The structure of reovirus

Abstract

The arrangement of the capsomers comprising the outer shell or capsid of reovirus has been determined by the computer modeling of images of reovirus replicas prepared by freeze-etching techniques. An optical low-pass spatial filter was used to improve the visibility of the virus images and two types of capsid subunit could then be seen which were termed ring and pentamer-crater capsomers. Sufficient of them could be identified on reovirus particles to make the computer modeling of the particle possible. The capsid was shown by these methods to have T=13,1 triangulated icosahedral symmetry. It is comprised of 60 ring capsomers and 12 pentamer craters surrounded by another 60 incomplete ring capsomers. The same T=13,1 symmetry was found for the single-shelled particles of the two rotaviruses, SA-11 and NCDV, which have 132 uniform capsomers. However, fragments of reovirus capsids, such as are sometimes seen in negatively stained reovirus preparations, were found which had non-T=13,1 symmetry, presumably as a result of capsomer rearrangement. The dimensions of a large number of the triangular subunits of these capsid fragments were measured and it was shown that triangles originating from different parts of the reovirus capsid had non-identical lengths. The lengths found were consistent with the T=13,1 model proposed for the virus. Conditions favouring the formation of crystalline aggregates of reovirus core particles have been found. The unit cell geometry of the two classes of crystals obtained was determined.