Bipartite signal sequence mediates nuclear translocation of the plant potyviral Nla protein

The Nla protein of certain plant potyviruses localizes to the nucleus of infected cells. Previous studies have shown that linkage of Nla to reporter protein μ-glucuronidase (GUS) is sufficient to direct GUS to the nucleus in transfected protoplasts and in cells of transgenic plants. In this study, we mapped sequences in Nla that confer karyophilic properties. A quantitative transport assay using transfected protoplasts, as well as an in situ localization technique using epidermal cells from transgenic plants, were employed. Two domains within Nla, one between amino acid residues 1 to 11 (signal domain I) and the other between residues 43 to 72 (signal domain II), were found to function additively for efficient localization of fusion proteins to the nucleus, although either region independently could facilitate a low level of translocation. Like signals from animal cells, both nuclear transport domains of Nla contain a high concentration of basic (arginine and lysine) residues. Nuclear transport signal domain II overlaps or is very near Tyr⁶², which is the residue that mediates covalent attachment of a subset of Nla molecules to the 5′ terminus of viral RNA within infected cells. The nature of the Nla nuclear transport signal and the possibility for regulation of Nla translocation are discussed.