Component of the proteasome core, a large protease complex with broad specificity involved in protein degradation. The R.erythropolis proteasomes are able to cleave oligopeptides after Tyr, Phe and Leu, very poorly after Arg but not after Glu. Thus, displays chymotrypsin-like activity, low trypsin-like activity but no caspase-like activity. The formation of the proteasomal ATPase ARC-20S proteasome complex, likely via the docking of the C-termini of ARC into the intersubunit pockets in the alpha-rings, may trigger opening of the gate for substrate entry. Interconversion between the open-gate and close-gate conformations leads to a dynamic regulation of the 20S proteasome proteolysis activity. The Vmax observed with the beta2-alpha1 proteasome subtype is 2.2-fold, 1.2-fold and 4-fold higher than that with the beta2-alpha2, beta1-alpha2 and beta1-alpha1 subtypes, respectively. Protein degradation; proteasomal Pup-dependent pathway. The 20S proteasome core is composed of 14 alpha and 14 beta subunits that assemble into four stacked heptameric rings, resulting in a barrel-shaped structure. The two inner rings, each composed of seven catalytic beta subunits, are sandwiched by two outer rings, each composed of seven alpha subunits. All four combinations of alpha- and beta-subunits (beta2-alpha1, beta2-alpha2, beta1-alpha2 and beta1-alpha1) yield fully assembled and proteolytically active proteasomes. The catalytic chamber with the active sites is on the inside of the barrel. Has probably a gated structure, the ends of the cylinder being occluded by the N-termini of the alpha-subunits. Is likely capped by the proteasome-associated ATPase, ARC. The N-terminus is blocked. Belongs to the peptidase T1A family.