Microtubules interact strongly with the viral movement protein (MP) of

The nucleolus and specific nucleolar proteins are involved in the life cycles of some plant and animal viruses, but the functions of these proteins and of nucleolar trafficking in virus infections are largely unknown. The ORF3 protein of the plant virus, groundnut rosette virus (an umbravirus), has been shown to cycle through the nucleus, passing through Cajal bodies to the nucleolus and then exiting back into the cytoplasm. This journey is absolutely required for the formation of viral ribonucleoprotein particles (RNPs) that, themselves, are essential for the spread of the virus to noninoculated leaves of the shoot tip. Here, we show that these processes rely on the interaction of the ORF3 protein with fibrillarin, a major nucleolar protein. Silencing of the fibrillarin gene prevents longdistance movement of groundnut rosette virus but does not affect viral replication or cell-to-cell movement. Repressing fibrillarin production also localizes the ORF3 protein to multiple Cajal bodylike aggregates that fail to fuse with the nucleolus. Umbraviral ORF3 protein and fibrillarin interact in vitro and, when mixed with umbravirus RNA, form an RNP complex. This complex has a filamentous structure with some regular helical features, resembling the RNP complex formed in vivo during umbravirus infection. The filaments formed in vitro are infectious when inoculated to plants, and their infectivity is resistant to RNase. These results demonstrate previously undescribed functions for fibrillarin as an essential component of translocatable viral RNPs and may have implications for other plant and animal viruses that interact with the nucleolus.Cajal bodies ͉ plant virus movement ͉ ribonucleoprotein particles T he nucleolus is a subnuclear domain and is the site of transcription and processing of rRNA and of ribosome biogenesis. In addition, the nucleolus also participates in other aspects of RNA metabolism and cell function (1, 2). The nucleolus is structurally and functionally associated with Cajal bodies (CBs), which are structures found in both animals and plants (3, 4). CBs contain different proteins including coilin, a protein essential for CB formation, and fibrillarin, a major nucleolar protein that is a core component of small nucleolar ribonucleoprotein particles (snoRNPs) and is required for rRNA processing (4-7). CBs are involved in the maturation of small nuclear RNPs (snRNPs) and snoRNPs, which traffic through CBs before accumulating in splicing speckles and the nucleolus, respectively (8, 9). Both the nucleolus and CBs have a role in RNA silencing in plants (10,11). 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...

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.

Salmonella Typhimurium specifically targets antigen-sampling microfold (M) cells to translocate across the gut epithelium. Although M cells represent a small proportion of the specialized follicular-associated epithelium (FAE) overlying mucosa-associated lymphoid tissues, their density increases during Salmonella infection, but the underlying molecular mechanism remains unclear. Using in vitro and in vivo infection models, we demonstrate that the S. Typhimurium type III effector protein SopB induces an epithelial-mesenchymal transition (EMT) of FAE enterocytes into M cells. This cellular transdifferentiation is a result of SopB-dependent activation of Wnt/β-catenin signaling leading to induction of both receptor activator of NF-κB ligand (RANKL) and its receptor RANK. The autocrine activation of RelB-expressing FAE enterocytes by RANKL/RANK induces the EMT-regulating transcription factor Slug that marks epithelial transdifferentiation into M cells. Thus, via the activity of a single secreted effector, S. Typhimurium transforms primed epithelial cells into M cells to promote host colonization and invasion.