Intracellular protein crystallization in living insect cells

Crystallization of recombinant proteins in living cells is an emerging approach complementing conventional crystallization techniques. Homogeneous microcrystals well suited for serial diffraction experiments at X-ray free-electron lasers and synchrotron sources can be produced in a quasi-native environment, without the need for target protein purification. Several protein structures have already been solved; however, exploiting the full potential of this approach requires a systematic and versatile screening strategy for intracellular crystal growth. Recently, we published InCellCryst, a streamlined pipeline for producing microcrystals within living insect cells. Here, we present the detailed protocol, including optimized target gene expression using a baculovirus vector system, crystal formation, detection, and serial X-ray diffraction directly in the cells. The specific environment within the different cellular compartments acts as a screening parameter to maximize the probability of crystal growth. If successful, diffraction data can be collected 24 days after the start of target gene cloning.