Influence of the bovine seminal plasma protein PDC-109 on cholesterol in the presence of phospholipids.

The interaction of the major bovine seminal plasma protein PDC-109 with cholesterol was studied by employing spin-labelled analogues. It could be shown that PDC-109 does not interact directly with cholesterol molecules. However, in the presence of phospholipids we found a strong reduction of cholesterol motion by PDC-109. The fraction of immobilized cholesterol was largest for phosphorylcholine-containing lipids. This is consistent with the preferential interaction between PDC-109 and phosphatidylcholine. It is concluded that a stronger association and interaction of PDC-109 with phosphatidylcholine leads to an enhanced fraction of immobilized cholesterol analogues, but not to a phospholipid-dependent specific interaction between the protein and cholesterol. Moreover, the interaction of PDC-109 with various spin-labelled analogues of phosphatidylcholine (lysoPC, diacylPC) was investigated. In membranes of lipid vesicles the protein caused an immobilization of the phosphatidylcholine analogues mainly in the outer membrane leaflet, with no differences between diacylPC and lysoPC. The results are of relevance for understanding the physiological role of PDC-109 in the genesis of sperm cells.