Interactions between geminivirus replication proteins.

Geminiviruses are small DNA viruses that replicate in the nuclei of infected plant cells. The closely related geminiviruses tomato golden mosaic virus and bean golden mosaic virus each encode a protein, AL1, that catalyzes the initiation of rolling-circle replication. Both viruses also specify a second replication protein, AL3, that greatly enhances the level of viral DNA accumulation. Using recombinant proteins produced in a baculovirus expression system, we showed that AL1 copurifies with a protein fusion of glutathione S-transferase (GST) and AL1, independent of the GST domain. Similarly, authentic AL3 cofractionates with a GST-AL3 fusion protein. These results demonstrated that both AL1 and AL3 form oligomers. Immunoprecipitation of protein extracts from insect cells expressing both AL1 and AL3 showed that the two proteins also complex with each other. None of the protein interactions displayed virus specificity; the tomato and bean golden mosaic virus proteins complexed with each other. The addition of heterologous replication proteins 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.

Geminiviruses: models for plant DNA replication, transcription, and cell cycle regulation.

Geminiviruses have small, single-stranded DNA genomes that replicate through double-stranded intermediates in the nuclei of infected plant cells. Viral double-stranded DNA also assembles into minichromosomes and is transcribed in infected cells. Geminiviruses encode only a few proteins for their replication and transcription and rely on host enzymes for these processes. However, most plant cells, which have exited the cell cycle and undergone differentiation, do not contain the replicative enzymes necessary for viral DNA synthesis. To overcome this barrier, geminiviruses induce the accumulation of DNA replication machinery in mature plant cells, most likely by modifying cell cycle and transcriptional controls. In animals, several DNA viruses depend on host replication and transcription machinery and can alter their hosts to create an environment that facilitates efficient viral replication. Analysis of these viruses and their proteins has contributed significantly to our understanding of DNA replication, transcription, and cell cycle regulation in mammalian cells. Geminiviruses have the same potential for plant systems. Plants offer many advantages for these types of studies, including ease of transformation, well-defined cell populations and developmental programs, and greater tolerance of cell cycle perturbation and polyploidy. Our knowledge of the molecular and cellular events that mediate geminivirus infection has increased significantly during recent years. The goal of this review is to summarize recent research addressing geminivirus DNA replication and its integration with transcriptional and cell cycle regulatory processes.

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 double-stranded DNA intermediates in plant nuclei using host replication machinery. 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 oligomers and interacts with the viral replication initiator AL1. Experiments reported here established that AL3 also interacts with a plant homolog of the mammalian tumor suppressor protein, retinoblastoma (pRb). Analysis of truncated AL3 proteins indicated that pRb and AL1 bind to similar regions of AL3, whereas AL3 oligomerization is dependent on a different region of the protein. Analysis of truncated AL1 proteins located the AL3-binding domain between AL1 amino acids 101 and 180 to a region that also includes the AL1 oligomerization domain and the catalytic site for initiation of viral DNA replication. Interestingly, the AL3-binding domain was fully contiguous with the 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.

Functional domains of a geminivirus replication protein.

Tomato golden mosaic virus, a member of the geminivirus family, has a single-stranded DNA genome that is replicated and transcribed in infected plant cells through the concerted action of viral and host factors. One viral protein, AL1, contributes to both processes by binding to a directly repeated, double-stranded DNA sequence located in the overlapping (+) strand origin of replication and AL1 promoter. The AL1 protein, which occurs as a multimeric complex in solution, also catalyzes DNA cleavage during initiation of rolling circle replication. To identify the tomato golden mosaic virus AL1 domains that mediate protein oligomerization, DNA binding, and DNA cleavage, a series of truncated AL1 proteins were produced in a baculovirus expression system and assayed for each activity. These experiments localized the AL1 oligomerization domain between amino acids 121 and 181, the DNA binding domain between amino acids 1 and 181, and the DNA cleavage domain between amino acids 1 and 120. Deletion of the first 29 amino acids of AL1 abolished DNA binding and DNA cleavage, demonstrating that an intact N terminus is required for both activities. The observation that the DNA binding domain includes the oligomerization domain suggested that AL1-AL1 protein interaction may be a prerequisite for DNA binding but not for DNA cleavage. The significance of these results for AL1 function during geminivirus replication and transcription is discussed.

Multiple cis elements contribute to geminivirus origin function.

The genome of the geminivirus tomato golden mosaic virus (TGMV) consists of two circular DNA molecules which are dissimilar in sequence except for a highly conserved 200-bp common region that includes the origin for rolling circle replication. To better characterize the plus-strand origin, we analyzed the capacities of various TGMV common region sequences to support episomal replication in tobacco protoplasts when the viral replication proteins AL1 and AL3 were supplied in trans. These experiments demonstrated that the minimal origin is located in 89-bp common region fragment that includes the known AL1 binding motif and a hairpin structure containing the DNA cleavage site. Analyses of mutant origin sequences identified two additional cis elements--one that is required for origin activity and a second that greatly enhances replication. In contrast, a conserved partial copy of the AL1 binding site did not contribute to origin function. Mutational analysis of the functional AL1 binding site showed that both spacing and sequence of this motif are important for replication in vivo and AL1/DNA binding in vitro. Spacing changes between the AL1 binding site and hairpin also negatively impacted TGMV origin function in a position-dependent manner. Together, these results demonstrated that the organization of TGMV plus-strand origin is complex, involving multiple cis elements that are likely to interact with each other during initiation of replication.