Component of the proteolytic cascade acting downstream of the 26S proteasome in the ubiquitin-proteasome pathway (By similarity). Efficiently cleaves Ala-Ala-Ala-polypeptide and Pro-Pro-Ala-polypeptide, Val-Leu-Lys-polypeptide only at high concentration. Does not cleave Ala-Phe-Pro-polypeptide nor Pro-Leu-Gly-polypeptide. Release of an N-terminal tripeptide from a polypeptide. Inhibited by phenylmethanesulfonyl fluoride (PMSF) and butabindide, but not by peptidase inhibitor pepstatin, EDTA, nor bestatin. Optimum pH is 7.5-7.8. Homooligomer; forms a complex of 6 MDa probably composed of 40 subunits. Forms a structure consisting of 2 segmented and twisted strands that form a spindle-shaped structure. Each strand is composed of 10 segments (a segment being a homodimer oriented head to head), stacking of these segments leads to the formation of a twisted single strand. 2 strands compose the fully assembled spindle. The limitation of proteolytic products to tripeptides is achieved by tailoring the size of the substrate-binding cleft: the two negatively charged residues Glu-399 and Glu-430 that are blocking position P4 limit the number of residues that can be accommodated in the binding cleft and thus create a molecular ruler. At the same time, they orient substrates so that the tripeptides are removed exclusively from the N-terminus (PubMed:20676100). Belongs to the peptidase S8 family.