Unusual long-distance movement strategies of Potato mop-top virus RNAs in Nicotiana benthamiana.

The Potato mop-top virus (PMTV) genome encodes replicase, movement, and capsid proteins on three different RNA species that are encapsidated within tubular rodshaped particles. Previously, we showed that the protein produced on translational readthrough (RT) of the coat protein (CP) gene, CP-RT, is associated with one extremity of the virus particles, and that the two RNAs encoding replicase and movement proteins can move long distance in the absence of the third RNA (RNA-CP) that encodes the capsid proteins, CP and CP-RT. Here, we examined the roles of the CP and CP-RT proteins on RNA movement using infectious clones carrying mutations in the CP and CP-RT coding domains. The results showed that, in infections established with mutant RNA-CP expressing CP together with truncated CP-RT, systemic movement of the mutant RNA-CP was inhibited but not the movement of the other two RNAs. Furthermore, RNA-CP long-distance movement was inhibited in a mutant clone expressing only CP in the absence of the CP-RT polypeptide. CP-RT was not necessary for particle assembly because virions were observed in leaf extracts infected with the CP-RT deletion mutants. RNA-CP moved long distance when protein expression was suppressed completely or when CP expression was suppressed so that only CP-RT or truncated CP-RT was expressed. CP-RT but not CP interacted with the movement protein TGB1 in the yeast two-hybrid system. CP-RT and TGB1 were detected by enzyme-linked immunosorbent assay in virus particles and the long-distance movement of RNA-CP was correlated with expression of CP-RT that interacted with TGB1; mutant RNA-CP expressing truncated CP-RT proteins that did not interact with TGB1 formed virions but did not move to upper noninoculated leaves. The results indicate that PMTV RNA-CP can move long distance in two distinct forms: either as a viral ribonucleoprotein complex or as particles that are most likely associated with CP-RT and TGB1.