Interaction of rat serum phosphorylcholine-binding protein with phospholipid-containing liposomes

Rat serum phosphorylcholine-binding protein (PCBP) has been shown to inhibit the Ca2+-modulated heparin-lipoprotein precipitation reaction. This effect of PCBP on the reaction is prevented by phosphorylcholine (P-choline). A stoichiometric relationship between the serum very low density lipoproteins and PCBP was also evident in the heparin-very low density lipoprotein precipitation reaction (Nagpurkar, A., and Mookerjea, S. (1981) J. Biol. Chem. 256, 7440-7448). A study on the binding of PCBP to artificial liposomes was initiated to understand the mechanism of interaction between PCBP and phospholipids in soluble lipoproteins. Radioiodinated PCBP was incubated with multilamellar liposomes prepared with egg phosphatidylcholine (PC) and lysophosphatidylcholine, and it was found that the binding of PCBP to multilamellar liposomes was Ca2+-dependent and required the incorporation of about 25% lysophosphatidylcholine into the liposomes. Furthermore, the binding could be inhibited by addition of P-choline. The optimum concentrations of Ca2+ and liposomes as well as time and temperature required for binding were established. Analysis of Scatchard binding data yielded an association constant K alpha of 1.8 X 10(6) M-1 and a total binding capacity of 0.96 nmol/mumol of phospholipid. Substitution of P-choline head groups by phosphorylethanolamine and phosphorylserine on the PC of liposomes reduced the binding considerably, whereas the substitution of fatty acyl moieties on the PC of liposomes was without any effect. Bovine serum albumin was required in the assay to prevent artifactual binding of 125I-PCBP to the assay tubes. The results suggest that the P-choline groups on the surface of liposomes play an important role in the binding to PCBP and this may provide a possible explanation of the effect of PCBP on the Ca2+-dependent heparin-lipoprotein precipitation reaction.