DEVELOPMENTS IN THE UNDERSTANDING OF THE PARTICLE STRUCTURE OF TOBRAVIRUSES

Particles of tobraviruses resemble those of tobacco mosaic tobamovirus (TMV) in having helical symmetry and in being rod-shaped. However, is olated tobravirus coat protein and TMV coat protein respond to changes in the ionic strength and pH of the solute in contrasting ways. The t ypes of aggregate formed in solutions of coat protein also differ whic h may be related to differences in the apparent mechanism of reconstit ution of virus particles from isolated protein and RNA. The amino acid sequences of tobravirus and tobamovirus coat proteins have been shown to be similar in some regions known to be important for the structure of TMV particles. These alignments also show that tobravirus proteins are larger than tobamoviral proteins in part because of extra residue s at the C-terminus. Tobravirus particles give a signal in proton NMR spectroscopy but TMV particles do not. The signal is caused by segment al mobility of the C-terminal peptide. This difference between TMV and tobraviruses may be related to a property not shared by tobraviruses and TMV and it is therefore speculated that the mobile C-terminal pept ide of tobravirus coat proteins may be important in the transmission o f tobravirus particles by nematode vectors.