CRYSTALLOGRAPHIC STUDY OF A FELINE CALICIVIRUS. Lan Zhou, Ming Luo, Department of Microbiology, Center for Macromolecular Crystallography, University of Alabama at Birmingham, Birmingham, AL USA

Feline calicivirus (FCV) is a nonenveloped, spherical animal virus of the Calicivirus family. It contains a positive sense, single-stranded RNA genome and one major capsid protein. The virion has a diameter of 405 Å and exhibits T=3 icosahedral symmetry. The FCV crystals belong to an orthorhombic crystal system with unit-cell dimensions a=889.0 Å, b=998.3 Å, c=437.6 Å. Based on the Vm value ( 3.4 ų/Da), it was estimated that there is one FCV particle in one crystallographic asymmetric unit, which implies the presence of 60-fold noncrystallographic redundancy.

The orientations of the FCV particles in the unit cell were determined with a self-rotation as well as the locked rotation function using the GLRF program. Data between 15-7 Å and 7-4 Å, respectively, were used to search for 5-, 3- and 2-fold axes. Two sets of 5-, 3-, and 2-fold noncrystallographic symmetry axes were obtained. The orientations of the 5-, 3-, and 2-fold noncrystallographic symmetry axes of each set were those expected for an icosahedron. This indicated that two different particle orientations exist in the crystallographic asymmetric unit. Combining this with the packing consideration and the Vm value, it was concluded that there is one unique particle content from two differently oriented FCV particles in one crystallographic asymmetric unit. The precise particle orientations were refined by optimizing the fit between the directions of the observed self-rotation function peaks from one of the virus particle and the corresponding axes of a standard icosahedron.

In order to precisely define the particle positions in the unit cell, heavy atom derivatives of FCV crystals are in pursuit.