An emerging role for thioester‐linked polyubiquitin chains in protein degradation

Abstract

The mechanisms of ubiquitin chain assembly are incompletely understood. The classical model proposes that ubiquitin is first added to a lysine residue within a substrate and then to the distal end of the growing ubiquitin chain. This model becomes less intuitively appealing when applied to substrates with long chains, because the newly added ubiquitin becomes structurally remote from original substrate. One hypothesis that tackles this structural dilemma proposes that ubiquitin chains are pre‐assembled on the catalytic cysteine of an E2 enzyme, prior to transfer onto a substrate. However, until recently no experimental evidence supported the existence of cysteine‐linked polyubiquitin chains in living cells.We found that Ubc7, a yeast E2 enzyme of the endoplasmic reticulum (ER) protein quality control system, is polyubiquitylated exclusively on its catalytic cysteine. The polyubiquitin polymer can then be transferred to a nearby lysine side chain. Moreover, thioester‐linked polybiquitylated Ubc7 undergoes proteasomal degradation when its levels exceed that of its binding partner Cue1, a transmembrane protein that tethers Ubc7 to the ER membrane. Unassembled, and thus mislocalized, Ubc7 is targeted to the proteasome by Ufd4, a HECT‐class E3 ligase. This ensures that no Ubc7 activity persists beyond the ER. Thus, cysteine‐linked polyubiquitin chains can function as an additional degradation signal.