The 3.6 Å cryo-em structure of the outer heptameric α-ring of human 26s immunoproteasome in the preactivation state

The 26S proteasome is a unique multicatalytic proteinase complex, together with a ubiquitination system, providing controlled degradation of most intracellular eukaryotic proteins. The problem of studying the proteasome is the multiplicity of its intracellular forms, which are formed due to the modularity of the proteasome assembly process. In this study, using cryoelectron microscopy, we described for the first time the structure of the 26S human immunoproteasome in comparison with its constitutive form with a resolution of 3.6 Å. A detailed analysis of the structural features of the two complexes revealed the opening of the entrance in the outer heptameric 20S ring of the immunoproteasome subunit due to the separation of the N-terminal regions of the PSMA4 and PSMA5 subunits and the formation of a π–π stacking between the amino acid residues Tyr5 and Phe9 of the PSMA5 and PSMA6 subunits, respectively. The revealed removal of steric obstruction in the central channel of the 20S subunit may indicate the preactivation phenotype of the 26S human immunoproteasome, even in the absence of a bound substrate.