Serine protease inhibitor that plays an essential role in male reproduction and fertility. Modulates the hydrolysis of SEMG1 by KLK3/PSA (a serine protease), provides antimicrobial protection for spermatozoa in the ejaculate coagulum, and binds SEMG1 thereby inhibiting sperm motility (By similarity). Monomer. Homodimer. Homomultimers. Interacts with SEMG1 (via 164-283 AA). Interacts with LTF. Found in a complex with LTF, CLU, EPPIN and SEMG1. Bound to the surface of testicular and on the head and tail of ejaculate spermatozoa.