The BSPH1 protein performs various roles in sperm function, primarily through binding of the sperm membrane. This is done through its Fn2 domains, which bind choline phospholipids on the membranes' surfaces.[8] Binding occurs in the epididymis prior to ejaculation, and acts to stabilize the sperm.
After ejaculation and entry of sperm into the oviduct, the protein facilitates capacitation, a key step in egg fertilization.[5] Here, it acts in conjunction with oviductal cholesterol acceptors, such as albumin and HDL, to remove cholesterol from the sperm membrane in a cholesterol efflux. With the loss of cholesterol, glycosylphosphatidylinositol-anchor proteins (GPI-APs) become destabilized, increasing membrane fluidity, resulting in the release of BSPH1.[9][10]
