Complex architecture of a closterovirus virion Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/h415pf028

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  • In this investigation we examined the molecular architecture and functions associated with the virion components of the Beet yellows virus (BYV), family Closteroviridae. The BYV virions are filamentous particles composed of two coat proteins, the major coat protein (CP), which encapsidates the majority of the genome, and the minor coat protein (CPm) that makes a small tail-like structure at one end of the virions. The bipolar architecture of closteroviruses is unique among the plant helical viruses, and suggests that CP and CPm evolved to fulfill different functions in the life cycle of closteroviruses. Current models propose that the CPm tails are specialized virion components that participate in the vector transmission and cell-to-cell movement of the virus. In addition to CP and CPm, the cell-to-cell movement of BYV requires the contributions of three highly conserved viral genes encoding a small 6.4-kDa hydrophobic protein (p6), a HSP7O homologue (HSP7Oh) and a 64-kDa protein (p64). Using nano-liquid chromatography/tandem mass spectrometry and biochemical analyses we demonstrate that HSP7Oh and p64 are associated with virions. This conclusion is based on the co-migration of HSP7Oh and p64 with BYV virions in sucrose density gradients, and on the co-immunoprecipitation of HSP7Oh, pM and BYV capsid protein using anti-HSP7Oh or anti-p64 serum. The association of HSP7Oh and p64 with the virions is resistant to high concentrations of sodium chloride, which normally disrupt non-covalent protein interactions. Low concentrations of sodium dodecyl sulfate (SDS) or lithium chloride (LiC1), and treatment with alkaline or acidic pH resulted in the simultaneous disassembly of virions and dissociation of HSP7Oh and p64. The N-terminal domain of p64 is exposed at the virion surface and is accessible to antibodies and mild trypsin digestion. In contrast, the C-terminal domain is embedded in the virion and is inaccessible to antibodies or trypsin. The C-terminal domain of p64 is shown to be homologous to CP and CPm. Mutation of the signature motifs of capsid proteins of filamentous RNA viruses in p64 results in the formation of defective virions, which are unable to move from cell to cell. These results reveal the dual function of HSP7Oh and p64 in tail assembly and BYV motility and support the concept of the virion tail as a specialized device for BYV cell-to-cell movement.
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