Homologous Superfamily

Protease propeptides/proteinase inhibitor I9 (IPR009020)

Short name: Peptidase/Inhibitor_I9


Proteinase propeptide inhibitors (sometimes refered to as activation peptides) are responsible for the modulation of folding and activity of the pro-enzyme or zymogen. The pro-segment docks into the enzyme moiety shielding the substrate binding site, thereby promoting inhibition of the enzyme. Several such propeptides share a similar topology [PMID: 12095256], despite often low sequence identities [PMID: 9811547]. The propeptide region has an open-sandwich antiparallel-alpha/antiparallel-beta fold, with two alpha-helices and four beta-strands with a (beta/alpha/beta)x2 topology.

This propeptide inhibitor domain occurs widely across all forms of life, and is found associated with the N-terminal region of a number of MEROPS peptidase families:

  • Metallopeptidase family M14A (carboxypeptidases).
  • These include carboxypeptidase A1, A2 [PMID: 9384570], A3, A4, A5, A6, U, insect gut carboxypeptidase and B [PMID: 12162965].

  • Serine peptidase family S8A (subtilisin family). Members of this group belong to MEROPS inhibitor family I9, clan I-.
  • Serine peptidase family S8B (kexin family).
  • The calcium-dependent serine peptidases belonging to MEROPS family S8B include the Kex2/subtilisin-like proprotein convertase (PC) family, which have been identified in all eukaryotes, these include furin, PC1/3, and PC2. The convertases are synthesised as an "inactive" precursor proteins or zymogens. Following the N-terminal signal peptide is the prodomain, consisting of between 80 to 115 residues; it is an integral part of the zymogen and acts as a steric chaperone to aid proper folding of the zymogen. An autocatalytic cleavage at the second dibasic site, R-X-K-R, liberates the prodomain, but which remains attached and acts to inhibit any further endopeptidase activity by binding to the catalytic domain. Inhibition is released when the maturing convertase is transported to the trans-Golgi network (TGN) where a decrease in pH causes a second autoproteolytic cleavage to occur at the first dibasic site within the prodomain fragment [PMID: 10842308].

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.