Towards correlative super-resolution fluorescence and electron cryo-microscopy.

Correlative light and electron microscopy (CLEM) has become a powerful tool in life sciences. Particularly cryo-CLEM, the combination of fluorescence cryo-microscopy (cryo-FM) permitting for non-invasive specific multi-colour labelling, with electron cryo-microscopy (cryo-EM) providing the undisturbed structural context at a resolution down to the Ångstrom range, has enabled a broad range of new biological applications. Imaging rare structures or events in crowded environments, such as inside a cell, requires specific fluorescence-based information for guiding cryo-EM data acquisition and/or to verify the identity of the structure of interest. Furthermore, cryo-CLEM can provide information about the arrangement of specific proteins in the wider structural context of their native nano-environment. However, a major obstacle of cryo-CLEM currently hindering many biological applications is the large resolution gap between cryo-FM (typically in the range of ∼400 nm) and cryo-EM (single nanometre to the Ångstrom range). Very recently, first proof of concept experiments demonstrated the feasibility of super-resolution cryo-FM imaging and the correlation with cryo-EM. This opened the door towards super-resolution cryo-CLEM, and thus towards direct correlation of structural details from both imaging modalities.