THE BIPARTITE GEMINIVIRUS COAT PROTEIN AIDS BR1 FUNCTION IN VIRAL MOVEMENT BY AFFECTING THE ACCUMULATION OF VIRAL SINGLE-STRANDED-DNA

The movement of bipartite geminiviruses such as squash leaf curl virus (SqLCV) requires the cooperative interaction of two essential virus-e ncoded movement proteins, BR1 and BL1, While the viral coat protein AR 1 is not essential for systemic infection, genetic studies demonstrate that its presence masks the defective phenotype of certain BR1 missen se mutants, thus suggesting that coat protein does interact with the v iral movement pathway. To further examine the mechanism of this intera ction, we have constructed alanine-scanning mutants of AR1 and studied them for the ability to mask the infectivity defects of appropriate B R1 mutants, for the ability to target to the nucleus and to bind viral single-stranded DNA (ssDNA) and multimerize, and for effects on the a ccumulation of replicated viral ssDNA, We identified a specific region of AR1 required for masking of appropriate BR1 mutants and showed tha t this same region of AR1 was also important for ssDNA binding and the accumulation of viral replicated ssDNA, This region of ARI also overl apped that involved in multimerization of the coat protein. We also fo und that the accumulation in protoplasts of single-stranded forms of a recombinant plasmid that included the SqLCV replication origin but wa s too large to be encapsidated was dependent on the presence of ARI bu t did not appear to require encapsidation. These findings extend our m odel for SqLCV movement, demonstrating that coat protein affects viral movement through its ability to induce the accumulation of replicated viral ssDNA genomes, They further suggested that encapsidation was no t required for the AR1-dependent accumulation of viral ssDNA.