During biogenesis secretory proteins are translocated into the first compartment of the secretory pathway, the endoplasmic reticulum (ER), through a channel formed by the Sec61 protein. Proteins which fail to fold in the ER lumen are toxic and need to be degraded. Degradation of these proteins is mediated by cytosolic proteasomes and requires protein transport across the ER membrane back into the cytosol. This retrograde transport is also dependent on the Sec61 protein, but the mechanism of retrograde protein unknown. – We asked whether proteasomes were actively involved in extracting misfolded proteins from the ER. We used a cell-free assay (based on yeast ER membranes, in vitro translated proteins and cytosol) that reconstitutes export of a non-glycosylated misfolded protein from the ER and proteasomal degradation. We found that in this assay, proteasomes could fully substitute for cytosol, and were sufficient to promote ER export and degradation of the substrate. We next investigated whether proteasomes were able to directly bind to protein translocation channels in the ER, similar to ribosomes during cotranslational protein import into the ER. We found that proteasomes bind to both yeast and mammalian ER membranes. Ribosomes and proteasomes competed with each other for ER binding, and proteoliposomes containing only Sec61 channels were sufficient for proteasome binding. ER membranes from two sec61 mutants, sec61-32 which is defective in protein export from the ER, and a new mutant, sec61-302, displayed reduced affinity for proteasomes. Binding of proteasomes to the channel was mediated by the 6 AAA-ATPases (Rpt proteins) in the base of the 19S regulatory particle of the proteasome. These have non-equivalent functions in proteasome-mediated protein turnover and form a hetero-hexameric ring. We purified 19S particles with equivalent point mutations in the ATP-binding sites of individual Rpt proteins; all rpt mutant 19S complexes were defective in binding to the ER, suggesting that ATP-binding to the hexameric AAA-ATPase complex of the 19S base is important for this interaction. Using cells expressing epitope-tagged proteasomes we asked if we could isolate protein export channels by solubilising ER membranes and affinity-purification of proteasomes and associated proteins. In addition to the Sec61 channel, we found the ER membrane protein Sec63p, the ER-lumenal Hsp70 BiP , and several subunits of the Hrd1p complex, which is required for ER-associated degradation of misfolded proteins with defects in the ER lumen, associated with ER-bound proteasomes. We conclude that proteasome 19S subparticles are actively involved in protein export from the ER and during export physically interact with the protein export channel in the ER membrane.