Interaction of the cellular prion protein with raft-like lipid membranes

Conversion of the cellular isoform of the prion protein (PrPc) into the disease-associated isoform (PrPSc) plays a key role in the development of prion diseases. Within its cellular pathway, PrPc undergoes several posttranslational modifications, i.e., the attachment of two N-linked glycans and a glycosyl phosphatidyl inositol (GPI) anchor, by which it is linked to the plasma membrane on the exterior cell surface. To study the interaction of PrPc with model membranes, we purified posttranslationally modified PrPc from transgenic Chinese hamster ovary (CHO) cells. The mono-, di- and oligomeric states of PrPc free in solution were analyzed by analytical ultracentrifugation. The interaction of PrPc with model membranes was studied using both lipid vesicles in solution and lipid bilayers bound to a chip surface. The equilibrium and mechanism of PrPc association with the model membranes were analyzed by surface plasmon resonance. Depending on the degree of saturation of binding sites, the concentration of PrPc released from the membrane into aqueous solution was estimated at between 10(-9) and 10(-7) m. This corresponds to a free energy of the insertion reaction of -48 kJ/mol. Consequences for the conversion of PrPc to PrPSc are discussed.