Dual targeting of a virus movement protein to ER and plasma membrane subdomains is essential for plasmodesmata localization

Plant virus movement proteins (MPs) localize to plasmodesmata (PD) to facilitate virus cell-to-cell movement. Numerous studies have suggested that MPs use a pathway either through the ER or through the plasma membrane (PM). Furthermore, recent studies reported that ER-PM contact sites and PM microdomains, which are subdomains found in the ER and PM, are involved in virus cell-to-cell movement. However, functional relationship of these subdomains in MP traffic to PD has not been described previously. We demonstrate here the intracellular trafficking of fig mosaic virus MP (MP(FMV)) using live cell imaging, focusing on its ER-directing signal peptide (SP(FMV)). Transiently expressed MP(FMV) was distributed predominantly in PD and patchy microdomains of the PM. Investigation of ER translocation efficiency revealed that SP(FMV) has quite low efficiency compared with SPs of well-characterized plant proteins, calreticulin and CLAVATA3. An MP(FMV) mutant lacking SP(FMV) localized exclusively to the PM microdomains, whereas SP chimeras, in which the SP of MP(FMV) was replaced by an SP of calreticulin or CLAVATA3, localized exclusively to the nodes of the ER, which was labeled with Arabidopsis synaptotagmin 1, a major component of ER-PM contact sites. From these results, we speculated that the low translocation efficiency of SP(FMV) contributes to the generation of ER-translocated and the microdomain-localized populations, both of which are necessary for PD localization. Consistent with this hypothesis, SP-deficient MP(FMV) became localized to PD when co-expressed with an SP chimera. Here we propose a new model for the intracellular trafficking of a viral MP. A substantial portion of MP(FMV) that fails to be translocated is transferred to the microdomains, whereas the remainder of MP(FMV) that is successfully translocated into the ER subsequently localizes to ER-PM contact sites and plays an important role in the entry of the microdomain-localized MP(FMV) into PD.