Selective labeling techniques of molecules and subcellular structures for cryo-electron tomography

Electron microscopy is one of the most effective methods for studying the fine structure of the cells with a resolution thousands of times higher than that of the visible light microscopy. The most advanced implementation of biological electron microscopy is EM tomography of samples stabilized by freezing without water crystallization (cryoET). By circumventing the drawbacks of chemical fixation and dehydration, this technique allows the investigation of cellular structures in three dimensions at molecular level, down to resolving the individual proteins and their sub-domains. However, the problem of effective identification and localization of the object of interest has not yet been solved, thus limiting the range of targets to easily recognizable or abundant subcellular components. Labeling techniques provide the only way for locating the subject of investigation in microscopic images. CryoET imposes conflicting demands on the labeling system. These requirements can be succinctly stated as the need to introduce particles, composed of substances, foreign to the cellular chemistry, into a living cell, conjugate it to the molecule of interest without disrupting its vital functions and the gross physiology of the cell. This review examines both the established and promising methods for selectively labeling of proteins and subcellular structures, enabling their localization in cryoET images.