Phospholipid bilayer deformations associated with interbilayer contact and fusion

Abstract

Bilayers incorporating phospholipids that are capable, in isolation, of forming non-bilayer structures show ‘pits and particles’ or ‘cusps’ by freeze-fracture electron microscopy^1–6. These points of very high curvature in the bilayer have been interpreted either as inverted micelles within a single bilayer^2,3 or intersecting bilayers^4,5, or as intermembrane sites of attachment^1,6. We have used extremely rapid freezing and cleavage at very low temperature to look at the cardiolipin–phosphatidylcholine system and report here a high incidence of cross-fractured liposomes, which shows that even at lower CaCl₂ concentrations they are filled with membranous structures. More interestingly, the arrays of pits and particles are closely aligned and associated with two further kinds of asymmetric bilayer deformation. Flat areas in the bilayer seem to represent sites of particularly adherent interbilayer contact and deep imaginations appear to represent sites where fusion with internal bilayers has occurred.