Inner-nuclear-membrane–associated degradation employs Dfm1-independent retrotranslocation and alleviates misfolded transmembrane-protein toxicity

Before their delivery to and degradation by the 26S proteasome, misfolded transmembrane proteins of the endoplasmic reticulum (ER) and inner–nuclear membrane (INM) must be extracted from lipid bilayers. This extraction process, known as retrotranslocation, requires both quality-control E3 ubiquitin ligases and dislocation factors that diminish the energetic cost of dislodging the transmembrane segments of a protein. Recently, we showed that retrotranslocation of all ER transmembrane proteins requires the Dfm1 rhomboid pseudoprotease. However, we did not investigate whether Dfm1 also mediated retrotranslocation of transmembrane substrates in the INM, which is contiguous with the ER but functionally separated from it by nucleoporins. Here, we show that canonical retrotranslocation occurs during INM-associated degradation (INMAD) but proceeds independently of Dfm1. Despite this independence, ER-associated degradation (ERAD)-M and INMAD cooperate to mitigate proteotoxicity. We show a novel misfolded-transmembrane-protein toxicity that elicits genetic suppression, demonstrating the cell’s ability to tolerate a toxic burden of misfolded transmembrane proteins without functional INMAD or ERAD-M. This strikingly contrasted the suppression of the dfm1Δ null, which leads to the resumption of ERAD-M through HRD-complex remodeling. Thus, we conclude that INM retrotranslocation proceeds through a novel, private channel that can be studied by virtue of its role in alleviating membrane-associated proteotoxicity.