Dfm1 forms distinct complexes with Cdc48 and the ER ubiquitin ligases and is required for ERAD.

Proteins imported into the endoplasmic reticulum (ER) are scanned for their folding status. Those that do not reach their native conformation are degraded via the ubiquitin-proteasome system. This process is called ER-associated degradation (ERAD). Der1 is known to be one of the components required for efficient degradation of soluble ERAD substrates like CPY(*) (mutated carboxypeptidase yscY). A homologue of Der1 exists, named Dfm1. No function of Dfm1 has been discovered, although a C-terminally hemagglutinin (HA)(3)-tagged Dfm1 protein has been shown to interact with the ERAD machinery. In our studies, we found Dfm1-HA(3) to be an ERAD substrate and therefore not suitable for functional studies of Dfm1 in ERAD. We found cellular, non-tagged Dfm1 to be a stable protein. We identified Dfm1 to be part of complexes which contain the ERAD-L ligase Hrd1/Der3 and Der1 as well as the ERAD-C ligase Doa10. In addition, ERAD of Ste6(*)-HA(3) was strongly dependent on Dfm1. Interestingly, Dfm1 forms a complex with the AAA-ATPase Cdc48 in a strain lacking the Cdc48 membrane-recruiting component Ubx2. This complex does not contain the ubiquitin ligases Hrd1/Der3 and Doa10. The existence of such a complex might point to an additional function of Dfm1 independent from ERAD.

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