INCOMPLETE ENDOPLASMIC-RETICULUM (ER) RETENTION IN IMMATURE THYMOCYTES AS REVEALED BY SURFACE EXPRESSION OF ER-RESIDENT MOLECULAR CHAPERONES

The folding and assembly of nascent proteins in the endoplasmic reticu lum (ER) is assisted by molecular chaperones that are themselves retai ned within the ER. We now report that a number of different ER protein s, including molecular chaperones, are selectively expressed on the su rface of immature thymocytes, but their surface expression is extingui shed upon further differentiation. Escape from the ER is only possible for newly synthesized ER proteins before they become permanently reta ined. Thus, the cellular process of ER retention is incomplete in imma ture thymocytes and provides an explanation for surface expression of partial receptor complexes that transduce differentiative signals duri ng thymic development.