Interaction of the major protein from bovine seminal plasma, PDC-109 with phospholipid membranes and soluble ligands investigated by fluorescence approaches

Abstract

The major protein from bovine seminal plasma, PDC-109 binds selectively to choline phospholipids on the sperm plasma membrane and plays a crucial role in priming spermatozoa for fertilization. The microenvironment and accessibility of tryptophans of PDC-109 in the native state, in the presence of phosphorylcholine (PrC) and phospholipid membranes as well as upon denaturation have been investigated by fluorescence approaches. Quenching of the protein intrinsic fluorescence by different quenchers decreased in the order: acrylamide > succinimide≫ Cs⁺ > I⁻. Ligand binding afforded considerable protection from quenching, with shielding efficiencies following the order: dimyristoylphosphatidylcholine (DMPC) > lysophosphatidylcholine (Lyso-PC) > PrC. This has been attributed to a partial penetration of the protein into the DMPC membranes and Lyso-PC micelles, as well as a further stabilization of the binding due to the interaction of PDC-109 with lipid acyl chains and the resulting tightening of the protein structure, leading to a decreased accessibility of the tryptophan residues. Red-edge excitation shift (REES) studies yielded REES values of 4 nm for both native and denatured PDC-109, whereas reduced and denatured protein gave a REES of only 0.5 nm, clearly indicating that the structural and dynamic features of the microenvironment around the tryptophan residues are retained even after denaturation, presumably due to the constraints imposed on the protein structure by disulfide bonds. Upon binding of PDC-109 to DMPC membranes and Lyso-PC micelles the REES values were reduced to 2.5 and 1.0 nm, respectively, which could be due to the penetration of some parts of the protein, especially the segment containing Trp-90 into the membrane interior, where the red-edge effects are considerably reduced.