Reovirus Exists in the Form of 13 Particle Species That Differ in Their Content of Protein σ1

Abstract

When electrophoresed in 0.7% agarose gels, populations of reovirus particles can be resolved into 13 well-defined bands that possess from 0 to 12 projection/spike-associated trimers of protein σ1. This state of affairs is not an artifact of purification, of the techniques used to demonstrate it, of aggregation, or of virus particle instability. Complexes of monoclonal antibody against protein σ1 with virus particles that possess only 1, or, to a lesser extent, 2 σ1 trimers are less stable (that is, more readily dissociated by sonication) than complexes of antibody and virus particles that possess 3 or more σ1 trimers. The specific infectivity of virus particles that possess 3 or more σ1 trimers is essentially the same; virus particles that possess only 2 σ1 trimers are about two-thirds as infectious; and particles that possess only 1 σ1 trimer still possess very significant infectivity (about one-third of maximum). Reovirus particles that possess no σ1 trimers (about 1 in 30) are essentially noninfectious. The reason reovirus particles do not possess a full complement of σ1 trimers is presumably the fact that only very small amounts of protein σ1 are synthesized in infected cells; and since possession of 3 such trimers is sufficient for maximal infectivity, and since the average number of σ1 trimers per reovirus particle is 7.1, there is presumably no selection for variants that synthesize larger amounts of σ1. On the contrary, such variants may well be at a selective disadvantage.