2 REDUNDANT-SYSTEMS MAINTAIN LEVELS OF RESIDENT PROTEINS WITHIN THE YEAST ENDOPLASMIC-RETICULUM

The Saccharomyces cerevisiae gene ERD2 is responsible for the retrieva l of lumenal resident proteins of the endoplasmic reticulum (ER) lost to the next secretory compartment. Previous studies have suggested tha t the retrieval of proteins by ERD2 is not essential. Here, we find th at ERD2-mediated retrieval is not an essential process only because, o n its failure, a second inducible system acts to maintain levels of ER proteins. The second system is controlled by the ER membrane-bound ki nase encoded by IRE1, We conclude that IRE1 and ERD2 together maintain normal concentrations of resident proteins within the ER.