Structure-function relationships for the IL 2-receptor system. IV. Analysis of the sequence and ligand-binding properties of soluble Tac protein.
Structure-function relationships for the IL 2-receptor system. IV. Analysis of the sequence and ligand-binding properties of soluble Tac protein.
复制标题
IL 2 受体系统的结构-功能关系。
作者：
R. Robb;R. Kutny
Tac蛋白是已知结合生长和分化因子白细胞介素2（IL 2）的至少两种糖蛋白之一。除了其在细胞表面上的位置，在那里它在高和低亲和力IL 2受体中起作用，Tac以可溶形式从活化的淋巴细胞中释放。我们观察到这种释放的Tac蛋白标记的生物合成和Tac蛋白标记的完整细胞的表面碘化。竞争结合研究表明，可溶性Tac蛋白保留了以低亲和力（Kd为11.1 nM）结合IL 2的能力。此外，结构分析表明，多肽链开始于位置1，并在全长分子的Cys-192处或之前结束。因此，该蛋白缺失其正常的跨膜和胞质内片段，这是其溶解性和细胞释放的原因。氨基酸序列中明显缺乏修饰和在Cys-192处终止与仅依赖于mRNA剪接的细胞释放机制不一致。相反，结果表明，蛋白水解可能伴随着可溶性Tac蛋白从表达IL 2受体的细胞中释放。
The Tac protein is one of at least two glycoproteins known to bind the growth and differentiation factor interleukin 2 (IL 2). In addition to its location on the cell surface, where it plays a part in high and low affinity IL 2 receptors, Tac is released from activated lymphocytes in a soluble form. We observed this release both for Tac protein labeled biosynthetically and for Tac protein labeled by surface iodination of intact cells. Competitive binding studies indicated that the soluble Tac protein retained an ability to bind IL 2 with a low affinity (Kd of 11.1 nM). In addition, structural analysis revealed that the polypeptide chain began at position 1 and ended at or just before Cys-192 of the full-length molecule. Thus, the protein was missing its normal transmembrane and intracytoplasmic segments, accounting for its solubility and cellular release. The apparent lack of modification in the amino acid sequence and the termination at Cys-192 are inconsistent with a mechanism of cellular release dependent only on alternate mRNA splicing. Instead, the results suggest that proteolysis may accompany the release of soluble Tac protein from cells expressing IL 2 receptors.