Recent advances in research of plant virus movement mediated by triple gene block

Abstract

The aim of this short review was to summarize recent advances in the field of viral cell-to-cell movement mediated by the triple gene block (TGB). The growing body of new research has uncovered links between virus cell-to-cell trafficking and replication, silencing suppression, virus spread over the plant, as well as suggested the roles of nucleus/nucleolus in plant virus transport and revealed protein-membrane associations occurring during subcellular targeting and cell-to-cell movement. In this context, our review briefly summarized current views on several potentially important functions of TGB proteins and on the development of new experimental systems that improved understanding of the molecular events during TGB-mediated virus movement.

Keywords: TGB; movement protein; plant virus; triple gene block; virus movement.

Figure 1

(A) Genome organization of the new TGB-containing virus HGSV. Boxes represent genome-encoded open reading frames. Replicase gene domains are shown in the yellow box: MT, methyltransferase; PRO, protease; HEL, RNA helicase; POL, RNA-dependent RNA polymerase. Green boxes represent the TGB. Blue box specifies the viral coat protein (CP). (B) Molecular organization of TGB1, TGB2, and TGB3 proteins. Nucleolar localization sequences and helicase domain regions of TGB1 are shown above the BSMV TGB1. Characteristic signature sequences in TGB2 and TGB3 are shown. Dark green boxes indicate hydrophobic transmembrane sequence segments. (C) General scheme of TGB-mediated intracellular movement and interactions of macromolecules. Processes specific for potex-like and hordei-like TGBs are shown by blue and red arrows, respectively. Note that the box ‘binding to chaperone SGT1’ means a functional interaction between TGB3 and SGT1 (Ye et al., 2012). Transport steps common for both potex- and hordei-like TGBs are shown by parallel arrows. Processes that are not proved to be involved directly in virus cell-to-cell movement are shown by dashed arrows. Numbered gray arcs indicate alternative pathways of intracellular trafficking. (1) TGB2 and TGB3 may travel to their destinations in specific membrane containers such as vesicles formed in a COPII-independent manner, or ER-specific membrane rafts (Verchot-Lubicz et al., ; this review). (2) Trafficking to the cell periphery of the TGB1 protein (and TGB1-containing RNPs) may exploit the cytoskeleton-based pathway with the immediate movement to PD-associated compartment, or via binding to TGB2/TGB3-containing membrane subdomains involved in cytoskeleton-dependent transport (Verchot-Lubicz et al., ; this review). (3) TGB2/3-specific membrane containers may bind movement-competent RNPs containing TGBp1. On the other hand, these complexes may be delivered directly to the neck region of PD through interactions with cytoskeleton (see above; Verchot-Lubicz et al., ; this review). For further details, see text.