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The Open Reading Frame VI Product of Cauliflower mosaic virus Is a Nucleocytoplasmic Protein: Its N Terminus Mediates Its Nuclear Export and Formation of Electron-Dense Viroplasms

^a Institut de Biologie Moléculaire des Plantes, Unité Propre de Recherche, Centre National de la Recherche Scientifique 2357, Université Louis Pasteur, 67084 Strasbourg Cedex, France
^b Friedrich Miescher Institute, CH-4002 Basel, Switzerland

³ To whom correspondence should be addressed. E-mail mario.keller{at}ibmp-ulp.u-strasbg.fr; fax 33-3-88-61-44-42.

The Cauliflower mosaic virus (CaMV) open reading frame VI product (P6) is essential for the viral infection cycle. It controls translation reinitiation of the viral polycistronic RNAs and forms cytoplasmic inclusion bodies (viroplasms) where virus replication and assembly occur. In this study, the mechanism involved in viroplasm formation was investigated by in vitro and in vivo experiments. Far protein gel blot assays using a collection of P6 deletion mutants demonstrated that the N-terminal -helix of P6 mediates interaction between P6 molecules. Transient expression in tobacco (Nicotiana tabacum) BY-2 cells of full-length P6 and P6 mutants fused to enhanced green fluorescent protein revealed that viroplasms are formed at the periphery of the nucleus and that the N-terminal domain of P6 is an important determinant in this process. Finally, this study led to the unexpected finding that P6 is a nucleocytoplasmic shuttle protein and that its nuclear export is mediated by a Leu-rich sequence that is part of the -helix domain implicated in viroplasm formation. The discovery that P6 can localize to the nucleus opens new prospects for understanding yet unknown roles of this viral protein in the course of the CaMV infection cycle.

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