Dual interaction of a geminivirus replication accessory factor with a viral replication protein and a plant cell cycle regulator

Geminiviruses replicate their small, single-stranded DNA genomes through do uble-stranded DNA intermediates in plant nuclei using host replication mach inery. Like most dicot-infecting geminiviruses, tomato golden mosaic virus encodes a protein, AL3 or C3, that greatly enhances Viral DNA accumulation through an unknown mechanism. Earlier studies showed that AL3 forms oligome rs and interacts with the viral replication initiator ALI. Experiments repo rted here established that AL3 also interacts with a plant homolog of the m ammalian tumor suppressor protein, retinoblastoma (pRb). Analysis of trunca ted AL3 proteins indicated that pRb and AL1 bind to similar regions of AL3, whereas AL3 oligomerization is dependent on a different region of the prot ein. Analysis of truncated AL1 proteins located the AL3-binding domain betw een AL1 amino acids 101 and 180 to a region that also includes the ALI. oli gomerization domain and the catalytic site for initiation of viral DNA repl ication. interestingly, the AL3-binding domain was fully contiguous with th e domain that mediates AL1/pRb interactions. The potential significance of AL3/pRb binding and the coincidence of the domains responsible for AL3, AL1 , and pRb interactions are discussed. (C) 2001 Academic Press.