On-grid labeling method for freeze-fracture replicas

Sodium dodecyl sulfate-treated freeze-fracture replica labeling (SDS-FRL) is an electron microscopic (EM) method that can define the two-dimensional distribution of membrane proteins and lipids in a quantitative manner. Despite its unsurpassed merit, SDS-FRL has been adopted in a limited number of labs, probably because it requires a laborious labeling process as well as equipment and technique for freeze-fracture. Here, we present a method that reduces the manual labor significantly by mounting freeze-fracture replicas on EM grids prior to labeling. This was made possible by the discovery that freeze-fracture replicas invariably adhere to the carbon-coated formvar membrane with their platinum-carbon side, ensuring that the membrane molecules retained in replicas are accessible to labeling solutions. The replicas mounted on EM grids can be stored dry until labeling, checked by light microscopy before labeling, and labeled in the same manner as tissue sections. This on-grid method will make SDS-FRL easier to access for many researchers.