Caulimoviridae Tubule-Guided Transport Is Dictated by Movement Protein Properties

Plant viruses move through plasmodesmata (PD) either as nucleoprotein complexes (NPCs) or as tubule-guided encapsidated particles with the help of movement proteins (MPs). To explore how and why MPs specialize in one mechanism or the other, we tested the exchangeability of MPs encoded by DNA and RNA virus genomes by means of an engineered alfalfa mosaic virus (AMV) system. We show that Caulimoviridae (DNA genome virus) MPs are competent for RNA virus particle transport but are unable to mediate NPC movement, and we discuss this restriction in terms of the evolution of DNA virus MPs as a means of mediating DNA viral genome entry into the RNA-trafficking PD pathway.Following virus entry and replication, successful infection of a host requires viral spread to distal parts of the organism through the vascular tissue. In plants, virus movement involves mostly symplastic trafficking of different viral components through the connections of plasmodesmata (PD) (13). With this aim, plant viruses encode one or more movement proteins (MPs), which allow viral genomes to cross the host cell wall by altering the size exclusion limit (SEL) or the structure of PD (6, 11). Plant viruses have evolved distinct mechanisms to move their genomes within the host. These mechanisms can be grouped into two general strategies: one in which the genome is transported in the form of a nucleoprotein complex (NPC) and another in which nucleic acids are encapsidated and move as virus particles. In both cases, besides altering PD SEL, MPs are involved either in NPC assembly or in forming tubules traversing modified PD and helping transport of either NPC or virions to the neighboring cell. Within these two major strategies, there exists a wide range of variability in terms of the number and type of viral and host proteins helping MPs to mediate virus spread within the host (11).In spite of such variability, several different MPs have been classified into a 30K superfamily; these MPs, from 20 genera including both RNA and DNA genome viruses, are structurally related to the 30-kDa MP of Tobacco mosaic virus (TMV), independent of the movement strategy followed (14). Members of this family have a common core of predicted secondary structure elements (α-helices and β-elements) containing a nucleic acid binding domain. Distinct MPs belong to this family, including several tubule-forming MPs, although these are phylogenetically separated from the other members (14). Thus, 30K superfamily MPs are closely related, and some of them are functionally interchangeable in the viral context (2, 20). In particular, MPs from five distinct genera with an RNA genome can successfully replace the corresponding gene of Alfalfa mosaic virus (AMV) (19), indicating that one or more basic and fundamental movement properties might be associated with the common 30K structural core.Among all known plant viruses, only three viral families have evolved a DNA genome: Geminiviridae, Caulimoviridae, and Nanoviridae (6). One possible explanation for this restriction is that endogenous cell-to-cell transport via PD is specialized to use RNA as the communication and signaling molecule (12). To circumvent this restriction, and to allow the efficient exploitation of endogenous transport machineries, DNA genome viruses have evolved appropriate mechanisms involving their MPs. Interestingly, Begomovirus and Caulimovirus MPs also belong to the 30K superfamily discussed above (14). The MP encoded by Cauliflower mosaic virus (CaMV), the type member of Caulimoviridae, forms tubules that guide the movement of encapsidated virus via an indirect MP-virion interaction (16, 21), whereas geminivirus MPs selectively bind their genomes and transport them as NPCs (6, 9, 17). In this study, we investigated the evolutionary convergence of MPs encoded by DNA and RNA viruses by testing their exchangeability in the viral context.