Freeze-fracture characterization of proteolipid protein and basic protein of central nervous system myelin incorporated in liposomes.

Proteolipid protein (PLP) and basic protein (BP) of central nervous system myelin were purified from calf brain white matter and incorporated in liposomes of L-dimyristoyl-alpha-phosphatidylcholine (DML) or in liposomes formed with an extract of natural lipids from myelin. Freeze-fracture replicas of the liposomes were prepared to study the number and size of intramembrane protein particles (IMP) in the fracture faces of the lipid bilayer. Globular and elongated IMP were observed in the freeze-fracture liposome membranes after incorporation of proteolipid protein. Globular IMP were the most frequently found (91-96% of the total IMP), and some of them showed a tiny black spot or pit on the top, suggesting the presence of hydrophilic channels in these particles. Globular and elongated IMP were also observed in the fractured membranes when basic protein was incorporated in liposomes. Again, globular IMP were the most frequent (92-95%) but no spots were present on the top. In addition, both globular and elongated IMP generated by basic protein were significantly larger than IMP generated by PLP. The proportion, size and form of globular and elongated particles generated by PLP and BP were unaffected by the amount of protein incorporated in liposomes (0.13-0.75 protein/lipid, w/w) nor by the type of lipid matrix used (DML or myelin natural lipid mixture). Intramembrane particles were absent from membranes of liposomes of pure lipid.