The C-Terminal Transmembrane Domain of Cowpea Mild Mottle Virus TGBp2 Is Critical for Plasmodesmata Localization and for Its Interaction With TGBp1 and TGBp3

The functions of triple gene block proteins and coat protein of apple stem pitting virus in viral cell-to-cell movement

Apple stem pitting virus is a species in the genus Foveavirus in the family Betaflexiviridae. Apple stem pitting virus (ASPV) commonly infects apple and pear plants grown worldwide. In this study, by integrating bimolecular fluorescence complementation, split-ubiquitin-based membrane yeast two-hybrid, and Agrobacterium-mediated expression assays, the interaction relationships and the subcellular locations of ASPV proteins TGBp1-3 and CP in Nicotiana benthamiana leaf cells were determined. Proteins CP, TGBp1, TGBp2, and TGBp3 were self-interactable, and TGBp2 played a role in the formation of perinuclear viroplasm and enhanced the colocalization of TGBp3 with CP and TGBp1. We found that the plant microfilament and endoplasmic reticulum structures were involved in the production of TGBp3 and TGBp2 vesicles, and their disruption decreased the virus accumulation level in the systemic leaves. The TGBp3 motile vesicles functioned in delivering the viral ribonucleoprotein complexes to the plasma membrane. Two cysteine residues at sites 35 and 49 of the TGBp3 sorting signal were necessary for the diffusion of TGBp3-marked vesicles. Furthermore, our results revealed that TGBp1, TGBp2, and CP could increase plasmodesmal permeability and move to the adjacent cells. This study demonstrates an interaction network and a subcellular location map of four ASPV proteins and for the first time provides insight into the functions of these proteins in the movement of a foveavirus.

Heavy Metal-Associated Isoprenylated Plant Proteins (HIPPs) at Plasmodesmata: Exploring the Link between Localization and Function

Heavy metal-associated isoprenylated plant proteins (HIPPs) are a metallochaperone-like protein family comprising a combination of structural features unique to vascular plants. HIPPs possess both one or two heavy metal-binding domains and an isoprenylation site, facilitating a posttranslational protein lipid modification. Recent work has characterized individual HIPPs across numerous different species and provided evidence for varied functionalities. Interestingly, a significant number of HIPPs have been identified in proteomes of plasmodesmata (PD)-nanochannels mediating symplastic connectivity within plant tissues that play pivotal roles in intercellular communication during plant development as well as responses to biotic and abiotic stress. As characterized functions of many HIPPs are linked to stress responses, plasmodesmal HIPP proteins are potentially interesting candidate components of signaling events at or for the regulation of PD. Here, we review what is known about PD-localized HIPP proteins specifically, and how the structure and function of HIPPs more generally could link to known properties and regulation of PD.

Subcellular localization and interactions among TGB proteins of cowpea mild mottle virus

Cowpea mild mottle virus (CPMMV) is a flexuous filamentous virus that belongs to the genus Carlavirus (family Betaflexiviridae). The CPMMV genome contains six open reading frames (ORFs), among which the triple gene block (TGB), encoded by ORFs 2 to 4, has been reported to encode movement proteins for different viruses. The subcellular localization of the TGB proteins of CPMMV isolate CPMMV:BR:MG:09:2 was analysed by transient expression of each protein fused to a fluorophore. Overall, the accumulation pattern and interactions among CPMMV TGB proteins (TGBp) were similar to those of their counterparts from the potex-like group. Considering these similarities, we evaluated the potential interactions between the TGB proteins of CPMMV and of potato virus X, which could complement cell-to-cell movement. The TGBp2 and TGBp3 of PVX had an effect on CPMMV TGBp1, directing it to the plasmodesmata, but the reverse was not true.