Hennecke S, Cosson P
Basel Institute for Immunology, Switzerland.
J Biol Chem. 1993 Dec 15;268(35):26607-12.
Previous studies have shown that CD8 can be present at the cell surface either as a disulfide-linked homodimer of CD8 alpha or as a disulfide-linked heterodimer of CD8 alpha and CD8 beta. Here we analyzed the assembly and intracellular transport of CD8 with particular emphasis on the role of the transmembrane domains. A chimeric protein (alpha T alpha) made by replacing the transmembrane domain of CD8 alpha with that of the interleukin-2 receptor alpha chain (Tac) exhibited reduced ability to form homodimers, while a mutant of Tac containing the CD8 alpha transmembrane domain (T alpha alpha) dimerized efficiently. Contrary to CD8 alpha, CD8 beta expressed alone was retained in the endoplasmic reticulum (ER). Only a small amount of CD8 beta formed homodimers, and these also remained in the ER. A mutant of CD8 beta that dimerized efficiently was also retained in the ER, thus proving that ER retention of CD8 beta is not due to its poor homodimerization. Rather, the extracellular domain of CD8 beta requires interaction with that of CD8 alpha to exit the ER. The transmembrane domain of CD8 beta was also shown to participate in ER retention by preventing exit of monomeric CD8 beta out of the ER. These findings demonstrate the role of transmembrane domains in assembly and intracellular transport of the CD8 molecule.
先前的研究表明，CD8可作为CD8α的二硫键连接的同二聚体或作为CD8α和CD8β的二硫键连接的异二聚体存在于细胞表面。在此，我们分析了CD8的组装和细胞内运输，特别强调了跨膜结构域的作用。通过用白细胞介素-2受体α链（Tac）的跨膜结构域替换CD8α的跨膜结构域而构建的嵌合蛋白（αTα）形成同二聚体的能力降低，而含有CD8α跨膜结构域的Tac突变体（Tαα）则能有效地二聚化。与CD8α相反，单独表达的CD8β保留在内质网（ER）中。只有少量的CD8β形成同二聚体，并且这些同二聚体也保留在ER中。一种能有效二聚化的CD8β突变体也保留在ER中，因此证明CD8β在内质网中的保留不是由于其同二聚化能力差。相反，CD8β的细胞外结构域需要与CD8α的细胞外结构域相互作用才能离开内质网。CD8β的跨膜结构域也被证明通过阻止单体CD8β离开内质网而参与内质网保留。这些发现证明了跨膜结构域在CD8分子组装和细胞内运输中的作用。
