogenic MC3T3E1 cells. We have isolated a homologue (xFRP) of the cDNA from Xenopus laevis to clarify the function in early embryogenesis. In the Xenopus embryos, xFRP mRNA expression started at the onset of gastrulation, and sustained through the tail-bud stage. The mRNAs Were localized in the Spemann organizer, notochord, neural floor plate, hypochord and somite. The similarities with the pattern of expression of Xenopus follistain mRNA suggests that xFRP may play a role in neuralization. Follistatin is an activin-binding protein and binds to heparin/heparansulfate carbohydrate moieties on the cell surface proteoglycans. Thus, a function of follistain on the neuralization may be performed by the inhibition of activin actions. We have also found that follistatin can mediate the activin metabolization after the internalization via surface heparansulfate-proteoglycans in various cultured cell lines.2. We have identified a mouse PDZ protein that interacts with the activin type IIA receptor (ActRIIA), which we named activin receptor-interacting protein 1 (ARIP1). ARlP1 had one guanylate kinase domain in the NH(2)-terminal region, followed by two WW domains and five PDZ domains (PDZ1-5), and interacted with ActRIIA through PDZ5. ARIP1 interacts specifically with ActRIIA among the receptors for the transforming growth factor beta family. Interestingly, ARIP1 also interacted with Smad3, which is an activin/transforming growth factor beta intracellular signaling molecule. The mRNA of ARIPI was more abundant in the brain than in other tissues. These findings suggest that ARIP1 has a significant role in assembling activin signaling molecules at specific subcellular sites and in regulating signal transduction in neuronal cells.