Component of the proteasome core, a large protease complex with broad specificity involved in protein degradation. The H.volcanii alpha1-beta proteasome is able to cleave oligopeptides after Phe, Tyr and Trp, poorly after Glu but not after Arg. Thus, displays chymotrypsin-like activity, low caspase-like activity but no trypsin-like activity. The formation of the proteasomal ATPase PAN-20S proteasome complex, via the docking of the C-termini of PAN into the intersubunit pockets in the alpha-rings, triggers 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 (By similarity). In vitro, the chymotrypsin-like activity of the alpha1-beta proteasome is potently inhibited by carbobenzoxyl-leucinyl-leucinyl-leucinal-H (MG132) and significantly by N-acetyl-leucinyl-leucinyl-norleucinal-H (calpain inhibitor I). Optimum pH is 7.0-9.3 for the Suc-LLVY-Amc hydrolyzing activity (with the alpha1-beta proteasome subtype). Optimum temperature is 75 degrees Celsius for the Suc-LLVY-Amc hydrolyzing activity (with the alpha1-beta proteasome subtype). 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. H.volcanii produces at least 2 types of 20S proteasomes: an alpha1-beta proteasome and a proteasome containing all three subunits (alpha1, alpha2, and beta) that appears to be asymmetrical with homo-oligomeric alpha1 and alpha2 rings positioned on separate ends. 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 at one or both ends by the proteasome regulatory ATPase, PAN. Up-regulated at the mRNA level during transition from exponential to stationary phase. However, at the protein level, PsmA 1 is expressed at a high and relatively constant level throughout growth. Acetylated. The acetylated form at Met-1 was shown to be in 100-fold excess of the unacetylated form with the initiator methionine removed in whole cells and purified 20S proteasomes. Strains lacking psmA1 gene alone display relatively normal growth rate and overall cell yield, but they are more sensitive to growth on organic versus inorganic nitrogen sources and hypo-osmotic stress, and they show limited growth in the presence of L-canavanine. Abolition of alpha1 subunit synthesis also has a severe impact on the ability of cells to withstand thermal stress. Moreover, depletion of psmA1 and psmA2 together renders the cells inviable. H.volcanii proteasome requires high concentrations of salt, similar to the extracellular environment and cytoplasm of this organism, for complex stability and optimal activity. Belongs to the peptidase T1A family.