On the damage done to the structure of the Thermoplasma acidophilum proteasome by electron radiation

It has long been known that proteins are damaged when they are exposed to the electron beam in an electron microscope. Here we show that exposure to electrons under cryo‐EM conditions leads to a small change in the quaternary structure of the Thermoplasma acidophilum proteasome, and that backbones atoms belonging to the α‐helices in this molecule appear to be particular prone to chemical damage. A chemical mechanism is proposed for this damage. Both this local chemical effect and the more global quaternary structure effect appear to heterogenize samples leading to a radiation dose‐dependent degradation of the resolution of the EM maps obtained from this molecule.