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

Natali Ozber; Paolo Margaria; Charles T. Anderson; Massimo Turina; Cristina Rosa

OuMV MP follows this model. Once OuMV MP reaches the plasma membrane, it is endocytosed, sorted into the TGN and then recycled back to the plasma membrane, bypassing the MVB/PVC-to-vacuole pathway, to maintain its presence at the plasma membrane and to reach PD. The retrieval of MP from the plasma membrane is possibly AP2M-independent and does not depend on the Y motif (AP2M binding site). On the other hand, the plasmodesmal targeting of MP relies on the LL motif, potentially through its function in endocytic pathways. The precise molecular mechanism of MP targeting is, however, still unknown. In fact, the N-terminal region of MP retaining the LL motif does not act as a plasmodesmal targeting, as opposed to the TMV MP, in which the first 50 amino acid residues were characterized as a plasmodesmal localization signal (Yuan et al., 2016) . Moreover, the C-terminus of OuMV has a predicted disordered region with . CC-BY-NC-ND 4.0 International license is made available under a The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/724716 doi: bioRxiv preprint many potential phosphorylation sites (not shown) that may be important for its regulation and interaction with host proteins. Although this model requires further examination to understand the mechanistic details of the targeting of OuMV MP to PD, to the best of our knowledge, this is the first study showing the role of a LL motif in plasmodesmal targeting of MPs. Movement proteins are often associated to the sites of viral replication, thus it is extremely hard to untangle if a disturbance in viral replication, as the one seen with our mutants, is due to mislocalization of the viral MP from the site of replication, or to modifications of the sites of replication. Our present efforts focus on identifying host factors that are directly involved in post-Golgi trafficking of OuMV MP and cell-to-cell movement of OuMV, to better elucidate the molecular mechanism of the virus movement. Simple viral systems, such as the one presented by OuMV, are an invaluable tool that can be advantageously used to identify missing components of highly complex plant trafficking pathways.