Varied Movement Strategies Employed by Triple Gene Block–Encoding Viruses

Abstract

Several RNA virus genera belonging to the Virgaviridae and Flexiviridae families encode proteins organized in a triple gene block (TGB) that facilitate cell-to-cell and long-distance movement. The TGB proteins have been traditionally classified as hordei-like or potex-like based on phylogenetic comparisons and differences in movement mechanisms of the Hordeivirus and Potexvirus spp. However, accumulating data from other model viruses suggests that a revised framework is needed to accommodate the profound differences in protein interactions occurring during infection and ancillary capsid protein requirements for movement. The goal of this article is to highlight common features of the TGB proteins and salient differences in movement properties exhibited by individual viruses encoding these proteins. We discuss common and divergent aspects of the TGB transport machinery, describe putative nucleoprotein movement complexes, highlight recent data on TGB protein interactions and topological properties, and review membrane associations occurring during subcellular targeting and cell-to-cell movement. We conclude that the existing models cannot be used to explain all TGB viruses, and we propose provisional Potexvirus, Hordeivirus, and Pomovirus models. We also suggest areas that might profit from future research on viruses harboring this intriguing arrangement of movement proteins.

Helical capsids of plant viruses: architecture with structural lability

“…On the other hand, TGB1 molecules can bind to the PVX virion end containing the 5¢-terminus of genomic RNA (see above). In addition to converting virions into a translatable form, TGB1 binding can be pertinent to virus transport in plants, since TGB1 can be directed to plasmodesmata by the TGB2 and TGB3 proteins (discussed in Morozov & Solovyev, 2003;Solovyev et al, 2012;Verchot-Lubicz et al, 2010). This plausible hypothesis is consistent with the observations that deletions in the potexvirus CP C-terminal region involved in interactions with the TGB1 protein, while having little effect on virion formation (see above), block virus cell-to-cell movement (Fedorkin et al, 2001;Forster et al, 1992).…”

Plasma Membrane Localization of Solanum tuberosum Remorin from Group 1, Homolog 3 Is Mediated by Conformational Changes in a Novel C-Terminal Anchor and Required for the Restriction of Potato Virus X Movement

“…This indicates that RemCA is not directly involved in the interaction with TGBp1 but rather that StREM1.3-TGBp1 interaction takes place at the PM. The subcellular localization of TGBp1 is complex and depends on the other TGB proteins, viral RNA, and the host plant species (Verchot-Lubicz et al, 2010). When expressed alone in tobacco, TGBp1 is mostly nuclear and cytoplasmic.…”

Experimental and bioinformatic evidence that raspberry leaf blotch emaravirus P4 is a movement protein of the 30K superfamily

“…The cell-to-cell movement of potato virus X (PVX) and some other viruses utilizes three viral proteins, comprising the triple gene block (TGB), which have no structural similarity to 30K family proteins (Verchot-Lubicz et al, 2010). However, underlying shared aspects of the movement process have been revealed by the finding that movement of TGB1-deficient PVX can be rescued by the 30K MP (Morozov et al, 1997) and that TGB1 can rescue movement of 30K MP-deficient Odontoglossum ringspot virus, a tobamovirus related to TMV (Ajjikuttira et al, 2005).…”