In vivo protein crystallization opens new routes in structural biology

Expression of a protein in Sf9 insect cells at high concentration triggers formation of in vivo crystals that can be analyzed by serial femtosecond X-ray crystallography. Protein crystallization in cells has been observed several times in nature. However, owing to their small size these crystals have not yet been used for X-ray crystallographic analysis. We prepared nano-sized in vivo–grown crystals of Trypanosoma brucei enzymes and applied the emerging method of free-electron laser-based serial femtosecond crystallography to record interpretable diffraction data. This combined approach will open new opportunities in structural systems biology.