Until recently, the degradation of aberrant and unassembled proteins retained in 
the endoplasmic reticulum (ER) was thought to involve unidentified ER-localized 
proteases. We now show that the ER-associated degradation (ERAD) of two mutant 
proteins that accumulate in the ER lumen is inhibited in a proteasome-defective 
yeast strain and when cytosol from this mutant is used in an in vitro assay. In 
addition, ERAD is limited in vitro in the presence of the proteasome inhibitors, 
3,4-dichloroisocoumarin and lactacystin. Furthermore, we find that an ERAD 
substrate is exported from ER-derived microsomes, and the accumulation of 
exported substrate is 2-fold greater when proteasome mutant cytosol is used in 
place of wild-type cytosol. We conclude that lumenal ERAD substrates are 
exported from the yeast ER to the cytoplasm for degradation by the proteasome 
complex.