Elucidation of the viral disassembly switch of tobacco mosaic virus

Stable capsid structures of viruses protect viral RNA while they also require controlled disassembly for releasing the viral genome in the host cell. A detailed understanding of viral disassembly processes and the involved structural switches is still lacking. Biochemically, this process has been extensively studied using the tobacco mosaic virus model system and carboxylate interactions have been proposed to play a critical part in this process. Here, we present two cryo-EM structures of the helical TMV assembly at 2.1 and 2.0 Å resolution in conditions of high Ca 2+ concentration at low pH and in water. Based on our atomic models, we identified the conformational details of the disassembly switch mechanism: in high Ca 2+ /acidic pH environment the virion is stabilized between neighboring subunits through carboxyl groups E95 and E97 in close proximity to a Ca 2+ binding site. Upon increase in pH and lower Ca 2+ levels, mutual repulsion of the E95/E97 pair and Ca 2+ removal destabilize the network of interactions at lower radius and release the switch of virus disassembly. Our TMV structures revealed the conformational details for one of the reference systems of viral assembly/disassembly and provide the mechanistic explanation of a plethora of experimental results that were acquired over decades. Significance Statement Tobacco mosaic virus presents the text-book example of virus structure and RNA release from a viral capsid through disassembly. Despite the wealth of structural and biochemical data on the assembly/disassembly properties generated from more than 80 years of research, the atomic-resolution structural details of the proposed conformational changes have not been resolved to date. The here determined high-resolution cryo-EM structures reveal the conformational details of the molecular disassembly switch. When the virus enters the cell, carboxylate repulsion and loss of calcium-ion coordination destabilize the switch region and can trigger RNA release through virus disassembly. The two determined structural states resolve a long-standing question on environment-driven virus disassembly switches.