Maize streak virus coat protein is karyophyllic and facilitates nuclear transport of viral DNA

Transport of maize streak virus (MSV) DNA into the nucleus of host cells is essential for virus replication and the presence of virus particles in the nuclei of infected cells implies that coat protein (CP) must enter the nuc leus. To see if CP is imported into the nucleus in the absence of other vir al gene products, the MSV CP gene was expressed in insect cells with a bacu lovirus vector system, and also in tobacco protoplasts with a cauliflower m osaic virus (CaMV) 35S promoter-driven transient gene expression vector. Im munofluorescent staining showed that the CP accumulated in the nuclei of bo th insect and tobacco cells. Mutagenesis of a potential nuclear localizatio n signal in the CP resulted in cytoplasmic accumulation of the mutant prote in. We have shown previously that the CP binds to single-stranded (ss) and double-stranded (ds) viral DNA, To investigate if CP might also be involved in viral DNA nuclear transport, Escherichia coli-expressed CP, together wi th TOTO-1-labeled viral ss or ds DNA, was microinjected into maize and toba cco epidermal cells. Both ss and ds DNA moved into the nucleus when co-inje cted with the CP but not with E. coli proteins alone. These results suggest that, in addition to entering the nucleus where it is required for encapsi dation of the viral ss DNA, the MSV CP facilitates the rapid transport of v iral (ss or ds) DNA into the nucleus.