TCRzeta is transported to and retained in the Golgi apparatus independently of other TCR chains: implications for TCR assembly.

It is generally assumed that TCR assembly occurs in the endoplasmic reticulum (ER), and ER retention/degradation signals have been identified in several of the TCR chains. These signals are probably responsible for retention of incompletely assembled TCR complexes and free TCR chains in the ER. This study focused on the intracellular localization and transport of partially assembled TCR complexes as determined by confocal microscopy analyses. We found that none of the TCR chains except for TCRzeta were allowed to exit the ER in T cell variants in which the hexameric CD3gammaepsilonTi alphabetaCD3 deltaepsilon complex was not formed. Interestingly, TCRzeta was exported from the ER independently of other TCR chains and was predominantly located in a compartment identified as the Golgi apparatus. Furthermore, in the TCRzeta-negative cell line MA5.8, the hexameric CD3gammaepsilonTi alphabetaCD3 deltaepsilon complex was allowed to exit the ER and was also predominantly located in the Golgi apparatus. However, neither hexameric TCR complexes nor TCRzeta chains were efficiently expressed at the cell surface without the other. The observations that TCRzeta and hexameric TCR complexes are transported from the ER to the Golgi apparatus independently of each other and that these partial TCR complexes are unable to be efficiently expressed at the cell surface suggest that final TCR assembly occurs in the Golgi apparatus.