Regulation of activin signal transduction by PDZ domain-containing proteins

激活素信号转导通过多种机制调节。我们已经确定了新的调节蛋白，控制激活素功能的细胞内或细胞外。作为细胞内分子，鉴定了与激活素II型受体（ActRII）特异性结合的PDZ（PSD-95/Dlg/ZO-1）蛋白。我们将这些分子命名为ARIPs（激活素受体相互作用蛋白）。ARIP 1在其NH 2末端具有两个WW结构域，在其COOH末端具有五个PDZ结构域。ARIP 1通过WW结构域与Smad蛋白相关，并通过PDZ结构域5与ActRIIA相关。ARIP 1可以募集Smad 3并与激活素受体结合，因此ARIP 1可能是细胞膜和膜下区域激活素-信号复合物的支架，参与信号转导的生理调节。与此雅阁一致，我们观察到ARIP 1的过表达抑制了小鼠海马衍生细胞中Smad 3增强的转录，并抑制了配体刺激引起的Smad 3核转位。ARIPs可能参与激活素受体的组装和/或转运，作为一种细胞外调节蛋白，我们发现了一种新的卵泡抑素样蛋白，命名为FLRG（follistatin-related gene）。与卵泡抑素类似，FLRG结合激活素和BMP（骨形态发生蛋白）并在细胞外控制其功能。有趣的是，FLRG仅包含两个卵泡抑素结构域，并且第二个卵泡抑素结构域包括COOH末端酸性区域，能够结合激活素。这种结合方式不同于卵泡抑素-激活素。由于卵泡抑素和FLRG以不同的方式表达，并且它们可能是激活素和BMP的局部调节剂，因此它们在体内的功能可能不同。已经制备了识别小鼠和人FLRG的特异性单克隆抗体以进一步表征FLRG的功能。

Activin signal transduction is regulated through multiple mechanisms. We have identified novel regulatory proteins that control activin functions either intracellularly or extracellularly. As intracellular molecules, PDZ (PSD-95/Dlg/ZO-1) proteins that specifically associate with activin type II receptors (ActRIIs) were identified. We have named the molecules as ARIPs (activin receptor-interacting proteins). ARIP1 has two WW domains at its NH2 terminus and five PDZ domains at its COOH-terminus. ARIP1 associated with Smad proteins via WW domains and associated with ActRIIAs via PDZ domain 5. ARIP1 can recruite Smad 3 as well as binding to activin receptors, thus ARIP1 may be the scaffold of the activin-signaling complex in the cell membrane and in the submembranous region, and participate in a physiological regulation of signal transduction. In accord with this speculation, we have observed that overexpression of ARIP1 suppressed Smad 3-augmented transcription in a mouse hippocampus-derived cells and inhibited Smad 3 nuclear translocation by ligand stimulation. ARIPs are likely to have a role of assembling and/or transporting of activin receptors.As an extracellular regulatory protein, we have identified a novel follistatin-like protein, named FLRG (follistatin-related gene). Like follistatins, FLRG binds activins and BMPs (bone morphogenetic proteins) and controls their functions extracellularly. Interestingly, FLRG contains only two follistatin domains, and second follistatin domain including COOH-terminal acidic region is able to bind activin. This mode of association is different from that of follistatin-activin. Since follistatin and FLRG are expressed in a different manner and they are likely to be local regulators of activin and BMPs, their functions in vivo are likely to be different. Specific monoclonal antibodies recognizing mouse and human FLRG have been made to characterize the functions of FLRG further.