COAT PROTEIN INTERACTIONS INVOLVED IN TOBACCO MOSAIC TOBAMOVIRUS CROSS-PROTECTION

To investigate the molecular role of the tobacco mosaic tobamovirus (T MV) coat protein (CP) in conferring cross-protection, a potato X potex virus (PVX) vector (S. Chapman, Plant J. 2, 549-557, 1992) was used to systemically express a set of TMV mutant CPs in Nicotiana benthamiana prior to challenge inoculation with TMV. PVX-expressed wild-type TMV CP delayed TMV accumulation for up to 2 weeks compared to unprotected plants or plants preinfected with the unmodified PVX vector. Similar d elays in TMV accumulation were obtained using TMV CPs that were defici ent: in Virion formation but competent to assemble into helical aggreg ates. In contrast, TMV CPs that were incapable of helical aggregation or unable to bind viral RNA did not delay the accumulation of TMV. Fur thermore, TMV CPs with enhanced intersubunit interactions that favor h elical aggregation produced significantly greater delays in the accumu lation of challenge TMV than obtained from the wild-type CP. Thus the capabilities of TMV GP to interact with viral RNA and self-associate i n a helical fashion appear to be essential to its ability to confer pr otection. Taken together, these findings support a model for GP-mediat ed resistance in which the protecting CP recoats the challenge virus R NA as it disassembles. (C) 1998 Academic Press.