Multiple modes of subunit association in the structures of simple spherical viruses

How can protein subunits associate with each other in more than one way? This is a central question in structural biochemistry: processes such as allosteric control and cytoskeletal assembly involve formation of specific protein contacts that can be switched from one to another in a regulated fashion. The organization of small, spherical viruses shows such. switching particularly clearly. This article describes the characteristics of protein-protein contacts in these structures. The crystal structures offour different RNA-containing plant viruses have been determined to a sufficiently high resolution that the polypeptide chain of the coat subunits can be traced and amino-acid residues assigned. Thev are: tomato bushy stunt virus (TBSVT), southern bean mosaic virus (SBMV)2, satellite of tobacco necrosis virus (STNV)3; and turnip crinkle virus (TCV)4. These particGs, like all wellcharacterized small spherical viruses, have icosahedral symmetry. This symmetry implies that there are sixty replicas of a fundamental structural motif, all packed identically in the shell of the particle. A simple example, using commas as the motif, is shown in Fig. la. STNV has a similar simple arrangement. The other structures determined have 180 identical subunits, and the 60fold replicated motif must therefore contain three subunits, necessarily packed in somewhat different ways (Fig. 1b).