Cellular adaptation to proteotoxic stress at the endoplasmic reticulum (ER) depends on Lys48-linked polyubiquitination by ER-associated ubiquitin ligases (E3s) and subsequent elimination of ubiquitinated retrotranslocation products by the proteasome. The ER-associated E3 gp78 ubiquitinates misfolded proteins by transferring preformed Lys48-linked ubiquitin chains from the cognate E2 Ube2g2 to substrates. Here we demonstrate that Ube2g2 synthesizes linkage specific ubiquitin chains by forming an unprecedented homodimer: The dimerization of Ube2g2, mediated primarily by electrostatic interactions between two Ube2g2s, is also facilitated by the charged ubiquitin molecules. Mutagenesis studies show that Ube2g2 dimerization is required for ER-associated degradation (ERAD). In addition to E2 dimerization, we show that a highly conserved arginine residue in the donor Ube2g2 senses the presence of an aspartate in the acceptor ubiquitin to position only Lys48 of ubiquitin in proximity to the donor E2 active site. These results reveal an unanticipated mode of E2 self-association that allows the E2 to effectively engage two ubiquitins to specifically synthesize Lys48-linked ubiquitin chains.
Asymmetric interactions within an E2 enzyme homodimer facilitates ubiquitin positioning for assembly and subsequent en-bloc transfer of linkage-specific polyubiquitin chains.
- Ube2g2 forms a novel asymmetric dimer that drives linkage specific ubiquitin chain formation on its active site.
- The dimerization of Ube2g2 is facilitated by the charged ubiquitin molecules and required for its function in ERAD in the cell.
- The donor Ube2g2 uses an ion pairing mechanism to communicate with the acceptor ubiquitin.
- The electrostatic interaction between donor Ube2g2 and acceptor ubiquitin determines linkage specificity.