A Plant Virus Movement Protein Forms Ringlike Complexes with the Major Nucleolar Protein, Fibrillarin, In Vitro

Abstract

Fibrillarin, one of the major proteins of the nucleolus, has methyltransferase activity directing 2'-O-ribose methylation of rRNA and snRNAs and is required for rRNA processing. The ability of the plant umbravirus, groundnut rosette virus, to move long distances through the phloem, the specialized plant vascular system, has been shown to strictly depend on the interaction of one of its proteins, the ORF3 protein (protein encoded by open reading frame 3), with fibrillarin. This interaction is essential for several stages in the groundnut rosette virus life cycle such as nucleolar import of the ORF3 protein via Cajal bodies, relocalization of some fibrillarin from the nucleolus to cytoplasm, and assembly of cytoplasmic umbraviral ribonucleoprotein particles that are themselves required for the long-distance spread of the virus and systemic infection. Here, using atomic force microscopy, we determine the architecture of these complexes as single-layered ringlike structures with a diameter of 18-22 nm and a height of 2.0+/-0.4 nm, which consist of several (n=6-8) distinct protein granules. We also estimate the molar ratio of fibrillarin to ORF3 protein in the complexes as approximately 1:1. Based on these data, we propose a model of the structural organization of fibrillarin-ORF3 protein complexes and discuss potential mechanistic and functional implications that may also apply to other viruses.

Control of Nuclear and Nucleolar Localization of Nuclear Inclusion Protein a of Picorna-LikePotato virus AinNicotianaSpecies

“…This interaction was detected initially by YTHS and subsequently studied in plant cells by BiFC. In GRV, interaction between the ORF3 protein and fibrillarin recruits some fibrillarin to the cytoplasm for assembly of virus-like particles that are competent for long-distance movement (Kim et al, 2007a(Kim et al, , 2007bCanetta et al, 2008). However, it seems unlikely that the interaction between VPg and fibrillarin would recruit fibrillarin to the cytoplasm because the interaction between VPg and fibrillarin was observed only in nucleoli and CBs but not in the cytoplasm.…”

“…In plant cells infected with Groundnut rosette virus (GRV; genus Umbravirus), the GRV ORF3 protein enters the nucleus and accumulates in the nucleolus and Cajal bodies (CBs), small subnuclear bodies, or structures (Ryabov et al, 2004;Kim et al, 2007b). This viral protein interacts with the nucleolar protein fibrillarin, which is required for long-distance viral movement in plants (Kim et al, 2007a(Kim et al, , 2007bCanetta et al, 2008).…”

Nucleoli: Composition, Function, and Dynamics

“…In the best-studied system to date, the ORF3 long-distance movement protein of Groundnut rosette virus trafficks to the nucleolus via CBs, causing reorganization of CBs into multiple CBlike structures that fuse with the nucleolus. ORF3 then recruits fibrillarin (an abundant nucleolar RNA binding protein, known to be an RNA methylase) for assembly of cytoplasmic infectious viral particles (Kim et al, 2007a(Kim et al, , 2007bCanetta et al, 2008). The nucleolar localization of ORF3 is essential for systemic infection.…”

The Nucleolar Fibrillarin Protein Is Required for Helper Virus-Independent Long-Distance Trafficking of a Subviral Satellite RNA in Plants

“…Fibrillarin is required for rRNA processing (Barneche et al, 2000;Sáez-Vasquez et al, 2004) and is a nucleolar-localized RBP required for systemic infection of GRV (Kim et al, 2007;Canetta et al, 2008), Potato leafroll virus (Haupt et al, 2005), and Rice stripe tenuivirus (Zheng et al, 2015). Nucleolar colocalization of fibrillarin and P20 protein (Figure 7), along with detection of fibrillarin, P20 protein, and satBaMV RNA in co-IP complexes (Figures 4 and 6) but not in the fibrillarin RNAi transgenic line (Supplemental Figure 5 and Supplemental Tables 1 and 2), is consistent with the evidence for interaction of fibrillarin with satBaMV-P20 RNP complexes ( Figure 4E, Table 2).…”

“…Viral spread from infected cells to neighboring cells requires that the PD size exclusion limits be increased through the action of viral movement protein (MP) (Gopinath and Kao, 2007;Canetta et al, 2008;Harries et al, 2009;Hipper et al, 2013). Changes in PD permeability are also thought to enable movement into the vascular system during systemic phloem-mediated trafficking.…”