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. 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.