TYPE-II DOMAINS OF BSP-A1 -A2 PROTEINS - BINDING-PROPERTIES, LIPID EFFLUX, AND SPERM CAPACITATION POTENTIAL/

Bovine seminal plasma contains of family of major proteins (collective ly called (BSP)-B-1 proteins) that potentiate sperm capacitation by bi nding to capacitation factors such as heparin and by stimulating sperm membrane cholesterol efflux. Here, we investigated the structure-func tion relationship of type II domains of BSP proteins. We isolated from a tryptic digest of citraconylated BSP-A1/-A2 proteins the intact sec ond type II domain (domain b or Db). Similar to native protein, Db bou nd to heparin-Sepharose, p-aminophelayl-phosphorylcholine-Sepharose an d liposomes containing phosphatidylcholine. When assessed for biologic al function, Db did not stimulate cholesterol efflux from human fibrob lasts, a cell model for Lipid efflux studies, and from bovine spermato zoa, or potentiate bovine sperm capacitation induced by heparin and hi gh-density lipoproteins. Therefore, type II motifs of BSP problems rep resent binding units for sperm membrane choline phospholipids and hepa rin but the second type II domain of BSP-A1/-A2 alone is not sufficien t to stimulate Lipid efflux nor is sufficient to potentiate bovine spe rm capacitation. Thus, the presence of both type II domains in BSP pro teins is essential for the expression of functional properties, namely lipid efflux and sperm capacitation. (C) 1998 Academic Press.