BIOPHYSICAL CHARACTERIZATION OF THE INTERACTION OF BOVINE SEMINAL PLASMA-PROTEIN PDC-109 WITH PHOSPHOLIPID-VESICLES

PDC-109 is the major protein of bovine seminal plasma. It binds to the bovine sperm surface at ejaculation and modulates sperm capacitation. PDC-109 displays phosphorylcholine- and heparin-binding activities wh ich are thought to account for its sperm surface coating and glycosami noglycan-induced sperm capacitating activities, respectively. We have characterized the interaction of isolated PDC-109 with membranes of ph ospholipid vesicles using a biophysical approach. Our results show tha t PDC-109 interacts not only with the solvent-exposed phosphorylcholin e head group but also with the hydrophobic core of liposomes. Binding of PDC-109 to membranes is a very rapid, biphasic process with half ti mes of less than one second. Maximal binding of PDC-109 to small unila mellar vesicles was achieved with a stoichiometric ratio of 10-11 phos phatidylcholine molecules/PDC-109 molecule. Incorporation of phosphati dylethanolamine or phosphatidylserine into phosphatidylcholine vesicle s reduced the binding of PDC-109, suggesting that both the density of phosphorylcholine groups and the surface charge determine the interact ion of the seminal plasma protein with the surface of the membrane. El ectron spin resonance measurements showed that binding of PDC-109 to p hosphatidylcholine vesicles caused a rigidification of the membrane. T he relevance of the data for describing the role of PDC-109 in the mod ulation of sperm capacitation is discussed.