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 ubiquitinB(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.