Structure Determination of Echovirus 1

The atomic structure of echovirus 1 (a member of the enterovirus genus of the picornavirus family) has been determined using cryo-crystallography and refined to 3.55 Å resolution. Echovirus 1 crystallizes in space group P22₁2₁ with a = 352.45, b = 472.15 and c = 483.20 Å. The crystals contain one full virus particle in the asymmetric unit allowing for 60-fold noncrystallographic symmetry averaging. The diffraction pattern shows strong pseudo-B-centering with reflections with h + l = 2n + 1 being systematically weak or absent below about 6 Å resolution. The size of the unit cell and presence of pseudo-B-centering placed strong constraints on the allowed packing of the icosahedral particle in the crystal lattice. These constraints greatly facilitated the determination of the orientation and position of the virus by reducing the dimensionality of the search, but interactions between the crystallographic and noncrystallographic symmetries rendered the choice of space group ambiguous until very late in the structure determination. This structure determination provides a striking example of the power of packing analysis in molecular replacement and illustrates how subtle interactions between crystallographic and noncrystallographic symmetries can be resolved.