The selective recognition and degradation of mislocalized secretory and membrane proteins has been reconstituted in vitro. In this system, secretory and membrane proteins are synthesized in their non-translocated state simply by omitting ER-derived rough microsomes from the reaction or, if they are included, by inhibiting translocation using cotransin. With either of these manipulations, non-translocated proteins are rapidly ubiquitinated in preparation for their degradation. We are employing both classical fractionation and affinity purification approaches to identify the machinery for the selective recognition and degradation of these non-translocated proteins.