INTERACTIONS BETWEEN GEMINIVIRUS REPLICATION PROTEINS

Geminiviruses are small DNA viruses that replicate in the nuclei of in fected plant cells, The closely related geminiviruses tomato golden mo saic virus and bean golden mosaic virus each encode a protein, AL1, th at catalyzes the initiation of rolling-circle replication, Both viruse s also specify a second replication protein, AL3, that greatly enhance s the level of viral DNA accumulation. Using recombinant proteins prod uced in a baculovirus expression system, we showed that AL1 copurifies with a protein fusion of glutathione S-transferase (GST) and AL1, ind ependent of the GST domain, Similarly, authentic AL3 cofractionates wi th a GST-AL3 fusion protein, These results demonstrated that both AL1 and AL3 form oligomers. Immunoprecipitation of protein extracts from i nsect cells expressing both AL1 and AL3 shelved that the two proteins also complex with each other, None of the protein interactions display ed virus specificity; the tomato and bean golden mosaic virus proteins complexed with each other, The addition of heterologous replication p roteins had no effect on the efficiency of geminivirus replication in transient-replication assays, suggesting that heteroprotein complexes might be functional. The significance of these protein interactions is discussed with respect to geminivirus replication in plant cells.