Intersubunit interactions allowing a carboxylate mutant coat protein to inhibit tobamovirus disassembly

Tobacco mosaic tobamovirus (TMV) coat protein (CP) mutant E50Q lacks a repulsive intersubunit carboxylate group and can effectively inhibit the disassembly of wild-type TMV (Culver et al, 1995, Virology 206,724). To investigate the ability of this mutant CP to block disassembly, a series of second-site amino acid substitutions were added to the E50Q CP. These second-site mutations were designed to disrupt specific intersubunit stabilizing interactions involving hydrophobic or polar residues, salt bridges, and CP-RNA contacts. Results showed substitutions disrupting intersubunit interactions that face the disassembling surface of the virion dramatically reduced the ability of CP E50Q to inhibit TMV disassembly. Substitutions that disrupted the CP inner loop, RNA binding capabilities, or intersubunit interactions that faced away from the disassembling surface did not dramatically interfere with CP E50Q's ability to inhibit disassembly. Taken together, these findings suggest that intersubunit interactions made by 5' terminal E50Q subunits, not associated with RNA, provide the stabilizing forces that prevent virion disassembly. The role of these stabilizing interactions in TMV disassembly and their potential use for creating disassembly inhibiting CPs are discussed.