TCR zeta 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 reticul um (ER), and ER retention/degradation signals have been identified in sever al 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 017 the intracellular localization and transport of partiall y assembled TCR complexes as determined by confocal microscopy analyses. We found that none of the TCR chains except for TCR zeta were allowed to exit the ER in T cell variants in which the hexameric CD3 gamma epsilon Ti alph a beta CD3 delta epsilon complex was not formed Interestingly, XCR zeta was exported from the ER independently of other TCR chains and was predominant ly located in a compartment identified as the Golgi apparatus. Furthermore, in the TCR zeta-negative cell line MA5.8, the hexameric CD3 gamma epsilon Ti alpha beta CD3 delta epsilon complex was allowed to exit the ER and was also predominantly located in the Golgi apparatus. However, neither hexamer ic TCR complexes nor TCR zeta chains were efficiently expressed at the cell surface without the other. The observations that TCR zeta and hexameric TC R complexes are transported from the ER to the Golgi apparatus independentl y of each other and that these partial TCR complexes are unable to be effic iently expressed at the cell surface suggest that final TCR assembly occurs in the Golgi apparatus.