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Varied Movement Strategies Employed by Triple Gene Block--Encoding Viruses

¹Oklahoma State University, Department of Entomology and Plant Pathology, Stillwater 74078, U.S.A.; ²Scottish Crop Research Institute, Invergowrie, by Dundee, DD2 5DA, Scotland, U.K.; ³Moscow State University, Department of Virology and A. N. Belozersky Institute of Physico-Chemical Biology, Moscow 119899 Russia; ⁴Department of Plant and Microbial Biology, University of California, Berkeley 94720, U.S.A.; ⁵Institut de biologie moléculaire des plantes, laboratoire propre du CNRS (UPR 2357) conventionné avec l'Université de Strasbourg, 12 rue du Général Zimmer, 67084 Strasbourg, France

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.