Intracellular transport of plant viruses: finding the door out of the cell

Abstract

Plant viruses are a class of plant pathogens that specialize in movement from cell to cell. As part of their arsenal for infection of plants, every virus encodes a movement protein (MP), a protein dedicated to enlarging the pore size of plasmodesmata (PD) and actively transporting the viral nucleic acid into the adjacent cell. As our knowledge of intercellular transport has increased, it has become apparent that viruses must also use an active mechanism to target the virus from their site of replication within the cell to the PD. Just as viruses are too large to fit through an unmodified plasmodesma, they are also too large to be freely diffused through the cytoplasm of the cell. Evidence has accumulated now for the involvement of other categories of viral proteins in intracellular movement in addition to the MP, including viral proteins originally associated with replication or gene expression. In this review, we will discuss the strategies that viruses use for intracellular movement from the replication site to the PD, in particular focusing on the role of host membranes for intracellular transport and the coordinated interactions between virus proteins within cells that are necessary for successful virus spread.

Figure 1.

Cellular Membranes and Virus Intra/Intercellular Movement. (A, B) Schematics of the host cell endomembrane system illustrate the association of virus proteins with a known movement function (black text) from both non-tubule-forming viruses (A) and tubule-forming viruses (B) with host membranes. Only those proteins whose localization has been demonstrated in plants are shown. Host proteins that are known to mediate such interactions are shown where applicable (green text). PVX and BSMV are shown as representatives of the potex-like and hordei-like viruses, respectively. There are additional virus proteins in these categories that associate with membranes but that are not included due to space limitations. DNAJ, class of chaperone proteins; Ara7, Rab5 GTPase ortholog and a marker for early endosomes; Syn, synaptotagmin, a clathrin-associated, SNARE-interacting protein; CRT, calreticulin; REM, remorin, a plasma membrane protein; KNOLLE, T-SNARE; PDLPs, plasmodesmal localized proteins; At-4/1, Arabidopsis protein localized to plasmodesmata. (C, D) The results of inhibiting secretory pathways with brefeldin A (BFA) treatments or dominant negative mutations (ex. Sar1, Arf1) on the movement of non-tubule-forming (C) and tubule-forming (D) viruses from the ER to the cell periphery. A blue arrow blocked with a red X denotes that these viruses do not move through the endomembrane secretory system, since BFA treatment and/or mutations did not inhibit the movement of these virus proteins to the cell periphery. An unblocked arrow indicates that these virus proteins do move through the secretory system, since BFA treatment and/or mutations did diminish movement. V, vacuole; G, Golgi apparatus; N, nucleus; ER, endoplasmic reticulum; C, chloroplast; Pex, peroxisome; EE, early endosome; LE, late endosome. * TOM1 associates with vacuolar and probably ER as well as other unidentified membranes. These membrane interactions may be mediated by the host membrane-associated protein TOM2A.