NIa and NIb of peanut stripe potyvirus are present in the nucleus of infected cells, but do not form inclusions

We investigated, by immunological and gene-fusion methods, whether the failure of peanut stripe potyvirus (PStV)-encoded nuclear inclusion proteins a (NIa) and b (NIb) to form nuclear inclusions is due to the lack of their in vivo accumulation or the inability of one or both proteins to be transported into the nucleus. NIa domains (NIa-VPg and NIa-proteinase), full-length NIb, and full-length cylindrical inclusion (CI) protein of PStV were cloned, expressed in Escherichia coli, and used for antisera production. Immunoblot analysis of accumulation of NIa, NIb, and CI in time course experiments revealed that they accumulated to similar levels in PStV-infected Nicotiana benthamiana. In immunocytochemical studies with electron microscopy, antiserum against NIa-VPg, NIa-Pro, and NIb specifically labeled NIa and NIb proteins throughout the nuclei of PStV-infected cells, in the absence of nuclear inclusions. Translational fusions were made between NIa and NIb to either the green fluorescence protein or the β-glucuronidase in vectors for transient gene expression or stable expression in transgenic plants, respectively. Fusion proteins containing NIa accumulated in the nucleus, whereas fusion proteins containing NIb accumulated in a punctate pattern in the cytoplasm. These data indicate that at least NIa possesses a nuclear localization signal.