Cell-to-cell movement of potexviruses: Evidence for a ribonucleoprotein complex involving the coat protein and first triple gene block protein

The triple gene block proteins (TGBp1-3) and coat protein (CP) of potexviru ses are required for cell-to-cell movement. Separate models have been propo sed for intercellular movement of two of these viruses, transport of intact virions, or a ribonucleoprotein complex (RNP) comprising genomic RNA, TGBp 1, and the CP, At issue therefore, is the form(s) in which RNA transport oc curs and the roles of TGBp1-3 and the CP in movement. Evidence is presented that, based on microprojectile bombardment studies, TGBp1 and the CP, but not TGBp2 or TGBp3 are co-translocated between cells with viral RNA. In add ition, cell-to-cell movement and encapsidation functions of the CP were sho wn to be separable, and the rate-limiting factor of potexvirus movement was shown not to be virion accumulation, but rather, the presence of TGBp1-3 a nd the CP in the infected cell, These findings are consistent with a common mode of transport for potexviruses, involving a non-virion RNP, and show t hat TGBp1 is the movement protein, whereas TGBp2 and TGBp3 are either invol ved in intracellular transport or interact with the cellular machinery/dock ing sites at the plasmodesmata.