Crystallization of human complement component C5

Human complement component C5 has been crystallized using a low-salt batch technique. The crystals are large hexagonal bi-pyramids often larger than 1.5 mm. Although these crystals were grown in low salt (0.1 M NaCl), they are remarkably stable for at least 2 months at 281 K and they are not dissolved in aqueous buffers containing up to 2 M sodium chloride. The space group is P3₁21 or P3₂21, and the cell parameters were determined to be a = 144.9, b = 144.9, c = 243.1 Å; α = 90°, β = 90, γ = 120°. At room temperature and cryo-temperatures the crystals diffract at best to 6 Å using rotating-anode X-ray sources. Using synchrotron radiation with cryoprotection using 40%(v/v) PEG 400 the resolution limit can be extended to 3.3 Å. In both cases the crystals show significant anisotropy, with relatively weaker reflections at higher resolution in the a*b* plane.