Abstract
GFP-tagging is widely used as a molecular tool to localize and visualize the trafficking of proteins in cells but interpretation is frequently limited by the low resolution afforded by fluorescence light microscopy. Although complementary thin-section immunogold electron microscopic techniques go some way in aiding interpretation, major limitations, such as relatively poor structural preservation of membrane systems, low labelling efficiency and the two-dimensional nature of the images, remain. Here we demonstrate that the electron microscopic technique freeze-fracture replica immunogold labelling overcomes these disadvantages and can be used to define, at high resolution, the precise location of GFP-tagged proteins in specific membrane systems and organelles of the cell. Moreover, this technique provides information on the location of the protein within the phospholipid bilayer, potentially providing insight into mis-orientation of tagged proteins compared to their untagged counterparts. Complementary application of the freeze-fracture replica immunogold labelling technique alongside conventional fluorescence microscopy is seen as a novel and valuable approach to verification, clarification and extension of the data obtained using fluorescent-tagged proteins. The application of this approach is illustrated by new findings on PAT-family proteins tagged with GFP transfected into fibroblasts from patients with Niemann-Pick type C disease.