Role of transmembrane domains in assembly and intracellular transport of the CD8 molecule.

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.