Kunitz domain found in the amyloid-beta precursor protein (ABPP) subfamily
This subfamily includes the amyloid-beta precursor protein (ABPP, also called APP, APPI, Alzheimer disease amyloid protein, amyloid-beta A4 protein, cerebral vascular amyloid peptide (CVAP), protease nexin II (PN2)), as well as amyloid-like protein 2 (APLP2, also called amyloid protein homolog or APPH), among others. ABPP/APPI is an inhibitor of serine proteases such as anionic and cationic trypsins. For example, APPI-4M is a variant that specifically inhibits Kallikrein (KLK)-related peptidase 6 (KLK6), which is highly upregulated in several types of cancer where its increased activity promotes cancer invasion and metastasis. Amyloid-like protein 2 (APLP2) inhibits trypsin, chymotrypsin, plasmin, factor XIA, and plasma and glandular kallikrein, and may play a role in the regulation of hemostasis. Proteins in this subfamily contain a single Kunitz domain, with a structure similar to those of other Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.
Jiyao W et al.(2023). "The conserved domain database in 2023.",