Two Plant–Viral Movement Proteins Traffic in the Endocytic Recycling Pathway

Abstract

Many plant viruses exploit a conserved group of proteins known as the triple gene block (TGB) for cell-to-cell movement. Here, we investigated the interaction of two TGB proteins (TGB2 and TGB3) of Potato mop-top virus (PMTV), with components of the secretory and endocytic pathways when expressed as N-terminal fusions to green fluorescent protein or monomeric red fluorescent protein (mRFP). Our studies revealed that fluorophore-labeled TGB2 and TGB3 showed an early association with the endoplasmic reticulum (ER) and colocalized in motile granules that used the ER-actin network for intracellular movement. Both proteins increased the size exclusion limit of plasmodesmata, and TGB3 accumulated at plasmodesmata in the absence of TGB2. TGB3 contains a putative Tyr-based sorting motif, mutations in which abolished ER localization and plasmodesmatal targeting. Later in the expression cycle, both fusion proteins were incorporated into vesicular structures. TGB2 associated with these structures on its own, but TGB3 could not be incorporated into the vesicles in the absence of TGB2. Moreover, in addition to localization to the ER and motile granules, mRFP-TGB3 was incorporated into vesicles when expressed in PMTV-infected epidermal cells, indicating recruitment by virus-expressed TGB2. The TGB fusion protein-containing vesicles were labeled with FM4-64, a marker for plasma membrane internalization and components of the endocytic pathway. TGB2 also colocalized in vesicles with Ara7, a Rab5 ortholog that marks the early endosome. Protein interaction analysis revealed that recombinant TGB2 interacted with a tobacco protein belonging to the highly conserved RME-8 family of J-domain chaperones, shown to be essential for endocytic trafficking in Caenorhabditis elegans and Drosophila melanogaster. Collectively, the data indicate the involvement of the endocytic pathway in viral intracellular movement, the implications of which are discussed.

Hitching a Ride on Vesicles: Cauliflower Mosaic Virus Movement Protein Trafficking in the Endomembrane System

“…Indirect support for this hypothesis comes also from the evidence that Arabidopsis PRA1, the small Rab GTPase interacting with MP (Huang et al, 2001), resides entirely in endosomal compartments/PVCs and colocalizes perfectly with ARA7 (Alvim Kamei et al, 2008). Taken together, these results suggest that CaMV MP becomes part of MVBs on the way to the vacuole, and evidence that Potato mop-top virus (PMTV) triple gene block2 also colocalizes in vesicles with ARA7 (Haupt et al, 2005) supports the hypothesis that viral proteins can exploit the endomembrane system for degradation.…”

“…With endocytosis established as a fundamental plant transport process, it is conceivable that, due to their obligate nature, plant viruses use such mechanisms to support essential functions like replication and movement. Although a few plant viral proteins have been shown to interact or colocalize with components of the endocytic pathway (Haupt et al, 2005;Lewis and Lazarowitz, 2010), the crucial interaction with an AP-2-like complex, which mediates protein internalization from the PM, remains to be proved not only for plant virus proteins but for any protein in plants.…”

“…The m-subunit of AP complexes is devoted to cargo protein selection via a specific and well-characterized interaction with a Tyrsorting signal, YXXF, where F is a bulky hydrophobic residue and X is any amino acid (Bonifacino and Dell'Angelica, 1999). YXXF motifs are present in the cytoplasmic tail of many proteins integral to the PM and TGN/EE and have been found in the movement proteins (MPs) of some viruses (Laporte et al, 2003;Haupt et al, 2005). Plant viruses are obligate parasites that exploit host components to move within the cell and from cell to cell into the vascular system for systemic invasion of the host.…”

In vitro expression and antiserum production against the movement protein of Citrus leprosis virus C (CiLV-C)

“…In the CiLV-C pathosystem, there is a particular interest to follow the MP expression and its fate in the plant cells to understand why the infection is localized and not systemic. It is important to point out that most of the studies related to MPs are concentrated in their function or loss of function through mutations, production of transgenic plants, cellular localization and increasing of the size exclusion limit of the plasmodesmata, and interaction with other proteins in protoplasts cells (Wolf et al, 1989;Deom et al, 1990;Haupt et al, 2005;Akamatsu et al, 2007). Moreover, the peculiarity of MPs as non structural proteins contributes to the scarcity of studies related to antibody production and in vivo detection.…”

“…MPs are present in the genomes of most plant viruses and are implicated in cell-to-cell movement of the infectious material associated with the plasmodesmata (Wolf et al, 1989;Deom et al, 1990;Haupt et al, 2005;Akamatsu et al, 2007). In the CiLV-C pathosystem, there is a particular interest to follow the MP expression and its fate in the plant cells to understand why the infection is localized and not systemic.…”