THE GEMINIVIRUS BR1 MOVEMENT PROTEIN BINDS SINGLE-STRANDED-DNA AND LOCALIZES TO THE CELL-NUCLEUS

Plant viruses encode movement proteins that are essential for infectio n of the host but are not required for viral replication or encapsidat ion. Squash leaf curl virus (SqLCV), a bipartite geminivirus with a si ngle-stranded DNA genome, encodes two movement proteins, BR1 and BL1, that have been implicated in separate functions in viral movement. To further elucidate these functions, we have investigated the nucleic ac id binding properties and cellular localization of BR1 and BL1. In thi s study, we showed that BR1 binds strongly to single-stranded nucleic acids, with a higher affinity for single-stranded DNA than RNA, and is localized to the nucleus of SqLCV-infected plant cells. In contrast, BL1 binds only weakly to single-stranded nucleic acids and not at all to double-stranded DNA. The nuclear localization of BR1 and the previo usly demonstrated plasma membrane localization of BL1 were also observ ed when these proteins were expressed from baculovirus vectors in Spod optera frugiperda insect cells. The biochemical properties and cellula r locations of BR1 and BL1 suggest a model for SqLCV movement whereby BR1 is involved in the shuttling of the genome in and/or out of the nu cleus and BL1 acts at the plasma membrane/cell wall to facilitate vira l movement across cell boundaries.