Abstract
Layers of quality control ensure proper protein folding and complex formation prior to exit from the endoplasmic reticulum. The fission yeast Dsc E3 ligase is a Golgi-localized complex required for sterol regulatory element-binding protein (SREBP) transcription factor activation that shows architectural similarity to endoplasmic reticulum-associated degradation E3 ligases. The Dsc E3 ligase consists of five integral membrane proteins (Dsc1-Dsc5) and functionally interacts with the conserved AAA-ATPase Cdc48. Utilizing an in vitro ubiquitination assay, we demonstrated that Dsc1 has ubiquitin E3 ligase activity that requires the E2 ubiquitin-conjugating enzyme Ubc4. Mutations that specifically block Dsc1-Ubc4 interaction prevent SREBP cleavage, indicating that SREBP activation requires Dsc E3 ligase activity. Surprisingly, Golgi localization of the Dsc E3 ligase complex also requires Dsc1 E3 ligase activity. Analysis of Dsc E3 ligase complex formation, glycosylation, and localization indicated that Dsc1 E3 ligase activity is specifically required for endoplasmic reticulum exit of the complex. These results define enzyme activity-dependent sorting as an autoregulatory mechanism for protein trafficking.
Highlights
Proteolytic activation of fungal sterol regulatory element-binding protein (SREBP) requires the five-subunit Golgi Dsc E3 ligase
Purified Dsc1 RING domain fused to GST was incubated with ubiquitin-activating enzyme E1 (Uba1) and cognate ubiquitinconjugating enzyme E2 (Ubc4) (Fig. 1C)
Multiple lines of evidence demonstrate that Dsc1 E3 ligase activity is required for ER exit of the Dsc complex, ensuring that only fully functional Dsc E3 ligase moves to the Golgi where it acts in SREBP activation
Summary
Proteolytic activation of fungal SREBP requires the five-subunit Golgi Dsc E3 ligase. The fission yeast Dsc E3 ligase is a Golgi-localized complex required for sterol regulatory element-binding protein (SREBP) transcription factor activation that shows architectural similarity to endoplasmic reticulum-associated degradation E3 ligases. Analysis of Dsc E3 ligase complex formation, glycosylation, and localization indicated that Dsc E3 ligase activity is required for endoplasmic reticulum exit of the complex. These results define enzyme activity-dependent sorting as an autoregulatory mechanism for protein trafficking. We demonstrate that Dsc is a functional E3 ligase and that Dsc E3 ligase activity is required for ER exit of the ligase complex, establishing enzyme activity-dependent sorting as a new mechanism for protein trafficking
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