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

Heterotetrameric adaptor protein (AP) complexes are key components of protein sorting in endocytic and post-Golgi secretory pathways (reviewed by Canagarajah et al., 2013) . Genetic approaches in Arabidopsis have revealed that AP complexes function in hormone signaling (Di Rubbo et al., 2013; Fan et al., 2013; Kansup et al., 2013; Wang et al., 2013) , cytokinesis (Teh et al., 2013) , gravitropism (Niihama et al. 2009 ), reproduction Yamaoka et al., 2013; Feng et al., 2018) , and immunity (Hatsugai et al., 2016) . AP1 mediates protein transport from the TGN/EE to the plasma AP-dependent protein sorting is mediated most commonly by the recognition of tyrosine (Y), YXXØ and dileucine (LL), [D/E]XXXL[L/I] motifs (where Ø is a bulky hydrophobic residue and X any amino acid) on cargo proteins. In plants, one of the most studied sorting signals is the Y motif. Several cell surface receptors, receptor-like kinases and receptor-like proteins (reviewed by Geldner and Robatzek, 2008) , transporters (BOR1; Takano et al., 2010) and vacuolar sorting receptors (Happel et al., 2004; Gershlick et al., 2014; Nishimura et al. 2016 ) carry Y motifs. There are, however, only a few reports showing the recognition of these signals by AP complexes (Holstein et al., 2002 : Happel et al., 2004 : Gershlick et al., 2014 . LL motifs in plants are far less studied. These motifs are important for sorting of tonoplast proteins (reviewed by Pedrazzini et al., 2013) , and in this context, the recognition of a LL motif by AP1, but not AP3, has been reported (Wang et al., 2014) . Whether this motif functions in plant endocytic pathways remains to be elucidated. Viral movement proteins (MPs) are critical for mediating virus movement through plasmodesmata (PD), as the size of virus nucleic acids and virions does not allow for passive intercellular movement. Indeed, localization of viral MPs to PD has been demonstrated, and some MPs can increase the PD size exclusion limit to allow for the passage of viruses into adjacent cells (reviewed by Benitez-Alfonso et al., 2010; Niehl and Henlein, 2011) . Viral MPs use various transport strategies to reach PD. One of these strategies is hijacking the host endomembrane system (reviewed by Pitzalis and Heinlein, 2017) . MPs of several viruses contain putative Y and LL motifs, but there are only few reports pertaining to the role of these sorting motifs in the targeting of viral MPs to PD. The MP of Grapevine fanleaf virus (GFLV, genus Nepovirus) contains putative Y and LL motifs near its N terminus and these motifs are also conserved in MPs of other nepoviruses (Laporte et al., 2003) . The triple gene block protein TGB3 of Poa semilatent virus (PSLV, genus Hordeivirus) harbors a conserved YQDLN motif, which conforms to a Y sorting motif, in the central hydrophilic region that is involved in targeting to the cell periphery (Solovyev et al., 2000) . Potato mop-top virus (PMTV, genus Pomovirus) TGB3 also carries the same motif in its central loop. The disruption of this motif abolishes the localization of PMTV TGB3 to the ER and to motile granules, and the targeting of PMTV TGB3 to plasmodesmata (Haupt et al., 2005; Tilsner et al., 2010) . Three Y motifs, YLPL/YGKF/YPKF, are present in Cauliflower mosaic virus (CaMV, genus Caulimovirus) MP and at least one of the three motifs is required for endosomal localization of MP and for tubule formation. Moreover, CaMV MP directly interacts with Arabidopsis AP2M through the three Y motifs (Carlucc