In vivo crystallography at X-ray freeelectron lasers: The next generation of structural biology?

The serendipitous discoveryof the spontaneous growth of protein crystals inside cells has opened the field of crystallography to chemically unmodified samples directly available from their natural environment. On the one hand, through in vivo crystallography, protocols for protein crystal preparation can be highly simplified, although the technique suffers from difficulties in sampling, particularly in the extraction of the crystals fromthe cells partly due to their small sizes. On the other hand, the extremely intense X-ray pulses emerging fromX-ray freeelectron laser (XFEL) sources, along with the appearance of serial femtosecond crystallography (SFX) is a milestone for radiation damage-free protein structural studies but requires micrometre-size crystals. The combination of SFX with in vivo crystallography has the potential to boost the applicability of these techniques, eventually bringing the field to the point where in vitro sample manipulations will no longer be required, and direct imaging of the crystals from within the cells will be achievable. To fully appreciate the diverse aspects of sample characterization, handling and analysis, SFX experiments at the Japanese SPring-8 angstromcompact free-electron laserwere scheduled on various types of in vivo grown crystals. The first experiments have demonstrated the feasibility of the approach and suggest that future in vivo crystallography applications at XFELs will be another alternative to nano-crystallography.