Structural basis of E2-25K/UBB+1 interaction leading to proteasome inhibition and neurotoxicity.

E2-25K/Hip2 is an unusual ubiquitin-conjugating enzyme that interacts with the frameshift mutant of ubiquitin B (UBB(+1)) and has been identified as a crucial factor regulating amyloid-β neurotoxicity. To study the structural basis of the neurotoxicity mediated by the E2-25K-UBB(+1) interaction, we determined the three-dimensional structures of UBB(+1), E2-25K and the E2-25K/ubiquitin, and E2-25K/UBB(+1) complex. The structures revealed that ubiquitin or UBB(+1) is bound to E2-25K via the enzyme MGF motif and residues in α9 of the enzyme. Polyubiquitylation assays together with analyses of various E2-25K mutants showed that disrupting UBB(+1) binding markedly diminishes synthesis of neurotoxic UBB(+1)-anchored polyubiquitin. These results suggest that the interaction between E2-25K and UBB(+1) is critical for the synthesis and accumulation of UBB(+1)-anchored polyubiquitin, which results in proteasomal inhibition and neuronal cell death.