Autolabo: an automated system for ligand-soaking experiments with protein crystals

Ligand soaking of protein crystals is important for the preparation of heavy-atom derivative crystals for experimental phasing as well as for large-scale ligand screening in pharmaceutical developments. To facilitate laborious large-scale ligand screening, to reduce the risk of human contact with hazardous ligand reagents and to increase the success rate of the soaking experiments, a protein crystallization robot 'Autolabo' has been developed and implemented in the high-throughput crystallization-to-structure pipeline at RIKEN SPring-8 Center. The main functions of this robotic system are the production of protein crystals for experiments, the ligand soaking of these crystals and the observation of soaked crystals. The separate eight-channel dispensers of Autolabo eliminate the cross-contamination of reagents which should be strictly avoided in the ligand-soaking experiment. Furthermore, the automated approach reduces physical damage to crystals during experiments when compared with the conventional manual approach, and thereby has the potential to yield better quality diffraction data. Autolabo's performance as a ligand-soaking system was evaluated with a crystallization experiment on ten proteins from different sources and a heavy-atom derivatization experiment on three proteins using a versatile cryoprotectant containing heavy-atom reagents as ligands. The crystallization test confirmed reliable crystal reproduction in a single condition and the capability for crystallization with nucleants to improve crystal quality. Finally, Autolabo reproducibly derivatized the test protein crystals with sufficient diffraction quality for experimental phasing and model building, indicating a high potentiality of this automated approach in ligand-soaking experiments.