Type II domains of BSP-A1/-A2 proteins: binding properties, lipid efflux, and sperm capacitation potential.

Bovine seminal plasma contains a family of major proteins (collectively called 1BSP proteins) that potentiate sperm capacitation by binding to capacitation factors such as heparin and by stimulating sperm membrane cholesterol efflux. Here, we investigated the structure-function relationship of type II domains of BSP proteins. We isolated from a tryptic digest of citraconylated BSP-A1/-A2 proteins the intact second type II domain (domain b or Db). Similar to native protein, Db bound to heparin-Sepharose, p-aminophenylphosphorylcholine-Agarose and liposomes containing phosphatidylcholine. When assessed for biological function, Db did not stimulate cholesterol efflux from human fibroblasts, a cell model for lipid efflux studies, and from bovine spermatozoa, or potentiate bovine sperm capacitation induced by heparin and high-density lipoproteins. Therefore, type II motifs of BSP proteins represent binding units for sperm membrane choline phospholipids and heparin but the second type II domain of BSP-A1/-A2 alone is not sufficient to stimulate lipid efflux nor is sufficient to potentiate bovine sperm capacitation. Thus, the presence of both type II domains in BSP proteins is essential for the expression of functional properties, namely lipid efflux and sperm capacitation.