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 (via C-terminus) with SEMG1 (via 164-283 AA). Interacts with LTF. Found in a complex with LTF, CLU, EPPIN and SEMG1. Present on the surface of spermatozoa both before and after capacitation. Expressed in differentiated spermatogonia in testis. Expressed in spermatogonia cell lines GC-1 spg and GC-2spd(ts) as well as in the Leydig tumor cell line MLTC-1 (at protein level). Expressed specifically in epididymis and testis. Expressed predominantly on the postacrosomal region of mouse spermatozoa, in Sertoli cells, Leydig cells, and round spermatids in the testis, and in the principal cells of the cauda epididymidis epithelium. Androgen dependent. Down-regulated in Sertoli cell-selective androgen receptor knockout mice.