Membrane insertion and lipid-protein interactions of bovine seminal plasmaprotein PDC-109 investigated by spin-label electron spin resonance spectroscopy

The interaction of the major acidic bovine seminal plasma protein, PDC-109, with dimyristoylphosphatidylcholine (DMPC) membranes has been investigated by spin-label electron spin resonance spectroscopy. Studies employing phos phatidylcholine spin labels, bearing the spin labels at different positions along the sn-2 acyl chain indicate that the protein penetrates into the hy drophobic interior of the membrane and interacts with the lipid acyl chains up to the 14th C atom. Binding of PDC-109 at high protein/lipid ratios (PD C-109:DMPC = 1:2, w/w) results in a considerable decrease in the chain segm ental mobility of the lipid as seen by spin-label electron spin resonance s pectroscopy. A further interesting new observation is that, at high concent rations, PDC-109 is capable of (partially) solubilizing DMPC bilayers. The selectivity of PDC-109 in its interaction with membrane lipids was investig ated by using different spin-labeled phospholipid and steroid probes in the DMPC host membrane. These studies indicate that the protein exhibits highe st selectivity for the choline phospholipids phosphatidylcholine and sphing omyelin under physiological conditions of pH and ionic strength. The select ivity for different lipids is in the following order: phosphatidylcholine a pproximate to sphingomyelin greater than or equal to phosphatidic acid (pH 6.0) > phosphatidylglycerol approximate to phosphatidylserine approximate t o androstanol > phosphatidylethanolamine greater than or equal to N-acyl ph osphatidylethanolamine much greater than cholestane. Thus, the lipids beari ng the phosphocholine moiety in the headgroup are clearly the lipids most s trongly recognized by PDC-109. However, these studies demonstrate that this protein also recognizes other lipids such as phosphatidylglycerol and the sterol androstanol, albeit with somewhat reduced affinity.