Structural studies of tobacco ringspot virus

Tobacco ringspot virus (TRSV) is a member of the nepovirus genus of icosahedral RNA plant viruses. Nepoviruses and comoviruses are classified in the picornavirus superfamily based on similarity to picornaviruses. The Picorna-like virus capsids are composed of sixty copies of three subunits (with a similar $\beta$-barrel fold) which occupy positions on the surface lattice comparable to the 180 copies of a single subunit in a $T=3$ virus capsid. The three different subunits are synthesized as a polyprotein, which is subsequently cleaved by a viral proteinase. Structures of comoviruses and picornaviruses suggested that picorna-like viruses have evolved from a $T=3$ virus by triplication of the gene encoding a $\beta$-barrel domain and the development of cleavage sites in the interdomain linking regions. The picornavirus capsid polyprotein is cleaved at two sites, while the comoviruses possess one cleavage site. Nepovirus capsids, composed of a single capsid protein species, may represent an early stage in the evolution of picornavirus capsids. Structural studies on TRSV were initiated to investigate the proposed evolutionary relationship. The 3.5 A resolution structure of TRSV has revealed that the capsid protein is folded into three $\beta$-barrel domains that are covalently linked by extended polypeptides. The quaternary structure of the three domains in the capsid protein is stabilized mainly by hydrophobic interactions. The order of connectivity of the domains in TRSV confirms the proposed connectivity in the precleaved como- and picornavirus capsid polyproteins. The structural differences between equivalent domains in TRSV and comoviruses are confined to the external surface loops, interdomain connecting polypeptides and N-termini. The three different domains within TRSV and comoviruses are structurally closer than the three domains within picornaviruses. The results of structural comparisons support the proposed divergent evolution of the capsid polyproteins of nepo-, como- and picornaviruses.