Ubiquitin ligase Hul5 is required for fragment-specific substrate degradation in endoplasmic reticulum-associated degradation

To identify new components of the protein quality control and degradation pathway of the endoplasmic reticulum (ER), we performed a growth-based genome-wide screen of about 5000 viable deletion mutants of the yeast Saccharomyces cerevisiae. As substrates we used two misfolded ER membrane proteins, CTL* and Sec61-2L, chimeric derivatives of the classical ER degradation substrates CPY* and Sec61-2. Both substrates contain a cytosolic Leu2 protein fusion, and stabilization of these substrates in ER-associated degradation-deficient strains enables a restored growth of the transformed LEU2-deficient deletion mutants. We identified the strain deleted for the ubiquitin chain elongating ligase Hul5 among the mutant strains with a strong growth phenotype. Here we show that Hul5 is necessary for the degradation of two misfolded ER membrane substrates. Although the degradation of their N-terminal parts is Hul5-independent, the breakdown of their C-terminal fragments requires the ubiquitin chain elongating ligase activity of Hul5. In the absence of Hul5, a truncated form of CTL* myc remains to a large extent embedded in the ER membrane. Hul5 activity promotes the interaction of this truncated CTL* myc with the AAA-ATPase Cdc48, which is known to pull proteins out of the ER membrane. This study unravels the stepwise elimination of the ER membrane-localized CTL* myc substrate. First, N-terminal, lumenal CPY* is transferred to the cytoplasm and degraded by the proteasome. Subsequently, the remaining C-terminal membrane-anchored part requires Hul5 for its effective extraction out of the endoplasmic reticulum and proteasomal degradation.