Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

Abstract

Most plant viruses code for movement proteins (MPs) targeting plasmodesmata to enable cell-to-cell and systemic spread in infected plants. Small membrane-embedded MPs have been first identified in two viral transport gene modules, triple gene block (TGB) coding for an RNA-binding helicase TGB1 and two small hydrophobic proteins TGB2 and TGB3 and double gene block (DGB) encoding two small polypeptides representing an RNA-binding protein and a membrane protein. These findings indicated that movement gene modules composed of two or more cistrons may encode the nucleic acid-binding protein and at least one membrane-bound movement protein. The same rule was revealed for small DNA-containing plant viruses, namely, viruses belonging to genus Mastrevirus (family Geminiviridae) and the family Nanoviridae. In multi-component transport modules the nucleic acid-binding MP can be viral capsid protein(s), as in RNA-containing viruses of the families Closteroviridae and Potyviridae. However, membrane proteins are always found among MPs of these multicomponent viral transport systems. Moreover, it was found that small membrane MPs encoded by many viruses can be involved in coupling viral replication and cell-to-cell movement. Currently, the studies of evolutionary origin and functioning of small membrane MPs is regarded as an important pre-requisite for understanding of the evolution of the existing plant virus transport systems. This paper represents the first comprehensive review which describes the whole diversity of small membrane MPs and presents the current views on their role in plant virus movement.

Keywords: DNA genome; RNA genome; evolution; hydrophobic protein; intercellular movement; plant virus; protein motifs; trans-membrane domain; triple gene block.

Figure 1.. Comparison of plant virus RNA genomes encoding multicomponent cell-to-cell transport systems. Genes are shown as boxes with the names of the encoded proteins. Genes of proteins involved in cell-to-cell movement are shown in green. Genes encoding small hydrophobic proteins are shown in blue. Replicative genes are shown in yellow. Arrows indicate read-through codons. PVY, Potato virus Y; BYV, Beet yellows virus; PVX, Potato virus X; BSMV, Barley stripe mosaic virus; BNYVV, Beet necrotic yellow vein virus; HGSV, Hibiscus green spot virus; CarMV, Carnation mottle virus; RAVA, Ribes americanum virus A; BYDV, Barley yellow dwarf virus.

Figure 2.. Comparison of plant virus DNA genomes encoding small hydrophobic MPs. Genes are shown as arrows with the names of the encoded proteins. MSV, Maize streak virus. LIR–long intergenic region; SIR – short intergenic region. For Nanoviridae, only four DNA components (DNA-R, -S, -M and -C) encoding the functionally characterized proteins are shown, and hairpin marks replication origin. The numbers indicate the sizes of circular genomic DNAs.