Japanese soil-borne wheat mosaic virus CP-RT protein associates with the endoplasmic reticulum network and mobile particles at the ER-Golgi interface during infection

Viruses transmitted by the obligate biotrophic parasite Polymyxa graminis cause substantial losses in agriculture worldwide. A better knowledge of the infection process may lead to the identification of novel strategies for virus control. The coat protein readthrough (CP-RT) protein or its homologs, present in all P. graminis-transmitted viruses, are transmission factors of the viruses. To better characterize CP-RT function, we modified an infectious Japanese soil-borne wheat mosaic virus (JSBWMV) cDNA clone to express fluorescent protein-tagged CP-RT and analyzed the protein during infection in the model host plant Nicotiana benthamiana. We found that JSBWMV CP-RT localized to and self-interacted at the endoplasmic reticulum (ER) network and in small cortical particles during infection. The particles moved along the ER network and associated with sub-ER sites, most likely ER import sites, as indicated by co-localization with Sar1-GTP:GFP and AtSYP72:GFP. Particles were still present after reabsorption of the Golgi into the ER network after Brefeldin A treatment, thus they were not an intrinsic part of the Golgi-ER compartment. In natural CP-RT deletion mutants, the localization to the ER network was lost, while localization to the particles was maintained. Consistent with earlier data our findings from transmission experiments in hydroponic barley culture indicate that an intact JSBWMV CP-RT may be important for virus transmission by the vector, as more wild type CP-RT forms compared to mutant CP-RT forms accumulated in newly infected plants. ER-association of CP-RT may thus be important for virus transmission and early infection processes.