Mutant ubiquitin UBB+1 is accumulated in sporadic inclusion-body myositis muscle fibers.

Mutant ubiquitin (UBB+1), a product of "molecular misreading," is toxic to cells because its ubiquitinated form inhibits the proteasome, contributing to accumulation of misfolded proteins and their ensuing toxicity. The authors demonstrate in 10 sporadic inclusion body myositis (s-IBM) muscle biopsies that UBB+1 is accumulated in aggregates containing amyloid-beta and phosphorylated-tau. In s-IBM, UBB+1 may be pathogenic by inhibiting proteasome, thereby promoting accumulation of cytotoxic misfolded amyloid-beta and phosphorylated-tau.