Objective: The 26S proteasome is a unique multicatalytic proteinase complex, which, together with the ubiquitination system, provides controlled degradation of most intracellular eukaryotic proteins. The main obstacle of studying the proteasome is its multiple intracellular forms formed due to the modularity of the proteasome assembly process and its different catalytic phenotypes. Methods: In the present study we report the first cryo-electron microscopy (cryo-EM) structure of the 26S human immunoproteasome in comparison with its constitutive form at 3.6 Å resolution. Results and Discussion: Detailed analysis of the structural features of the two complexes revealed an unveiled entrance in the outer heptameric 20S a-ring of the 26S immunoproteasome in comparison with constitutive 26S, arose due to the disjunction of the N-terminal regions of the PSMA4 and PSMA5 subunits and emerged π–π stacking interaction between the Tyr5 and Phe9 amino acid residues of the PSMA5 and PSMA6 subunits, respectively. Conclusions: We suggest that observed abolition of steric hindrance in the central channel of the 20S particle may indicate the preactivation phenotype of the human 26S immunoproteasome, even in the absence of a bound substrate.