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.

Keywords: Golgi; carmovirus; endoplasmic reticulum; intracellular movement; mitochondria; movement proteins.

FIGURE 1

(A) The organization of the carmovirus genome. ORF1: auxiliary replicase; ORF2: RNA dependent RNA polymerase; ORF3: double gene protein 1 (DGBp1); ORF4: double gene protein 2 (DGBp2); ORF 5: coat protein. (B) The organization of the DGB genes. The first DGB ORF encodes DGBp1 having a central RNA binding domain (RBD) rich in basic residues. DGBp2 is encoded in the second DGB ORF and has one (MNSV-like) or two (CarMV-like) transmembrane domains. Topology and membrane orientation of Melon necrotic spot virus (MNSV), Turnip crinkle virus (TCV), and Carnation mottle virus (CarMV) DGBp2s is shown.

FIGURE 2

Intracellular transport of double gene block proteins of Melon necrotic spot virus. MNSV DGBp1 and DGBp2 likely use two independent pathways to reach the cellular periphery and PD. Self-interacting DGBp1 molecules may bind vRNA to form RNPs that associate with actin microfilaments and move to cellular periphery in an actin-dependent manner (1). After cotranslational/signal recognition particle (SRP)-dependent and translocon-assisted insertion into ER membranes (2), MNSV DGBp2 is exported to the Golgi apparatus in a COPII-dependent pathway (3). Next, DGBp2 is targeted to PD via an unidentified post-Golgi route, or alternatively it can reach the PD by lateral diffusion along the ER (4).