Crystallization and X-ray structure determination of an RNA-dependent hexameric helicase.

Hexameric helicases couple the energy of ATP hydrolysis to processive movement along nucleic acids and are critical components of cells and many viruses. Molecular motion derives from ATP hydrolysis at up to six distinct catalytic centers, which is coupled to the coordinated action of translocation loops in the center of the hexamer. Due to the structural dynamics and catalytic complexity of hexameric helicases, few have been crystallized with a full complement of bound substrates, and instead tend to form crystals belonging to high-symmetry space groups that obscure the differences among catalytic subunits. We were able to overcome these difficulties and solve an asymmetric structure of the Rho transcription termination factor from Escherichia coli bound to ATP mimics and RNA. Here, we present some considerations used for crystallization of this hexameric helicase, discuss the utility of substrate-centric crystal-screening strategies, and outline a crystal-aging screen that allowed us to overcome the adverse effects of nonmerohedral twinning.