An optimal exposure strategy for cryoprotected virus crystals with lattice constants greater than 1000 A

Studies of icosahedral virus capsids provide insights into the function of supramolecular machines. Virus capsid crystals have exceptionally large unit cells; as a result, they diffract weakly compared with protein crystals. HK97 is a dsDNA lambda-like bacteriophage whose 13 MDa capsid expands from 550 A to 650 A with large subunit conformational changes during virus maturation. The HK97 penultimate maturation intermediate was crystallized in a tetragonal unit cell that has lattice constants of 1010 A x 1010 A x 730 A. The crystals could be cryoprotected, but diffracted to a modest resolution of 5 A at a bending-magnet beamline. When these crystals were optimally exposed with two orders-of-magnitude more photons from a new insertion-device beamline, data extending to better than 3.8 A resolution were obtained. Here, the strategies to collect and process such data are described. These strategies can be adapted for other crystals with large unit cells and for microcrystals.