The biogenesis of secreted and resident proteins of the secretory pathway is an essential cellular function of all organisms. As a highly regulated process, the cell has evolved mechanisms to assure only properly folded and assembled proteins reach their ultimate destinations. This 'quality control mechanism' retains aberrant proteins at the site of synthesis, the endoplasmic reticulum (ER), and targets them for degradation. The importance of faithfully regulating protein biosynthesis is underscored by the numerous human diseases including Alzheimer's, Cystic Fibrosis, Huntington's, Creutzfeldt-Jakob, and familial insomnia caused by aberrant protein conformation. The long-term objectives of this research are to understand the regulation and mechanisms governing secretory protein biogenesis. This project focuses on a novel cellular mechanism required to tolerate aberrant proteins until they can be degraded. This is an active mechanism requiring activation of an intracellular signal transduction pathway known as the unfolded protein response. Surprisingly, the tolerance mechanism is functional in the absence of ER protein degradation making both facets of ER quality control particularly amenable for investigation. The primary goals of the project include 1) using genetic and biochemical approaches to identify genes required for tolerance, 2) purifying and characterizing aberrant protein complexes from tolerant and intolerant cells, 3) ultrastructural analysis of cells expressing aberrant proteins to precisely localize these complexes and study their morphological consequences in tolerant and intolerant cells, 4) defining physiological conditions requiring a tolerance function, and 5) isolating novel ER protein degradation mutants and their genes with an emphasis on common factors to establish a genetic linkage to tolerance. The simple eukaryote Saccharomyces cerevisiae used for the studies is particularly well-suited. It provides a combination of powerful genetic and biochemical approaches necessary to gain a comprehensive under- standing of the mechanisms underlying the detoxification and degradation of aberrant proteins.