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.