Order and disorder in crystals of hexameric NTPases from dsRNA bacteriophages

The packaging of genomic RNA in members of the Cystoviridae is performed by P4, a hexameric protein with NTPase activity. Across family members such as Φ6, Φ8 and Φ13, the P4 proteins show low levels of sequence identity, but presumably have similar atomic structures. Initial structure-determination efforts for P4 from Φ6 and Φ8 were hampered by difficulties in obtaining crystals that gave ordered diffraction. Diffraction from crystals of full-length P4 showed a variety of disorder and anisotropy. Subsequently, crystals of Φ13 P4 were obtained which yielded well ordered diffraction to 1.7 Å. Comparison of the packing arrangements of P4 hexamers in different crystal forms and analysis of the disorder provides insights into the flexibility of this family of proteins, which might be an integral part of their biological function.