Unusual features of pomoviral RNA movement

Abstract

Potato mop-top pomovirus (PMTV) is one of a few viruses that can move systemically in plants in the absence of the capsid protein (CP). Pomoviruses encode the triple gene block genetic module of movement proteins (TGB 1, 2, and 3) and recent research suggests that PMTV RNA is transported either as ribonucleoprotein (RNP) complexes containing TGB1 or encapsidated in virions containing TGB1. Furthermore, there are different requirements for local or systemic (long-distance) movement. Research suggests that nucleolar passage of TGB1 may be important for the long-distance movement of both RNP and virions. Moreover, and uniquely, the long-distance movement of the CP-encoding RNA requires expression of both major and minor CP subunits and is inhibited when only the major CP sub unit is expressed. This paper reviews pomovirus research and presents a current model for RNA movement.

Keywords: BSBV; BVQ; PMTV; Pomovirus; TGB; microtubules; nucleolus; nucleus.

Figure 1

Folding predictions for 5′ UTRs of Potato mop-top virus (PMTV), Beet soil-borne virus (BSBV) and Beet virus Q (BVQ) RNAs, using Mfold program with pre-selected parameters (Zuker, 2003). Analyzed sequences were limited to the 5′ proximal 180 nucleotides and structures with optimal dG+ max 0.8 kcal/mol were selected. For the RNA-Reps, the amplified sections contain conserved sequences that are highlighted in light-gray: AAAUA in the large loops with UU at the extremities. The conservation of these common nucleotides and their positions in the different RNA-Reps secondary structures contrasts with the very low sequence similarity between the 5′ UTRs of the other PMTV, BSBV, and BVQ RNAs.

Figure 2

(A,B) Systemic infection of Chenopodium quinoa by BVQ RNA transcripts. (A) Upper non-inoculated leaves show chlorosis and necrosis and (B) local lesions induced on inoculated leaves. (C,D) Single confocal microscope sections showing nuclear and nucleolar accumulation of YFP-TGB1 expressed from the PMTV reporter clone. [(C) left panel) YFP-TGB1 and (D): YFP-del84-TGB1. [(C,D) right panels], co-expression of the respective proteins with monomeric red fluorescent protein-labeled Arabidopsis fibrillarin 2 (magenta) to distinguish the nucleolus. Scale bar = 5 μm. (E) Model of PMTV RNA movement. (1) Viral RNP complexes are transported intra-cellularly by TGB2 and TGB3 on the endoplasmic reticulum-actin network to the PD. (2) TGB1 enters the nucleolus where it either acquires a host factor or is modified in some way, or acts to modify the host to facilitate long-distance movement of RNP or (3), tipped virions in the vasculature. (4) Virions that do not contain CP-RT or that contain mutant CP-RT that does not interact with TGB1 are not competent for long-distance movement.