Kunitz domain of epididymal protease inhibitor eppin and similar proteins
This subfamily includes the Kunitz inhibitor domain protein eppin (also called Cancer/testis antigen 71 or CT71, epididymal protease inhibitor, protease inhibitor WAP7, serine protease inhibitor-like with Kunitz and WAP domains 1, or WAP four-disulfide core domain protein 7) as well as WAP four-disulfide core domain proteins 6A and 6B in mice, and similar proteins. Eppin is a serine protease inhibitor that plays an essential role in male reproduction and fertility. It modulates the hydrolysis of seminal fluid protein semenogelin 1 (SEMG1) by the serine protease kallikrein-related peptidase 3 (KLK3, PSA), provides antimicrobial protection for spermatozoa in the ejaculate coagulum, and binds SEMG1, thereby inhibiting sperm motility. Thus, eppin could potentially be used as a target for male contraception. These domains are similar to Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.
Jiyao W et al.(2023). "The conserved domain database in 2023.",