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Molecular Dissection of the Mechanism by Which Potexvirus Triple Gene Block Proteins Mediate Cell-to-Cell Transport of Infectious RNA

August 1998 , Volume 11 , Number  8
Pages  801 - 814

Tony J. Lough , 1 , 2 Khalid Shash , 1 Beatriz Xoconostle-Cázares , 1 Katrina R. Hofstra , 2 David L. Beck , 2 Ezequiel Balmori , 2 Richard L. S. Forster , 2 and William J. Lucas 1

1Section of Plant Biology, Division of Biological Sciences, University of California, Davis 95616, U.S.A.; 2Horticulture and Food Research Institute of New Zealand, Molecular Genetics Group, Private Bag 92 169, Auckland, New Zealand

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Accepted 6 May 1998.

The triple gene block (TGB; consisting of proteins TGB1--3) and coat protein (CP) of white clover mosaic potexvirus (WClMV) are required for cell-to-cell movement of viral RNA. Cell-to-cell spread of WClMV mutants in which the TGB open reading frames had been mutated was rescued in transgenic plants expressing specific TGB proteins (TGBPs). This indicated that there are no requirements for the synthesis in cis of viral TGBPs. These transgenic plants provided an experimental framework to explore the roles performed by the TGBPs and CP in cell-to-cell movement of WClMV RNA. Microinjection experiments established that TGB1 functions as the WClMV cell-to-cell movement protein (MP). Furthermore, combined microinjection and dual-channel confocal laser scanning microscopy provided direct evidence that infectious transcripts of WClMV move cell to cell as a ribonucleoprotein complex, consisting of single-stranded RNA, TGB1, and CP. Movement of this ribonucleoprotein complex displayed an absolute requirement for the presence of both TGB2 and TGB3. A model consistent with these findings is presented.

Additional keywords: macromolecular trafficking, plasmodesmata.

© 1998 The American Phytopathological Society