Mutations in the central domain of potato virus X TGBp2 eliminate granular vesicles and virus cell-to-cell trafficking.

Most RNA viruses remodel the endomembrane network to promote virus replication, maturation, or egress. Rearrangement of cellular membranes is a crucial component of viral pathogenesis. The PVX TGBp2 protein induces vesicles of the granular type to bud from the endoplasmic reticulum network. Green fluorescent protein (GFP) was fused to the PVX TGBp2 coding sequence and inserted into the viral genome and into pRTL2 plasmids to study protein subcellular targeting in the presence and absence of virus infection. Mutations were introduced into the central domain of TGBp2, which contains a stretch of conserved amino acids. Deletion of a 10-amino-acid segment (m2 mutation) overlapping the segment of conserved residues eliminated the granular vesicle and inhibited virus movement. GFP-TGBp2m2 proteins accumulated in enlarged vesicles. Substitution of individual conserved residues in the same region similarly inhibited virus movement and caused the mutant GFP-TGBp2 fusion proteins to accumulate in enlarged vesicles. These results identify a novel element in the PVX TGBp2 protein which determines vesicle morphology. In addition, the data indicate that vesicles of the granular type induced by TGBp2 are necessary for PVX plasmodesmata transport.