Organization of glycosphingolipids in phosphatidylcholine bilayers: use of antibody molecules and Fab fragments as morphologic markers.

The techniques of ultrafast freezing and freeze-etch electron microscopy have been successfully employed to visualize IgG molecules and Fab fragments specifically bound to the neutral glycosphingolipids Forssman and asialo-GM1 incorporated into phosphatidylcholine liposomes. Monovalent Fab is the superior marker because of its small size and because it does not cause liposomal aggregation with concomitant glycolipid reorganization. Analysis of Fab labeling of liposomes containing these neutral glycosphingolipids leads to the conclusion that the Forssman glycosphingolipid is dispersed in clusters of not more than several molecules when present at low mole fraction in fluid-phase 1-palmitoyl-2-oleoylphosphatidylcholine liposomes. In contrast to this, asialo-GM1 under the same conditions is present in clusters of about 15 molecules in this phospholipid matrix.