TCRζ 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 TCRζ were allowed to exit the ER in T cell variants in which the hexameric CD3γ ϵTiα βCD3δ ϵ complex was not formed. Interestingly, TCRζ 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 TCRζ‐negative cell line MA5.8, the hexameric CD3γ ϵTiα βCD3δ ϵ complex was allowed to exit the ER and was also predominantly located in the Golgi apparatus. However, neither hexameric TCR complexes nor TCRζ chains were efficiently expressed at the cell surface without the other. The observations that TCRζ 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.