The Power of Freeze-Fracture Electron Microscopy in Membrane Exploration

Abstract

Freeze-fracture transmission electron microscopy (ff-TEM) as a cryo-fixation, replica technique enables us to monitor highly dynamic nano-scale events in micro-scale biological as well as artificial membrane assemblies. Furthermore, freeze-fracturing allows a unique look at the interior of cell membranes because the fracture plane follows the area of weakest forces and splitting membrane bilayers into two opposite, complementary sheets. Therefore it is still the only way to obtain detailed structural information about lateral inhomogeneities in cell as well as model membranes in a probe-free mode. Applying this unique technique we are able to study formation and transformation of lipid-, protein-, toxin-, as well as drug micro domains. Examples will be given for lipid domains in liposomal bilayer and their transformation by temperature, incorporation of amphiphilic drugs, and proteins. Furthermore, domain initiation by toxin and phage protein will be shown. Lipid-induced modulation of 2-D crystals of bacteriorhodopsin in liposomal bilayer demonstrates an extreme example for domain formation of membrane-spanning proteins. Additionally to domain explorations, we were able to study unique membranes made of bola-type Archaebacteria lipids and to identify transitions between their horseshoe- and membrane-spanning arrangements. Furthermore applying this unique TEM technique, we investigated transformations to non-bilayer structures such as hexagonal and cubic phases.