Interaction of C-reactive protein with artificial phosphatidylcholine bilayers and complement.

We have examined the interaction of C-reactive protein (CRP) with model membranes and complement. Binding of CRP to multilamellar liposomes or unilamellar vesicles of egg-phosphatidylcholine required the presence of lysophosphatide in the bilayer. The binding was Ca++-dependent, could be inhibited by phosphocholine, and resulted in activation of the classical complement pathway. A weak interaction between CRP and agarose was observed, which was also CA++-dependent and could be inhibited by phosphocholine and galactose. In addition, incorporation of galactocyl cerebroside in phosphatidylcholine:lysophosphatidylcholine liposomes enhanced the binding of CRP. Binding constants of 1.9 X 10(-5) M and 7.1 X 10(-5) M were calculated for liposomes containing and lacking the glycolipid, respectively. Furthermore, CRP bound to galactocyl cerebroside-containing liposomes bound approximately twice as much C1q as the same amount of CRP bound to liposomes lacking the glycolipid. We conclude that: 1) An alteration of the normal organization of phosphatidylcholine bilayers is necessary for binding of CRP. 2) The presence of galactosyl residues on the surface of the bilayer enhances the binding of CRP, perhaps through interaction with a putative secondary binding site on the protein.