Binder of sperm protein homolog 1 is a protein that in humans is encoded by the BSPH1 gene. It contains two fibronectin type II domains (Fn2 domains) linked in tandem by disulfide bonds. BSPH1 is a member of the BSP protein superfamily along with bovine PDC-109.
Function 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. 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. 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.
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