Cajal bodies and the nucleolus are required for a plant virus systemic infection

Abstract

The nucleolus and Cajal bodies (CBs) are prominent interacting subnuclear domains involved in a number of crucial aspects of cell function. Certain viruses interact with these compartments but the functions of such interactions are largely uncharacterized. Here, we show that the ability of the groundnut rosette virus open reading frame (ORF) 3 protein to move viral RNA long distances through the phloem strictly depends on its interaction with CBs and the nucleolus. The ORF3 protein targets and reorganizes CBs into multiple CB-like structures and then enters the nucleolus by causing fusion of these structures with the nucleolus. The nucleolar localization of the ORF3 protein is essential for subsequent formation of viral ribonucleoprotein (RNP) particles capable of virus long-distance movement and systemic infection. We provide a model whereby the ORF3 protein utilizes trafficking pathways involving CBs to enter the nucleolus and, along with fibrillarin, exit the nucleus to form viral 'transportcompetent' RNP particles in the cytoplasm.

Interaction of a plant virus-encoded protein with the major nucleolar protein fibrillarin is required for systemic virus infection

“…In elucidating the nuclear pathway of the ORF3 protein, we also observed the partial relocalization of the nucleolar protein, fibrillarin, to the cytoplasmic inclusions containing viral RNPs, whereas normally, fibrillarin does not accumulate in cytoplasm (14). Fibrillarin is one of the major proteins of the nucleolus.…”

“…The CBs are then reorganized into multiple smaller structures (CB-like aggregates, CBLs) that move to and fuse with the nucleolus by an unknown mechanism (14). The ORF3 protein is exported from the nucleus, leading to the formation of cytoplasmic viral RNP particles that are transported to the rest of the plant via the phloem.…”

“…Finally, a number of animal and plant viruses including the RNAcontaining tobacco etch virus and the DNA-containing tomato yellow leaf curl virus have a nucleolar phase in their life cycle (12,13). Recently, we have shown that the ability of the umbravirus, groundnut rosette virus (GRV), to move long distances through the phloem, the specialized vascular system used by plants for the transport of assimilates and macromolecules, depends strictly on the interaction of one of its proteins, the ORF3 protein, with CBs and the nucleolus (14).…”

“…respectively, resulting in failure to form viral RNPs, and of their long-distance movement (14) (Fig. 1).…”

“…1) (16,20). The R-rich domain is involved in nuclear import, and the L149 residue, in addition to the R-rich domain, is essential for nucleolar targeting of the ORF3 protein (14,20). The whole L-rich region also acts as a nuclear export signal (20), suggesting that the ORF3 protein traffics between the nucleus (nucleolus) and cytoplasm of infected cells.…”