Plasmodesmal endoplasmic reticulum proteins regulate intercellular trafficking of cucumber mosaic virus in Arabidopsis (May, erad190, 2023)

Plasmodesmata (PD) are plasma membrane (PM)-lined cytoplasmic nanochannels that mediate in cell-to-cell communication across the cell wall. A range of proteins are embedded in the PD PM and endoplasmic reticulum (ER) and function in regulating PD-mediated symplasmic trafficking. However, knowledge of the nature and function of the ER-embedded proteins, in the intercellular movement of non-cell-autonomous proteins, is limited. Here, we report the functional characterization of two ER luminal proteins, AtBiP1/2, and two ER integral membrane proteins, AtERdj2A/B, which are located within the PD. These PD proteins were identified as interacting proteins with Cucumber mosaic virus (CMV) movement protein (MP) in coimmunoprecipitation studies, using an Arabidopsis-derived plasmodesmal-enriched cell wall protein preparation (PECP). The AtBiP1/2 PD location was confirmed by transmission electron microscopy-based immunolocalization, and their AtBiP1/2 signal peptides (SPs) function in PD targeting. In vitro/in vivo pull-down assays revealed the association between AtBiP1/2 and CMV MP, mediated by AtERdj2A, through the formation of an AtBiP1/2-AtERdj2-CMV MP complex within PD. The role of this complex in CMV infection was established, as systemic infection was retarded in bip1/bip2w and erdj2b mutants. Our findings provide a model for a mechanism by which the CMV MP mediates in cell-to-cell trafficking of its viral ribonucleoprotein complex.