Ubiquitination of TMV Coat Protein Aggregates in Infected Tobacco Leaves

Protein extracts from Tobacco mosaic virus (TMV) infected young tobacco leaves exhibiting strong symptoms contain higher amounts of ubiquitin conjugates in comparison with non-infected control plants presumably due to stress reactions of host cells. Western blots with antibodies against TMV indicate as well an increase in coat protein or in ubiquitinated coat protein subunits. Immunogold labelling of infected leaf material revealed the accumulation of large amounts of coat protein in opaque inclusions which also reacted strongly with ubiquitin antibodies. These inclusions appear in close vicinity to chloroplasts in chlorotic areas of the leaf. It is concluded that most of the ubiquitin conjugates found in Western blots are due to ubiquitinated coat protein. As a consequence, a considerable amount of coat protein cannot be used for correct capsid construction as the stabilizing lysine residues are blocked by ubiquitination.