Cytosolic tripeptidyl-peptidase that releases N-terminal tripeptides from polypeptides and is a component of the proteolytic cascade acting downstream of the 26S proteasome in the ubiquitin-proteasome pathway. It plays an important role in intracellular amino acid homeostasis (By similarity). Stimulates adipogenesis (PubMed:17932511). Release of an N-terminal tripeptide from a polypeptide. Translocates to the nucleus in response to gamma-irradiation. Expressed in the brain, skeletal muscle, gonadal and mesenteric white adipose tissue and brown adipose tissues. Homozygous mutant mice die in utero before embryonic day 9.0. Heterozygous mice display normal food intake but appear lean with a significant reduction in body fat, smaller adipocytes, decreased plasma insulin levels and less white adipose tissue in the gonad, groin and mesenteric regions. The limitation of proteolytic products to tripeptides is achieved by tailoring the size of the substrate-binding cleft: the two negatively charged residues Glu-305 and Glu-331 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 (By similarity). Belongs to the peptidase S8 family.