The role of multiubiquitination in dislocation and degradation of the α subunit of the T cell antigen receptor

Unassembled alpha subunits of the T cell receptor (TCR alpha) are degraded by proteasomes following their dislocation from the endoplasmic reticulum membrane. We previously demonstrated that a variant of TCR alpha lacking lysines (K alpha R) is degraded by this pathway with kinetics indistinguishable from those of the wild type protein (Yu, H., Kaung, G., Kobayashi, S., and Kopito, R. R. (1997) J. Biol. Chem. 272, 20800-20804), demonstrating that ubiquitination on lysines is not required for TCR alpha degradation by the proteasome. Here, we show that dislocation and degradation of TCR alpha and K alpha R are sup pressed by dominant negative ubiquitin coexpression and by mutations in the ubiquitin activating enzyme, indicating that their degradation requires a functional ubiquitin pathway. A cytoplasmic TCR alpha variant that mimics a dislocated degradation intermediate was degraded 5 times more rapidly than full-length TCR alpha, suggesting that dislocation from the endoplasmic reticulum membrane is the rate-limiting step in TCR alpha degradation. We conclude that ubiquitination is required both for dislocation and for targeting TCR alpha chains to the proteasome.