The Packing Structure of Crystalline RI-APC Virus

In a paper in the preceding issue of this Journal Morgan, Howe, Rose, and Moore (1) have described their studies of electron micrographs of sections of tissue infected with RI-APC viruses. The virus particle aggregates appear most frequently as well ordered two-dimensional arrays. Studies of serial sections have shown that these two-dimensional nets are planes through three-dimensional virus crystals. We have had the opportunity to examine these electron micrographs. Our study has suggested that the viral particles are packed in a cubic body-centered lattice. The determination of the probable crystal lattice structure of a virus from section micrographs cannot, for several reasons, be as certain as a determination using replica or pseudo-replica techniques (2). In the electron micrograph of a replica, the simultaneous view of two non-parallel faces which intersect along an edge may provide powerful visual evidence concerning the intermolecular packing in three dimensions. The cutting of sections, as in this study, of thickness less than the diameter of a single particle will in general produce marked distortion either in particle shape or in interparticle alignment, perhaps in both. The extent of the distortion will be variable since it will depend on many different factors. These will include the nature of the plane of section and the orientation of the cutting edge within the plane. In general the most marked changes in the virus shape will occur when particle distortion is the only means of relieving the compression. The most marked changes in the shape and size of the lattice unit will occur when rows of virus molecules can relieve the compression by moving with respect to each other. The extent of the distortion has been observed directly in one instance by Morgan and his co-workers. A thick section of tissue was examined in the light microscope and a photomicrograph was taken of a single diamond-shaped crystal embedded in a cell. The same crystal was then examined electron microscopically in a contiguous thin section. The angles measured in the original photograph were approximately 100 ° and 80 °. In the thin section electron micrograph, the angles were approximately 135 ° and 45 ° respectively. Morgan and his coworkers have shown that the particle distortion appears as a lengthening in a direction parallel to the knife edge, roughly proportional to the shortening in the direction perpendicular to it. From their studies they have concluded that the virus molecules are spherical …