Sorting of a multi-subunit ubiquitin ligase complex in the endolysosome system

Abstract

The yeast Dsc E3 ligase complex has long been recognized as a Golgi-specific protein ubquitination system. It shares a striking sequence similarity to the Hrd1 complex that plays critical roles in the ER-associated degradation pathway. Using biochemical purification and mass spectrometry, we identified two novel Dsc subunits, which we named as Gld1 and Vld1. Surprisingly, Gld1 and Vld1 do not coexist in the same complex. Instead, they compete with each other to form two functionally independent Dsc subcomplexes. The Vld1 subcomplex takes the AP3 pathway to reach the vacuole membrane, whereas the Gld1 subcomplex travels through the VPS pathway and is cycled between Golgi and endosomes by the retromer. Thus, instead of being Golgi-specific, the Dsc complex can regulate protein levels at three distinct organelles, namely Golgi, endosome, and vacuole. Our study provides a novel model of achieving multi-tasking for transmembrane ubiquitin ligases with interchangeable trafficking adaptors.

Endosome and Golgi‐associated degradation ( EGAD ) of membrane proteins regulates sphingolipid metabolism

“…Gld1 binding sorts the Dsc complex to the Golgi and to endosomes, while Vld1 diverts the Dsc complex from the Golgi via the AP‐3 pathway directly to the limiting membrane of the vacuole. In gld1Δ mutants, the Dsc complex is exclusively detected on the limiting membrane of vacuoles (Yang et al , ). Disrupting Gld1 resulted in GFP‐Orm2 accumulation at the ER and post‐ER compartments (Fig A, fully penetrant phenotype, n = 153 cells), similar to tul1∆ .…”

“…The deletion of Vld1 did not alter the ER localization of GFP‐Orm2 (Fig A). In vld1Δ mutants, the Dsc complex is no longer detected on the limiting membrane of the vacuole, but still localizes to Golgi and endosomes (Yang et al , ). These results suggested that the Gld1‐Dsc complex prevented the accumulation of Orm2 on endosomes and Golgi.…”

“…Among the best scoring candidates are Rnf122 and Rnf24, two little characterized transmembrane ubiquitin ligases that have been reported to localize to the ER and to the Golgi (Wang et al, 2006;Lussier et al, 2008). In budding yeast, the Dsc complex localizes to the Golgi, endosomes, and the limiting membrane of vacuoles (Reggiori & Pelham, 2002;Stewart et al, 2011;Li et al, 2015b;Yang et al, 2018), where it ubiquitinates its substrates. The Dsc substrates are thought to be exclusively degraded inside vacuoles via the ESCRT pathway (Reggiori & Pelham, 2002;Tong et al, 2014;Dobzinski et al, 2015;Li et al, 2015a;Yang et al, 2018).…”

“…In budding yeast, the Dsc complex localizes to the Golgi, endosomes, and the limiting membrane of vacuoles (Reggiori & Pelham, 2002;Stewart et al, 2011;Li et al, 2015b;Yang et al, 2018), where it ubiquitinates its substrates. The Dsc substrates are thought to be exclusively degraded inside vacuoles via the ESCRT pathway (Reggiori & Pelham, 2002;Tong et al, 2014;Dobzinski et al, 2015;Li et al, 2015a;Yang et al, 2018). It is difficult to rationalize the synthetic genetic interaction of vps4D with the Dsc complex, if the Dsc complex would only function upstream of the ESCRT machinery.…”

EGAD ! There is an ERAD doppelganger in the Golgi

“…This observation is at odds with work in budding yeast, where Tul1 substrates appeared to be degraded exclusively via the lysosome (Migliano & Teis, ). Moreover, a recent study (Yang et al , ) describing two mutually exclusive Dsc cofactors, Vld1 and Gld1, has defined Dsc complexes with distinct subcellular distribution: While Gld1‐containing Dsc complexes localize to Golgi/endosomes, Vld1‐containing complexes are found at the membrane of the vacuole, the yeast equivalent of the lysosome (Yang et al , ).…”