Subcellular localization of the HSP70-homolog encoded by beet yellows closterovirus

Closteroviridae is the only viral family coding for a homolog of HSP70 (HSP 70h). Polyclonal antiserum to recombinant beet yellows closterovirus (BW) H SP70h was generated and used for immunogold labeling of the leaf samples de rived from the infected Nicotiana benthamiana plants. Ultrastructural analy sis revealed the preferential accumulation of BW in phloem, although occasi onal infection of the leaf mesophyll cells was also observed. The strongest HSP70h-specific labeling was associated with Virion aggregates and vesicle s harboring scattered virions. HSP70h was also observed in close proximity of plasmodesmatal and inside the plasmodesmatal channels. The possible role of the BW HSP70h in RNA encapsidation was tested in tobacco protoplasts. A BW mutant possessing an inactivated HSP70h gene exhibited no detectable en capsidation defects. Collectively, the obtained results suggested that clos teroviral HSP70h escorts the virions to their destinations inside the infec ted cells and possibly participates in the intercellular translocation of B YV. (C) 1999 Academic Press.