Phase contrast cryo-electron microscopy

Phase contrast microscopy has been used in light microscopy for imaging unstained biological material for more than 80 years, opens up new possibilities in structural biology by cryo-electron microscopy (cryo-EM). Recently, “Volta phase plate cryo-EM of the small protein complex Prx3” was shown that it is feasible to obtain near atomic resolution of a very small protein complex (250 kDa) with phase contrast single particle analysis.

Images of unstained and frozen-hydrated biological material produce very little contrast, since such samples interact weakly with high-energy electrons. The only way to produce contrast in cryo-EM (until recently) was to intentionally defocus the image at the expense of altering high-resolution structural information. Despite the various efforts made in the development of EM phase plates, the technique was not readily applicable for cryo-EM, mainly due to the very short ‘life time’ of a phase plate and its complicated use.

However, we have developed the new generation of the thin film phase plate at Max-Planck Institute of Biochemistry in Martinsried, Germay, which succeeded in creating a device called the Volta phase plate (VPP) that nowadays is routinely used for data collection in cryo-EM and cryo-tomography. As a result, it creates contrast for observation of weak phase objects by boosting the low frequency information, without the need of defocusing.

Here, I will show how in-focus phase contrast cryo-EM enables single particle analysis of very challenging samples in terms of size, heterogeneity and flexibility that are difficult to solve by any other approach. Because of the high contrast it provides, VPP imaging could moreover become ideal tool for quick sample screening and/or initial model building. Moreover, I will present a recent published paper “Subtomogram analysis using the Volta phase plate” which shows the capability and contribution of this technique in the field of cryo-electron tomography at sub-nanometer resolution. This method could open a new pathway for cellular cryo-electron tomography to have better understanding about localization of macromolecular complex within a cell in their natural context.