Interaction of bovine seminal plasma proteins with model membranes and sperm plasma membranes

Abstract

PDC-109 is the major protein of the bovine seminal plasma, which binds to the plasma membrane of spermatozoa upon ejaculation and plays a critical role in priming the sperm cells for fertilization to take place. Experimental studies from the last 15 years have shown that the cell-surface receptor for PDC-109 is a choline phospholipid and shed light on the molecular details of the interaction of this protein with model membranes containing phosphatidylcholine. PDC-109 contains two fibronectin type 2 domains and each domain binds one ligand molecule. Single crystal X-ray diffraction studies have shown that the specific binding of the soluble head group moiety of the choline phospholipids, phosphorylcholine, with PDC-109 is mediated by a cation–p interaction of the quaternary ammonium group with the indole side chain of a core tryptophan residue. The binding is further stabilized by hydrogen bonds of the phosphate group with the side chains of different amino acid residues or with the main chain amide groups of several residues. Spinlabel ESR studies have shown that although PDC-109 does not bind to membranes lacking a choline-containing lipid, it does exhibit some, albeit weaker, interaction with other phospholipids such as phosphatidylglycerol and phosphatidylserine as well as cholesterol. Surface plasmon resonance studies on the mechanism of interaction with phosphatidylcholine membranes has shown that the tight binding of PDC-109 is characterized by a very slow dissociation process. Analysis of the activation parameters obtained from the SPR studies indicated that binding of PDC-109 to phosphatidylcholine membranes is favoured by a strong entropic contribution, whereas negative entropic contribution is primarily responsible for the rather weak interaction of this protein with phosphatidic acid and phosphatidylglycerol.These observations are discussed in the light of the role of PDC-109 in cholesterol efflux from sperm plasma membranes, which is a necessary event before capacitation, and subsequently fertilization, can take place.