An Update on the Intracellular and Intercellular Trafficking of Carmoviruses

Abstract

Despite harboring the smallest genomes among plant RNA viruses, carmoviruses have emerged as an ideal model system for studying essential steps of the viral cycle including intracellular and intercellular trafficking. Two small movement proteins, formerly known as double gene block proteins (DGBp1 and DGBp2), have been involved in the movement throughout the plant of some members of carmovirus genera. DGBp1 RNA-binding capability was indispensable for cell-to-cell movement indicating that viral genomes must interact with DGBp1 to be transported. Further investigation on Melon necrotic spot virus (MNSV) DGBp1 subcellular localization and dynamics also supported this idea as this protein showed an actin-dependent movement along microfilaments and accumulated at the cellular periphery. Regarding DGBp2, subcellular localization studies showed that MNSV and Pelargonium flower break virus DGBp2s were inserted into the endoplasmic reticulum (ER) membrane but only MNSV DGBp2 trafficked to plasmodesmata (PD) via the Golgi apparatus through a COPII-dependent pathway. DGBp2 function is still unknown but its localization at PD was a requisite for an efficient cell-to-cell movement. It is also known that MNSV infection can induce a dramatic reorganization of mitochondria resulting in anomalous organelles containing viral RNAs. These putative viral factories were frequently found associated with the ER near the PD leading to the possibility that MNSV movement and replication could be spatially linked. Here, we update the current knowledge of the plant endomembrane system involvement in carmovirus intra- and intercellular movement and the tentative model proposed for MNSV transport within plant cells.

ScreeningLegionellaeffectors for antiviral effects reveals Rab1 GTPase as a proviral factor coopted for tombusvirus replication

“…Rab1 and COPII vesicles are also coopted by different viruses for various purposes, from replication, intracellular trafficking of viral proteins, to particle assembly (39)(40)(41)(42). For example, Rab1 facilitates the interaction of the hepatitis C virus (HCV) NS5A protein with lipid droplets to alter lipid metabolism that favors HCV replication (43).…”

Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

“…Cell-to-cell movement of many viruses belonging to the family Tombusviridae requires the coordinated actions of two small proteins encoded in the central region of their genomes ( Figure 1 ) [18] , [53] , [57] , [170] – [173] . After TGB, this gene block is referred to as the double gene block (DGB) [51] .…”

“…The second MP in carmoviruses, DGB2, has been classified into two groups having one or two potential transmembrane domains (TMD) ( Table S5 ) [18] , [178] . DGB2 proteins can be co-translationally inserted in vitro into ER-derived microsomes and participate in a signal recognition particle-dependent and translocon-assisted processes [18] , [57] , [178] – [181] . The membrane topology of these proteins has been determined both in vitro and in vivo [18] , [179] , [180] , [182] .…”

“…A lateral diffusion of DGB2 along the ER membranes, probably driven by the ER-associated actin system, is required for the protein ER export [178] . The initially proposed model for carmovirus movement implies that the DGB2 cytosolic region can interact with DGB1/RNA complex, and this ternary complex is transported from the replication sites to PD through the endomembrane system [18] , [57] . However, some DGB1 proteins have a nuclear localization mediated by two nuclear localization signals that are necessary for virus cell-to-cell movement [18] , [57] , [176] .…”

Allelic RNA Motifs in Regulating Systemic Trafficking of Potato Spindle Tuber Viroid

“…RNA intercellular trafficking, including short distance cell-to-cell and long distance systemic movement, has been demonstrated as a critical means for cellular communications in various organisms [ 1 , 2 , 3 , 4 ]. Viruses and sub-viral agents also exploit such pathways to successfully invade a host [ 5 , 6 , 7 , 8 ]. Being a special group of sub-viruses and made up of only circular noncoding RNAs, viroids entirely depend on their RNA structural motifs to interact with host factors for function [ 7 , 9 , 10 ].…”