Editors' ChoiceUbiquitination

Using Cysteine to Set the Stage for Its Own Degradation

Science's STKE  10 Apr 2007:
Vol. 2007, Issue 381, pp. tw124
DOI: 10.1126/stke.3812007tw124

The elimination of misfolded or misassembled proteins through endoplasmic reticulum-associated degradation (ERAD) depends on their polyubiquitination and proteolytic degradation. Thus, it is not surprising that ER stress in yeast leads to an increase in the abundance of the ubiquitin-conjugating enzyme Ubc7 as well as that of Cue1 (coupling of ubiquitin conjugation to ER degradation), which binds Ubc7 and targets it to the ER. Loss of Cue1 leads to Ubc7 mislocalization and, through a mechanism that has been unclear, to its subsequent degradation. Ravid and Hochstrasser found that overexpressed Ubc7 was rapidly degraded under conditions in which endogenous Ubc7 was stable. Degradation was blocked by the overexpression of either wild-type Cue1 or a mutant form that bound Ubc7 but did not associate with the ER. Pharmacological analysis indicated that Ubc7 degradation was mediated by the proteasome. Furthermore, analysis of yeast strains that lacked ubiquitination enzymes indicated that Ubc7 degradation depended on the E3 ligase Ufd4 but not on additional E2 ligases. Mutational analysis indicated that Ubc7 degradation depended on a cysteine residue in its active site (Cys89) but not on the presence of any of its lysine residues. The association of ubiquitin chains with Ubc7 was sensitive to the reducing agent dithiothreitol, which indicates that they were associated with cysteine rather than lysine; moreover, ubiquitin chains were absent from Ubc7 in which alanine was substituted for Cys89. Although Ufd4 was required for Ubc7 degradation, it was not necessary for its polyubiquitination. Thus, the authors conclude that Ubc7 degradation depends on its autoubiquitination on Cys89.

T. Ravid, M. Hochstrasser, Autoregulation of an E2 enzyme by ubiquitin-chain assembly on its catalytic residue. Nat. Cell Biol. 9, 422-427 (2007). [PubMed]