Determining the Functional Specificity of Beet Yellows Closterovirus Heat Shock Protein 70KDa Homolog

Speaker: Jonathan Reed

Abstract: Molecular chaperones that belong to the ~70 kDa heat shock protein (HSP70) family are ubiquitous cellular proteins, providing for a variety of functions. HSP70 possess an ATPase domain that supplies the energy for their chaperone activity. The plant virus family, Closteroviridae, is unique among viruses in that it encodes for its own HSP70 homolog (HSP70h) that functions in viral cell-to-cell movement.
Our model system involved the beet yellows closterovirus (BYV) tagged via insertion of the gene encoding green florescent protein (GFP). The cell-to-cell movement of BYV·GFP was evaluated using a florescent microscope. To examine the level of functional specialization of HSP70h and its ATPase domain, we replaced BYV gene or part thereof with the corresponding coding sequences of the peaHSP71.2, peaHSC70.0, or HSP70h of beet yellows stunt virus (BYSV), a related closterovirus.
Hybrid BYV clones expressing each of the peaHSP70’s or corresponding ATPase domains failed to move from cell-to-cell. This result suggests that BYV HSP70 diverged from its cellular ancestors to acquire specific capability to translocate BYV.
To our surprise, the replacement experiments with BYSV HSP70h and its ATPase domain also resulted in movement-incompetent virus, suggesting an extreme level of functional specialization. This latter result may be due to the phloem-limited nature of BYSV. Our system can only introduce BYV hybrids to the mesophyll cells. It is possible that BYSV HSP70h is functional only in a phloem tissue. We are currently developing an experimental system for testing this possibility via introducing BYV hybrids into phloem cells.