Elucidation of Functional Implication of Post-translational Tyrosine Suifation

现已知，酪氨酸硫化的翻译后蛋白质修饰在多细胞真核生物的蛋白质中广泛存在。鉴于已鉴定的绝大多数酪氨酸硫化(TyrS)蛋白是分泌型蛋白，而催化蛋白质酪氨酸硫化反应的酪氨酸硫基转移酶(TPST)位于高尔基体，对这种独特的蛋白质修饰的功能相关性的初步推测集中在蛋白质分泌方面。进一步阐述了酪氨酸硫化在将分泌蛋白包装成分泌颗粒中的假想作用。沿着同样的思路，我们一直在研究一种假定的TyrS受体的存在，该受体参与介导酪氨酸硫化蛋白的靶向和/或细胞内运输。利用亲和凝胶分级技术，我们在m…的胆酸钠中检测到一个175kDa的酪氨酸-O-硫酸盐结合蛋白牛肝和胰腺以及犬肝和胰腺的微体膜组分较多。Western印迹分析表明，牛肝酪氨酸结合蛋白在体内和体外均存在于酪氨酸-硫化蛋白的复合体中，提示前者可能是后者的受体。提出了一个假说模型，在MDCK细胞中，TyrS残基在酪氨酸硫化蛋白的生物合成运输过程中作为顶端靶向信号，由TyrS受体介导。TyrS蛋白被TyrS受体识别和结合，并被包装到从跨高尔基体膜室萌发出来的运输小泡中。运输囊泡与卷曲状的膜室融合，在膜室中，由于低pH环境可能通过ATPase/H+泵的作用，酪氨酸硫化蛋白与TyrS受体解离。携带游离TyrS蛋白的囊泡从卷曲状的膜室发芽，向质膜的顶端区域迁移。与顶端质膜融合后，酪氨酸硫化蛋白从顶端表面分泌出来。较少

Post-translational protein modification by tyrosine sulfation is now known to have a widespread occurrence among proteins of multicellular eukaiyotic organisms. I view of that the vast majority of tyrosine-sulfated (TyrS) proteins identified are secretory proteins and that the tyrosyiprotein sulfotransferase (TPST), which catalyzes the protein tyrosine sulfation reaction, is located in the Golgi, the initial speculation on the functional relevance of this unique protein modification was focused on the aspect of protein secretion. A hypothetical role of tyrosine sulfation in the packaging of secretory proteins into secretory granules was further elaborated. Along the same line, we have been investigating the presence of a putative TyrS receptor involved in mediating the targeting and/or intracellular transport of tyrosine-sulfated proteins. By employing the affinity gel fraction technique, we have detected a 175 kDa tyrosine-O-.sulfate-binding protein in sodium choleate etracts of the m … More icrosomal membrane fractions of bovine liver and pancreas, as well as canine liver and pancreas. Western blot analysis revealed the presence of the bovine liver TyrS-binding protein in complexes with tyrosine-sulfated proteins both 1.in vivo and in vitro, suggesting the putative role of the former being the receptor for the latter. A hypothetical model for TyrS residues serving as an apical targeting signal during the biosynthetic transport of tyrosine-sulfated proteins, as mediated by the TyrS receptor, in MDCK cells is proposed. TyrS proteins are recognized and bound by the TyrS receptor, and packaged into the transport vesicle budded off from the trans Golgi membrane compartment. The transport vesicle fuses with a CURL-like membrane compartment in which tyrosine-sulfated proteins become dissociated from the TyrS receptor due to the low pH environment presumably generated through the action of ATPase/H+ pump. The vesicle carrying free TyrS proteins buds off from the CURl-like membrane compartment and migrates toward the apical domain of the plasma membrane. Upon fusion with the apical plasma membrane, tyrosine-sulfated proteins become secreted from the apical surface. Less