The role of post-Golgi transport pathways and sorting motifs in the plasmodesmal targeting of the movement protein (MP) of Ourmia melon virus (OuMV)

Abstract

Plants have a highly sophisticated endomembrane system targeted by plant viruses for cell-to-cell movement. The movement protein (MP) of ourmia melon virus (OuMV) is delivered to plasmodesmata (PD) and forms tubules to facilitate cell-to-cell movement. Although several functionally important regions for correct subcellular localization of OuMV MP have been identified, little is known about the pathways OuMV MP hijacks to reach the PD. Here, we demonstrate that OuMV MP localizes to the trans-Golgi network (TGN), but not to the multivesicular body/prevacuolar compartment or Golgi, and carries two putative sorting motifs, a tyrosine (Y) and a dileucine (LL) motif, near its N-terminus. Glycine substitutions in these motifs result in loss of OuMV infectivity in Nicotiana benthamiana and Arabidopsis. Live cell imaging of GFP-labeled sorting motif mutants shows that Y motif mutants fail to localize to the TGN, plasma membrane, and PD. Mutations in the LL motif do not impair plasma membrane targeting of MP but affect its ability to associate with callose deposits at the PD. Taken together, these results suggest that both motifs are indispensable for targeting OuMV MP to PD and for efficient systemic infection but show differences in functionality. Co-immunoprecipitation assays coupled with mass spectrometry identified a series of host factors that could interact with the OuMV MP and link the MP with various pathways, in particular vesicle trafficking and membrane lipids. This study provides new insights into the intracellular targeting of MPs and pathways that plant viruses hijack for cell-to-cell movement.