Cryo-EM Reveals Unanchored M1-Ubiquitin Chain Binding at hRpn11 of the 26S Proteasome.

The 26S proteasome is specialized for regulated protein degradation and formed by a dynamic regulatory particle (RP) that caps a hollow cylindrical core particle (CP) where substrates are proteolyzed. Its diverse substrates unify as proteasome targets by ubiquitination. We used cryogenic electron microscopy (cryo-EM) to study how human 26S proteasome interacts with M1-linked hexaubiquitin (M1-Ub) unanchored to a substrate and E3 ubiquitin ligase E6AP/UBE3A.