A new plant-specific syntaxin-6 protein may define an intracytoplasmic route for begomoviruses

Due to limited free diffusion in the cytoplasm, viruses must use active transport mechanisms to move intracellularly. Nevertheless, how the plant ssDNA begomovirus hijacks the host intracytoplasmic transport machinery to move from the nucleus to the plasmodesma remains enigmatic. Here, we identified nuclear shuttle protein (NSP)-interacting proteins by probing a protein microarray and demonstrated that viral NSP, a facilitator of the nucleocytoplasmic trafficking of viral (v)DNA, interacts with a new endosomal vesicle-localized plant-specific syntaxin-6 protein, designated NISP in planta . We also showed that begomovirus infection requires the NISP-NSP interaction; NISP displays a pro-viral function, but not the syntaxin-6 paralog AT2G18860 that failed to interact with NSP. Consistent with these findings, nisp-1 mutant plants were less susceptible to begomovirus infection, a phenotype reversed by NISP complementation, whereas overexpressing lines accumulated higher levels of viral DNA than wild-type. Furthermore, NISP interacts with NIG, another pro-viral factor that accessorizes the NSP-vDNA nucleocytoplasmic translocation. Additionally, the NISP-NIG interaction is enhanced by NSP. We also showed that NISP associates with vDNA and might assemble a NISP-NIG-NSP-vDNA- complex. NISP may function as a docking site for recruiting NIG and NSP into trafficking vesicles, providing a mechanism for the intracytoplasmic translocation of the NSP-vDNA complex towards the cell periphery.