Pre-Golgi degradation of newly synthesized T-cell antigen receptor chains: intrinsic sensitivity and the role of subunit assembly

The T cell antigen receptor (TCR) is a multisubunit complex composed of at least seven transmembrane chains. The predominant species in most T cells has the composition alpha beta gamma delta epsilon zeta 2. The roles of subunit assembly in transport out of the ER and in the recently described process of pre-Golgi degradation of newly synthesized TCR chains were analyzed in a T-cell line deficient in the synthesis of delta chains (delta 2) and in COS-1 fibroblasts transfected with genes encoding individual TCR chains. Studies with the delta-deficient T-cell line showed that, in the absence of delta, the other TCR chains were synthesized at normal rates, but, instead of being transported to the cell surface, they were retained in the ER. Analysis of the fate of TCR chains retained in the ER showed that they were degraded at vastly different rates by a nonlysosomal pathway. Whereas the alpha chains were degraded rapidly, gamma, zeta, and epsilon were relatively long-lived. To analyze whether this selective degradation was because of different intrinsic susceptibility of the individual chains to degradation or to the formation of resistant oligomers, TCR chains were expressed alone or in combinations in COS-1 fibroblasts. These studies showed that (a) individual TCR chains were degraded at different rates when expressed alone in COS-1 cells, and (b) sensitive chains could be stabilized by coexpression with a resistant chain. Taken together, these observations indicate that both intrinsic sensitivity and subunit assembly play a role in determining the rates at which newly synthesized TCR chains are degraded in the ER.