Sequential Poly-ubiquitylation by Specialized Conjugating Enzymes Expands the Versatility of a Quality Control Ubiquitin Ligase

Abstract

SummaryThe Doa10 quality control ubiquitin (Ub) ligase labels proteins with uniform lysine 48-linked poly-Ub (K48-pUB) chains for proteasomal degradation. Processing of Doa10 substrates requires the activity of two Ub conjugating enzymes. Here we show that the non-canonical conjugating enzyme Ubc6 attaches single Ub molecules not only to lysines but also to hydroxylated amino acids. These Ub moieties serve as primers for subsequent poly-ubiquitylation by Ubc7. We propose that the evolutionary conserved propensity of Ubc6 to mount Ub on diverse amino acids augments the ubiquitylation sites within a substrate and thereby increases the target range of Doa10. Our work provides new insights on how the consecutive activity of two specialized conjugating enzymes facilitates the attachment of poly-Ub to very heterogeneous client molecules. Such stepwise ubiquitylation reactions most likely represent a more general cellular phenomenon that extends the versatility, yet sustains the specificity of the Ub conjugation system.

Quality Control of Protein Complex Assembly by a Transmembrane Recognition Factor

“…A similar priming followed by Ubc7-dependent extension was observed for Doa10-dependent ubiquitination. In this case, priming was mediated by the conjugating enzyme Ubc6 and was particularly important for the ubiquitination of lysine-poor substrates, as Ubc6 facilitates ubiquitin conjugation to lysine and hydroxylated residues (Weber et al, 2016).…”

Differential use of E2 ubiquitin conjugating enzymes for regulated degradation of the rate-limiting enzymes HMGCR and SQLE in cholesterol biosynthesis

“…However, interestingly, UBE2G2 KO cells also show a minor decrease in their ability to degrade SQLE in response to cholesterol. This may represent a vestigial function maintained in mammalian UBE2G2, as its yeast homolog Ubc7 acts as a promiscuous E2 ligase and interacts with gp78, Hrd1 and the yeast MARCH6-homologue Doa10 [20,33].…”

The evolving role of ubiquitin modification in endoplasmic reticulum-associated degradation

“…While Hrd1 functions in collaboration with a variety of other proteins, Doa10 acts predominantly with three different components in a ubiquitination complex: Ubc6, Ubc7, and Cue1 [102, 147, 148]. Recent data suggest that Ubc6 initially monoubiquitinates the substrate, whereas Ubc7 creates the polyubiquitin chain by adding subsequent ubiquitin moieties [149]. In contrast with Hrd1, which ubiquitinates substrates containing misfolded lesions in the ER lumen and ER membrane, Doa10 substrates contain a cytosolic lesion (ERAD-C) [101].…”