Quaternary and Tertiary Structures of Isometric RNA Viruses

Small spherical RNA viruses infecting members of all five biological kingdoms have been subjects of biophysical studies for decades (Kaper, 1975; Argos and Johnson, 1984). Isolated from their hosts, these obligate parasites are homogeneous chemical entities that are now studied at atomic resolution using x-ray crystallography. In the crystal the virus exists in a resting or dormant state, however, particles released from dissolved crystals are fully infectious. Many viruses form crystalline inclusion bodies within their hosts (Martelli and Russo, 1977), suggesting that crystalline aggregates are a natural and stable state for storing virus particles. In the dormant state the viral capsid protects the nucleic acid from degradation and is essentially a storage protein. During other stages of the virus life cycle, the capsid protein participates in a variety of functions; some are listed in Table I. Although relatively few viruses have been investigated at atomic resolution (Table II), a clear pattern has emerged relating the quaternary structures of different virus capsids (Fig. 1) and the tertiary structures from different virus subunits (Fig. 2). Beyond the striking similarities there are differences in these virus structures that reflect unique strategies evolved for accomplishing required functions. In this paper the current understanding of the relationship between the structures of simple RNA viruses and their function will be discussed using, as examples, three structures recently determined in our laboratory (Hosur et al., 1987; Stauffacher et al., 1987; Chen et al., 1988). An introductory section on the structure determination of one of these viruses (beanpod mottle virus) will describe some of the modern methods of virus x-ray crystallography.