Degradation of proteins that are retained in the quality control apparatus of the endoplasmic reticulum (ER) has been attributed to a third proteolytic system, distinct from the lysosomal and the cytoplasmic ubiquitindependent proteosomal proteolytic pathways. However, several recent studios have shown that ER degradation of a mutant membrane protein, CFTRΔF508, is at least in part mediated from the cytoplasmic side by the 26 S proteasome. In this study, we examined the possibility that ER degradation of mutant secretory protein α 1-antitrypsin (α 1-AT) Z, the mutant protein associated with infantile liver disease and adult-onset emphysema of α 1-AT deficiency, is mediated by the proteasome. The results show that a specific proteasome inhibitor, lactacystin, inhibits ER degradation of α 1-ATZ in transfected human fibroblast cell lines and in a cell-free microspinal translocation system. Although it is relatively easy to conceptualize how a transmembrane protein like CFTRAF508 might be accessible on the cytoplasmic aspect of the ER membrane for ubiquitination and degradation by the proteasome, it is more difficult to conceptualize how this might occur for a luminal polypeptide. The results show that, once within the lumen of the ER, α 1-ATZ interacts with the transmembrane molecular chaperone calnexin and specifically induces the polyubiquitination of calnexin. The results, therefore, provide evidence that the proteasome, from its cytoplasmic localization, induces the degradation of the luminal α 1-ATZ molecule by first attacking the cytoplasmic tail of calnexin molecules that are associated with α 1-ATZ.