Carboxylate interactions involved in the disassembly of tobacco mosaic tobamovirus.

Structural studies of tobacco mosaic tobamovirus (TMV) have identified two coat protein (CP) intersubunit carboxyl-carboxylate interactions and one CP carboxylate-RNA phosphate interaction whose electrostatic repulsion is believed to drive virion disassembly. In this study, the involvement of each interaction in the disassembly process was examined. Site-directed mutagenesis was used to replace selected negatively charged CP residues, E or D, with neutral residues, Q or N, respectively. Purified mutant CPs were assayed for their ability to inhibit wild-type TMV disassembly both in vitro and in vivo. Results indicate that the lateral carboxylate interaction made by residue E106 is much more complex than previously thought, involving three residues, E95, E97, and D109, from an adjacent subunit. Mutations at all three residues are required to inhibit disassembly significantly. Different mutant coat proteins inhibited disassembly of the wild-type virus to varying degrees. Mutant E50Q, which modified the axial intersubunit interaction, had the greatest ability to inhibit disassembly followed by mutants E95Q/E97Q/D109N and D116N, which modified the lateral and CP-RNA interactions, respectively. Within each set of interacting carboxylate groups, mutations in the face opposite the disassembling surface of the TMV virion conferred the greatest ability to inhibit disassembly. This observation is consistent with the polar nature of TMV disassembly and confirms that repulsive intersubunit interactions derived from the 5' terminal subunits provide the key controlling mechanisms for virion disassembly.