Structure-function relationship between the tobamovirus TMV-Cg coat protein and the HR-like response.

The tobamovirus TMV-Cg induces an HR-like response in Nicotiana tabacum cv. Xanthi nn sensitive plants lacking the N or N' resistance genes. This response has been characterized by the appearance of necrotic lesions in the inoculated leaf and viral systemic spread, although the defence pathways are activated in the plant. A previous study demonstrated that the coat protein (CP) of TMV-Cg (CPCg) was the elicitor of this HR-like response. We examined the influence of four specific amino acid substitutions on the structure of CPCg, as well as on the development of the host response. To gain insights into the structural implications of these substitutions, a set of molecular dynamic experiments was performed using comparative models of wild-type and mutant CPCg as well as the CP of the U1 strain of TMV (CPU1), which is not recognized by the plants. A P21L mutation produces severe changes in the three-dimensional structure of CPCg and is more unstable when this subunit is laterally associated in silico. This result may explain the observed incapacity of this mutant to assemble virions. Two other CPCg mutations (R46G and S54K) overcome recognition by the plant and do not induce an HR-like response. A double CPCg mutant P21L-S54K recovered its capacity to form virions and to induce an HR-like response. Our results suggest that the structural integrity of the CP proteins is important for triggering the HR-like response.