soybean trypsin inhibitor (STI)-like domain, beta-trefoil fold, found in Solanum tuberosum cysteine protease inhibitor (CPI), serine protease inhibitor (SPI), aspartic protease inhibitor (API) and similar proteins
This subfamily includes Solanum tuberosum CPI, SPI, API, and similar proteins. CPI is a Kunitz-type potato cathepsin D inhibitor. It acts as a potent inhibitor of cathepsin l (cysteine protease) but does not inhibit trypsin or chymotrypsin (serine proteases). SPI is a potent inhibitor of serine proteases (chymotrypsin and trypsin). It inhibits tightly human leukocyte elastase (HLE). It does not inhibit papain, pepsin nor cathepsin D (cysteine and aspartic proteases). API functions as the inhibitor of cathepsin D (aspartic protease). It may also inhibit trypsin and chymotrypsin (serine proteases). CPI, SPI, and API protect plants by inhibiting proteases of invading organisms. Members of this subfamily contain an STI-like domain with a beta-trefoil fold, which is characterized by 12 beta strands folded into three similar trefoil subdomains (alpha, beta, and gamma) associated to give an overall structure with pseudo-3-fold symmetry.
Jiyao W et al.(2023). "The conserved domain database in 2023.",