Structure determination of echovirus 1

The atomic structure of echovirus 1 (a member of the enterovirus genus of t he picornavirus family) has been determined using cryo-crystallography and refined to 3.55 Angstrom resolution. Echovirus 1 crystallizes in space grou p P22(1)2(1) with a = 352.45, b = 472.15 and c = 483.20 Angstrom. The cryst als 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 + I = 2n + 1 being system atically weak or absent below about 6 Angstrom resolution. The size of the unit cell and presence of pseudo-B-centering placed strong constraints on t he allowed packing of the icosahedral particle in the crystal lattice. Thes e constraints greatly facilitated the determination of the orientation and position of the virus by reducing the dimensionality of the search, but int eractions between the crystallographic and noncrystallographic symmetries r endered the choice of space group ambiguous until very late in the structur e determination. This structure determination provides a striking example o f the power of packing analysis in molecular replacement and illustrates ho w subtle interactions between crystallographic and noncrystallographic symm etries can be resolved.