COMPARATIVE-STUDIES OF T=3 AND T=4 ICOSAHEDRAL RNA INSECT VIRUSES

Crystallographic and molecular biological studies of T = 3 nodaviruses (180 identical subunits in the particle) and T = 4 tetraviruses (240 identical subunits in the particle) have revealed similarity in both t he architecture of the particles and the strategy for maturation. The comparative studies provide a novel opportunity to examine an apparent evolution of particle size, from smaller (T = 3) to larger (T = 4), w ith both particles based on similar subunits. The BBV and FHV nodaviru s structures are refined at 2.8 Angstrom and 3 Angstrom respectively, while the N omega V structure is at 6 Angstrom, resolution. Neverthele ss, the detailed comparisons of the noda and tetravirus X-ray electron density maps show that the same type of switching in subunit twofold contacts is used in the T = 3 and T = 4 capsids, although differences must exist between quasi and icosahedral threefold contacts in the T = 4 particle that have not yet been detected. The analyses of primary a nd tertiary structures of noda and tetraviruses show that N omega V su bunits undergo a post assembly cleavage like that observed in nodaviru ses and that the cleaved 76 C-terminal residues remain associated with the particle.