Serial Femtosecond Crystallography of G Protein–Coupled Receptors

G Structures G protein–coupled receptors (GPCRs) are eukaryotic membrane proteins that have a central role in cellular communication and have become key drug targets. To overcome the difficulties of growing GPCRs crystals, Liu et al. (p. 1521) used an x-ray free-electron laser to determine a high-resolution structure of the serotonin receptor from microcrystals. The structure of a human serotonin receptor was solved using a free-electron laser to analyze microcrystals. X-ray crystallography of G protein–coupled receptors and other membrane proteins is hampered by difficulties associated with growing sufficiently large crystals that withstand radiation damage and yield high-resolution data at synchrotron sources. We used an x-ray free-electron laser (XFEL) with individual 50-femtosecond-duration x-ray pulses to minimize radiation damage and obtained a high-resolution room-temperature structure of a human serotonin receptor using sub-10-micrometer microcrystals grown in a membrane mimetic matrix known as lipidic cubic phase. Compared with the structure solved by using traditional microcrystallography from cryo-cooled crystals of about two orders of magnitude larger volume, the room-temperature XFEL structure displays a distinct distribution of thermal motions and conformations of residues that likely more accurately represent the receptor structure and dynamics in a cellular environment.