Detection and possible functions of African cassava mosaic virus DNA B gene products

Polyclonal antisera raised against synthetic oligopeptides have been used to detect the DNA B gene products BV1 and BC1 of the geminivirus African cassava mosaic virus following SDS-PAGE fractionation of Nicotiana benthamiana extracts. BV1 antiserum detected a soluble protein of 29 kDa, consistent with the size predicted from sequence data. BC1 antiserum detected proteins of 37, 39, and 42 kDa in addition to variable, less abundant species, all of which are larger than the predicted size of 34 kDa. BC1 antiserum detected a single protein of 35-36 kDa following in vitro translation in reticulocyte lysate, suggesting that BC1 is post-translationally modified in plants. The nature of the modification was not resolved, although neither glycosylation nor association with nucleic acids is involved. In common with putative spread proteins of several other plants viruses, BC1 co-fractionated with the cell wall. The replication of both genomic components in N. tabacum protoplasts was unaffected by the introduction of frameshift mutations into BV1 and BC1 coding regions. In inoculated N. benthamiana leaves, however, the accumulation of a BV1 mutant was significantly reduced compared to the levels attained by co-inoculated, complementing BV1 and BC1 mutants. In contrast, the accumulation of a BC1 mutant was unaffected, although symptom induction in inoculated leaves and systemic infection occurred only in the presence of both BV1 and BC1. The results are consistent with a role for BV1 in localized cell-to-cell spread and for BC1, possibly together with BV1, in long-distance vascular spread of the virus.