Biogenesis of secretory proteins requires their translocation into the endoplasmic reticulum (ER) through the Sec61 channel. Proteins that fail to fold are transported back into the cytosol and are degraded by proteasomes. For many substrates this retrograde transport is affected by mutations in the Sec61 channel, and can be promoted by ATP and the 19S regulatory particle of the proteasome, which binds directly to the Sec61 channel via its base. Here, we identify mutations in SEC61 which reduce proteasome binding to the channel, and demonstrate that proteasomes and ribosomes bind differently to cytosolic domains of the channel. We found that Sec63p and BiP coprecipitate with
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