Figure 5. Freeze-fracture electron micrographs of membrane vesicles showing convex and concave surfaces. Left. Outer surface of membrane, fracture plane and view of inner leaflet of bilayer with membrane-embedded proteins appearing as bumps. Right. Outer leaflet of bilayer viewed from inside with pits where membrane proteins have been removed with the inner leaflet. Convex and concave surfaces always exhibit the same “texture”, and the convex surface is identical to the appearance of the cytoplasmic membrane in intact cells. Therefore, vesicles prepared by osmotic lysis retain same orientation as membrane in intact cell [i.e., they are right-side-out (RSO)]. This conclusion is supported quantitatively by binding studies with antibodies to proteins known to be on the cytoplasmic face of the membrane. By comparing intact and disrupted vesicles, no more than 2% of the vesicles prepared by osmotic lysis are either inverted or sufficiently leaky to allow antibody access to the cytoplasmic surface.
It is also possible to obtain vesicle preparations that are inside-out (ISO) However, ISO vesicles are about one-tenth the diameter of RSO vesicles.