Thiol protease. Has dipeptidylpeptidase activity. Active against a broad range of dipeptide substrates composed of both polar and hydrophobic amino acids. Proline cannot occupy the P1 position and arginine cannot occupy the P2 position of the substrate. Can act as both an exopeptidase and endopeptidase. Activates serine proteases such as elastase, cathepsin G and granzymes A and B. Can also activate neuraminidase and factor XIII. Release of an N-terminal dipeptide, Xaa-Yaa-|-Zaa-, except when Xaa is Arg or Lys, or Yaa or Zaa is Pro. Binds 1 Cl(-) ion per heavy chain. Strongly inhibited by the cysteine peptidase inhibitors mersalyl acid, iodoacetic acid and cystatin. Inhibited by N-ethylmaleimide, Gly-Phe-diazomethane, TLCK, TPCK and, at low pH, by dithiodipyridine. Not inhibited by the serine peptidase inhibitor PMSF, the aminopeptidase inhibitor bestatin, or metal ion chelators. High activity at pH 4.5-6.8. Tetramer of heterotrimers consisting of exclusion domain, heavy- and light chains. Ubiquitous. Highly expressed in lung, kidney and placenta. Detected at intermediate levels in colon, small intestine, spleen and pancreas. Up-regulated in lymphocytes by IL2/interleukin-2. N-glycosylated. While glycosylation at Asn-53, Asn-119 and Asn-276 is mediated by STT3A-containing complexes, glycosylation at Asn-29 is mediated STT3B-containing complexes. In approximately 50% of the complexes the exclusion domain is cleaved at position 58 or 61. The two parts of the exclusion domain are held together by a disulfide bond. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase C1 family. Extended N-terminus. CTSC mutation db