Both bipartite geminivirus movement proteins define viral host range, but only BL1 determines viral pathogenicity

Bipartite geminiviruses such as squash leaf curl virus (SqLCV) encode two movement proteins (MPs), BR1 and BL1, that are essential for virus movement and systemic infection of host plants. BR1 has been implicated in the host range properties of the virus, and BL1 in viral pathogenic properties. To more precisely examine the roles of each MP, we have introduced missense and deletion mutations into the coding sequence of both BR1 and BL1, and examined the effects of these mutations on viral infectivity and the production of disease symptoms in pumpkin, squash, and Nicotiana benthamiana. For each MP, a range of mutant phenotypes from partially to fully defective was observed that affected the overall level and rate of infectivity. However, only mutations in BL1, and not BR1, affected the severity of disease symptoms, confirming our earlier finding that BL1 is responsible for the production of disease symptoms. For all mutants, the cucurbit hosts were found to be more permissive for viral movement than was N. benthamiana, and several mutations in both BL1 and BR1 produced host-specific phenotypes, retaining high levels of infectivity in pumpkin and squash, but abolishing infectivity for N. benthamiana. Unexpectedly, functional SqLCV coat protein (AR1) was found to specifically mask the phenotypes of certain BR1 mutations, suggesting some redundancy of function between coat protein and the BR1 MP and an interaction of AR1 with the viral movement pathway. AR1 and BR1 have similar nucleic acid binding affinities, suggesting a possible mechanism for the observed effects of the viral coat protein on viral movement.