The KDEL receptor mediates a retrieval mechanism that contributes to quality control at the endoplasmic reticulum

Newly synthesized proteins in the endoplasmic reticulum (ER) must fold and assemble correctly before being transported to their final cellular destina tion. While some misfolded or partially assembled proteins have been shown to exit the ER, they fail to escape the early secretory system entirely, be cause they are retrieved from post-ER compartments to the ER. We elucidate a mechanistic basis for this retrieval and characterize its contribution to ER quality control by studying the fate of the unassembled T-cell antigen receptor (TCR) alpha chain. While the steady-state distribution of TCR alph a is in the ER, inhibition of retrograde transport by COPI induces the accu mulation of TCR alpha in post-ER compartments, suggesting that TCR alpha is cycling between the ER and post-ER compartments. TCR alpha associates with BiP, a KDEL protein. Disruption of the ligand-binding function of the KDEL receptor releases TCR alpha from the early secretory system to the cell su rface, so that TCR alpha is no longer subject to ER degradation. Thus, our findings suggest that retrieval by the KDEL receptor contributes to mechani sms by which the ER monitors newly synthesized proteins for their proper di sposal.