The triple-gene-block protein 3 (TGBp3) of Bamboo mosaic virus (BaMV) is an integral endoplasmic reticulum (ER) membraneprotein which is assumed to form a membrane complex to deliver the virus intracellularly. However, the virus entity that isdelivered to plasmodesmata (PD) and its association with TGBp3-based complexes are not known. Results from chemicalextraction and partial proteolysis of TGBp3 in membrane vesicles revealed that TGBp3 has a right-side-out membranetopology; i.e., TGBp3 has its C-terminal tail exposed to the outer surface of ER. Analyses of the TGBp3-specificimmunoprecipitate of Sarkosyl-extracted TGBp3-based complex revealed that TGBp1, TGBp2, TGBp3, capsid protein (CP),replicase and viral RNA are potential constituents of virus movement complex. Substantial co-fractionation of TGBp2, TGBp3and CP, but not TGBp1, in the early eluted gel filtration fractions in which virions were detected after TGBp3-specificimmunoprecipitation suggested that the TGBp2- and TGBp3-based complex is able to stably associate with the virion. Thisnotion was confirmed by immunogold-labeling transmission electron microscopy (TEM) of the purified virions. In addition,mutational and confocal microscopy analyses revealed that TGBp3 plays a key role in virus cell-to-cell movement byenhancing the TGBp2- and TGBp3-dependent PD localization of TGBp1. Taken together, our results suggested that the cellto-cell movement of potexvirus requires stable association of the virion cargo with the TGBp2- and TGBp3-based membranecomplex and recruitment of TGBp1 to the PD by this complex
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