Graduate Thesis Or Dissertation
 

Genetic requirements for the assembly and cell-to-cell movement of the beet yellows virus

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  • Beet yellows virus (BYV) is a filamentous, positive-strand RNA virus that belongs to the family Closteroviridae. BYV particles encapsidate a 15.5 kb RNA and posses complex polar architecture. A long virion body is formed by the major capsid protein(CP), whereas the minor capsid protein (CPm) assembles a short tail that encapsidates the 5'-terminal region of BYV RNA. In addition to proteins required for viral RNA replication and encapsidation, BYV encodes four proteins whose role in the virus life cycle was unknown. These proteins include a small, 6-kDa, hydrophobic protein (p6), a homolog of the cellular 70-kDa heat shock proteins (Hsp7Oh), a 64-kDa protein (p64), and a 20-kDa protein (p20). It was found recently that Hsp7Oh, p64, and p20 are incorporated into BYV virions, and that Hsp7Oh is required for the virus movement from cell to cell. In this study, we characterized genetic requirements for BYV assembly and cell-to-cell movement, and determined relationships between these two processes. It was demonstrated that in addition to Hsp7Oh, p6, p64, CP, and CPm are each essential, but not sufficient for virus movement. These results indicated that five-component movement machinery of BYV is the most complex among plant viruses. Extensive mutational analysis of CP and CPm revealed strong correlation between abilities of BYV to assemble tailed virions and to move from cell to cell, suggesting that formation of functional virions is a prerequisite for virus translocation. We have found that CPm, Hsp7Oh, and p64 are necessary for the efficient virion tail formation. Assembly of the virion tails and bodies was shown to occur independent of each other and likely to involve two separate packaging signals within the genomic RNA. Our work demonstrated that BYV encodes one conventional movement protein, p6, whose only known function is to mediate virus movement. The other four movement associated proteins of BYV, CP, CPm, Hsp7Oh, and p64 are the virion components, each of which is required for assembly of the tailed, movement-competent virions. Based on these and other data, we propose that BYV and other closteroviruses evolved virion tails as a specialized device for the directional cell-to-cell movement of large RNA genomes.
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